~dkuhlman/python-training-materials/Materials

2to3 is available in the standard library as <a class="reference internal" href="library/2to3.html#module-lib2to3" title="lib2to3: the 2to3 library"><code class="xref py py-mod docutils literal">lib2to3</code></a>; a standalone

entry point is provided as <code class="file docutils literal">Tools/scripts/2to3</code>. See

<a class="reference internal" href="library/2to3.html#to3-reference">2to3 - Automated Python 2 to 3 code translation</a>.

</dd>

<dt id="term-abstract-base-class">abstract base class</dt>

<dd>Abstract base classes complement <a class="reference internal" href="#term-duck-typing">duck-typing</a> by

providing a way to define interfaces when other techniques like

<a class="reference internal" href="library/functions.html#hasattr" title="hasattr"><code class="xref py py-func docutils literal">hasattr()</code></a> would be clumsy or subtly wrong (for example with

<a class="reference internal" href="reference/datamodel.html#new-style-special-lookup">magic methods</a>). ABCs introduce virtual

subclasses, which are classes that don’t inherit from a class but are

still recognized by <a class="reference internal" href="library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal">isinstance()</code></a> and <a class="reference internal" href="library/functions.html#issubclass" title="issubclass"><code class="xref py py-func docutils literal">issubclass()</code></a>; see the

<a class="reference internal" href="library/abc.html#module-abc" title="abc: Abstract base classes according to PEP 3119."><code class="xref py py-mod docutils literal">abc</code></a> module documentation. Python comes with many built-in ABCs for

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data structures (in the <a class="reference internal" href="library/collections.html#module-collections" title="collections: High-performance datatypes"><code class="xref py py-mod docutils literal">collections</code></a> module), numbers (in the

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<a class="reference internal" href="library/numbers.html#module-numbers" title="numbers: Numeric abstract base classes (Complex, Real, Integral, etc.)."><code class="xref py py-mod docutils literal">numbers</code></a> module), and streams (in the <a class="reference internal" href="library/io.html#module-io" title="io: Core tools for working with streams."><code class="xref py py-mod docutils literal">io</code></a> module). You can

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create your own ABCs with the <a class="reference internal" href="library/abc.html#module-abc" title="abc: Abstract base classes according to PEP 3119."><code class="xref py py-mod docutils literal">abc</code></a> module.</dd>

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<dt id="term-argument">argument</dt>

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<dd>A value passed to a <a class="reference internal" href="#term-function">function</a> (or <a class="reference internal" href="#term-method">method</a>) when calling the

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function. There are two types of arguments:

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<ul>

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<li>keyword argument: an argument preceded by an identifier (e.g.

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<code class="docutils literal">name=</code>) in a function call or passed as a value in a dictionary

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preceded by <code class="docutils literal">**</code>. For example, <code class="docutils literal">3</code> and <code class="docutils literal">5</code> are both keyword

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arguments in the following calls to <a class="reference internal" href="library/functions.html#complex" title="complex"><code class="xref py py-func docutils literal">complex()</code></a>:

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<div class="highlight-python"><div class="highlight"><pre>complex(real=3, imag=5)

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complex(**{'real': 3, 'imag': 5})

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</pre></div>

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</div>

115

</li>

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<li>positional argument: an argument that is not a keyword argument.

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Positional arguments can appear at the beginning of an argument list

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and/or be passed as elements of an <a class="reference internal" href="#term-iterable">iterable</a> preceded by <code class="docutils literal">*</code>.

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For example, <code class="docutils literal">3</code> and <code class="docutils literal">5</code> are both positional arguments in the

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following calls:

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<div class="highlight-python"><div class="highlight"><pre>complex(3, 5)

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complex(*(3, 5))

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</pre></div>

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</div>

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</li>

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</ul>

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Arguments are assigned to the named local variables in a function body.

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See the <a class="reference internal" href="reference/expressions.html#calls">Calls</a> section for the rules governing this assignment.

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Syntactically, any expression can be used to represent an argument; the

130

evaluated value is assigned to the local variable.

131

See also the <a class="reference internal" href="#term-parameter">parameter</a> glossary entry and the FAQ question on

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<a class="reference internal" href="faq/programming.html#faq-argument-vs-parameter">the difference between arguments and parameters</a>.

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</dd>

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<dt id="term-attribute">attribute</dt>

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<dd>A value associated with an object which is referenced by name using

136

dotted expressions. For example, if an object o has an attribute

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a it would be referenced as o.a.</dd>

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<dd>Benevolent Dictator For Life, a.k.a. <a class="reference external" href="https://www.python.org/~guido/">Guido van Rossum</a>, Python’s creator.</dd>

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<dt id="term-bytes-like-object">bytes-like object</dt>

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<dd>An object that supports the <a class="reference internal" href="c-api/buffer.html#bufferobjects">buffer protocol</a>,

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like <a class="reference internal" href="library/functions.html#str" title="str"><code class="xref py py-class docutils literal">str</code></a>, <a class="reference internal" href="library/functions.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal">bytearray</code></a> or <a class="reference internal" href="library/stdtypes.html#memoryview" title="memoryview"><code class="xref py py-class docutils literal">memoryview</code></a>.

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Bytes-like objects can be used for various operations that expect

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binary data, such as compression, saving to a binary file or sending

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over a socket. Some operations need the binary data to be mutable,

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in which case not all bytes-like objects can apply.</dd>

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<dt id="term-bytecode">bytecode</dt>

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<dd>Python source code is compiled into bytecode, the internal representation

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of a Python program in the CPython interpreter. The bytecode is also

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cached in <code class="docutils literal">.pyc</code> and <code class="docutils literal">.pyo</code> files so that executing the same file is

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faster the second time (recompilation from source to bytecode can be

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avoided). This “intermediate language” is said to run on a

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<a class="reference internal" href="#term-virtual-machine">virtual machine</a> that executes the machine code corresponding to

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each bytecode. Do note that bytecodes are not expected to work between

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different Python virtual machines, nor to be stable between Python

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releases.

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A list of bytecode instructions can be found in the documentation for

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<a class="reference internal" href="library/dis.html#bytecodes">the dis module</a>.

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</dd>

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<dt id="term-class">class</dt>

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<dd>A template for creating user-defined objects. Class definitions

162

normally contain method definitions which operate on instances of the

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class.</dd>

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<dt id="term-classic-class">classic class</dt>

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<dd>Any class which does not inherit from <a class="reference internal" href="library/functions.html#object" title="object"><code class="xref py py-class docutils literal">object</code></a>. See

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<a class="reference internal" href="#term-new-style-class">new-style class</a>. Classic classes have been removed in Python 3.</dd>

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<dt id="term-coercion">coercion</dt>

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<dd>The implicit conversion of an instance of one type to another during an

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operation which involves two arguments of the same type. For example,

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<code class="docutils literal">int(3.15)</code> converts the floating point number to the integer <code class="docutils literal">3</code>, but

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in <code class="docutils literal">3+4.5</code>, each argument is of a different type (one int, one float),

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and both must be converted to the same type before they can be added or it

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will raise a <code class="docutils literal">TypeError</code>. Coercion between two operands can be

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performed with the <code class="docutils literal">coerce</code> built-in function; thus, <code class="docutils literal">3+4.5</code> is

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equivalent to calling <code class="docutils literal">operator.add(*coerce(3, 4.5))</code> and results in

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<code class="docutils literal">operator.add(3.0, 4.5)</code>. Without coercion, all arguments of even

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compatible types would have to be normalized to the same value by the

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programmer, e.g., <code class="docutils literal">float(3)+4.5</code> rather than just <code class="docutils literal">3+4.5</code>.</dd>

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<dt id="term-complex-number">complex number</dt>

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<dd>An extension of the familiar real number system in which all numbers are

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expressed as a sum of a real part and an imaginary part. Imaginary

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numbers are real multiples of the imaginary unit (the square root of

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<code class="docutils literal">-1</code>), often written <code class="docutils literal">i</code> in mathematics or <code class="docutils literal">j</code> in

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engineering. Python has built-in support for complex numbers, which are

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written with this latter notation; the imaginary part is written with a

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<code class="docutils literal">j</code> suffix, e.g., <code class="docutils literal">3+1j</code>. To get access to complex equivalents of the

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<a class="reference internal" href="library/math.html#module-math" title="math: Mathematical functions (sin() etc.)."><code class="xref py py-mod docutils literal">math</code></a> module, use <a class="reference internal" href="library/cmath.html#module-cmath" title="cmath: Mathematical functions for complex numbers."><code class="xref py py-mod docutils literal">cmath</code></a>. Use of complex numbers is a fairly

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advanced mathematical feature. If you’re not aware of a need for them,

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it’s almost certain you can safely ignore them.</dd>

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<dt id="term-context-manager">context manager</dt>

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<dd>An object which controls the environment seen in a <a class="reference internal" href="reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal">with</code></a>

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statement by defining <a class="reference internal" href="reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal">__enter__()</code></a> and <a class="reference internal" href="reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal">__exit__()</code></a> methods.

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See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0343">PEP 343</a>.</dd>

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<dt id="term-cpython">CPython</dt>

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<dd>The canonical implementation of the Python programming language, as

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distributed on <a class="reference external" href="https://www.python.org">python.org</a>. The term “CPython”

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is used when necessary to distinguish this implementation from others

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such as Jython or IronPython.</dd>

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<dt id="term-decorator">decorator</dt>

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<dd>A function returning another function, usually applied as a function

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transformation using the <code class="docutils literal">@wrapper</code> syntax. Common examples for

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decorators are <a class="reference internal" href="library/functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal">classmethod()</code></a> and <a class="reference internal" href="library/functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal">staticmethod()</code></a>.

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The decorator syntax is merely syntactic sugar, the following two

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function definitions are semantically equivalent:

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<div class="highlight-python"><div class="highlight"><pre>def f(...):

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...

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f = staticmethod(f)

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@staticmethod

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def f(...):

211

...

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</pre></div>

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</div>

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The same concept exists for classes, but is less commonly used there. See

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the documentation for <a class="reference internal" href="reference/compound_stmts.html#function">function definitions</a> and

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<a class="reference internal" href="reference/compound_stmts.html#class">class definitions</a> for more about decorators.

217

</dd>

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<dt id="term-descriptor">descriptor</dt>

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<dd>Any new-style object which defines the methods <a class="reference internal" href="reference/datamodel.html#object.__get__" title="object.__get__"><code class="xref py py-meth docutils literal">__get__()</code></a>,

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<a class="reference internal" href="reference/datamodel.html#object.__set__" title="object.__set__"><code class="xref py py-meth docutils literal">__set__()</code></a>, or <a class="reference internal" href="reference/datamodel.html#object.__delete__" title="object.__delete__"><code class="xref py py-meth docutils literal">__delete__()</code></a>. When a class attribute is a

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descriptor, its special binding behavior is triggered upon attribute

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lookup. Normally, using a.b to get, set or delete an attribute looks up

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the object named b in the class dictionary for a, but if b is a

224

descriptor, the respective descriptor method gets called. Understanding

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descriptors is a key to a deep understanding of Python because they are

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the basis for many features including functions, methods, properties,

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class methods, static methods, and reference to super classes.

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For more information about descriptors’ methods, see <a class="reference internal" href="reference/datamodel.html#descriptors">Implementing Descriptors</a>.

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</dd>

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<dt id="term-dictionary">dictionary</dt>

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<dd>An associative array, where arbitrary keys are mapped to values. The

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keys can be any object with <a class="reference internal" href="reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal">__hash__()</code></a> and <a class="reference internal" href="reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal">__eq__()</code></a> methods.

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Called a hash in Perl.</dd>

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<dt id="term-dictionary-view">dictionary view</dt>

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<dd>The objects returned from <a class="reference internal" href="library/stdtypes.html#dict.viewkeys" title="dict.viewkeys"><code class="xref py py-meth docutils literal">dict.viewkeys()</code></a>, <a class="reference internal" href="library/stdtypes.html#dict.viewvalues" title="dict.viewvalues"><code class="xref py py-meth docutils literal">dict.viewvalues()</code></a>,

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and <a class="reference internal" href="library/stdtypes.html#dict.viewitems" title="dict.viewitems"><code class="xref py py-meth docutils literal">dict.viewitems()</code></a> are called dictionary views. They provide a dynamic

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view on the dictionary’s entries, which means that when the dictionary

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changes, the view reflects these changes. To force the

239

dictionary view to become a full list use <code class="docutils literal">list(dictview)</code>. See

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<a class="reference internal" href="library/stdtypes.html#dict-views">Dictionary view objects</a>.</dd>

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<dt id="term-docstring">docstring</dt>

242

<dd>A string literal which appears as the first expression in a class,

243

function or module. While ignored when the suite is executed, it is

244

recognized by the compiler and put into the <code class="xref py py-attr docutils literal">__doc__</code> attribute

245

of the enclosing class, function or module. Since it is available via

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introspection, it is the canonical place for documentation of the

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object.</dd>

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<dt id="term-duck-typing">duck-typing</dt>

249

<dd>A programming style which does not look at an object’s type to determine

250

if it has the right interface; instead, the method or attribute is simply

251

called or used (“If it looks like a duck and quacks like a duck, it

252

must be a duck.”) By emphasizing interfaces rather than specific types,

253

well-designed code improves its flexibility by allowing polymorphic

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substitution. Duck-typing avoids tests using <a class="reference internal" href="library/functions.html#type" title="type"><code class="xref py py-func docutils literal">type()</code></a> or

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<a class="reference internal" href="library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal">isinstance()</code></a>. (Note, however, that duck-typing can be complemented

256

with <a class="reference internal" href="#term-abstract-base-class">abstract base classes</a>.) Instead, it

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typically employs <a class="reference internal" href="library/functions.html#hasattr" title="hasattr"><code class="xref py py-func docutils literal">hasattr()</code></a> tests or <a class="reference internal" href="#term-eafp">EAFP</a> programming.</dd>

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<dd>Easier to ask for forgiveness than permission. This common Python coding

260

style assumes the existence of valid keys or attributes and catches

261

exceptions if the assumption proves false. This clean and fast style is

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characterized by the presence of many <a class="reference internal" href="reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal">try</code></a> and <a class="reference internal" href="reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal">except</code></a>

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statements. The technique contrasts with the <a class="reference internal" href="#term-lbyl">LBYL</a> style

264

common to many other languages such as C.</dd>

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<dt id="term-expression">expression</dt>

266

<dd>A piece of syntax which can be evaluated to some value. In other words,

267

an expression is an accumulation of expression elements like literals,

268

names, attribute access, operators or function calls which all return a

269

value. In contrast to many other languages, not all language constructs

270

are expressions. There are also <a class="reference internal" href="#term-statement">statement</a>s which cannot be used

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as expressions, such as <a class="reference internal" href="reference/simple_stmts.html#print"><code class="xref std std-keyword docutils literal">print</code></a> or <a class="reference internal" href="reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal">if</code></a>. Assignments

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are also statements, not expressions.</dd>

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<dt id="term-extension-module">extension module</dt>

274

<dd>A module written in C or C++, using Python’s C API to interact with the

275

core and with user code.</dd>

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<dt id="term-file-object">file object</dt>

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<dd>An object exposing a file-oriented API (with methods such as

278

<code class="xref py py-meth docutils literal">read()</code> or <code class="xref py py-meth docutils literal">write()</code>) to an underlying resource. Depending

279

on the way it was created, a file object can mediate access to a real

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on-disk file or to another type of storage or communication device

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(for example standard input/output, in-memory buffers, sockets, pipes,

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etc.). File objects are also called file-like objects or

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streams.

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There are actually three categories of file objects: raw binary files,

285

buffered binary files and text files. Their interfaces are defined in the

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<a class="reference internal" href="library/io.html#module-io" title="io: Core tools for working with streams."><code class="xref py py-mod docutils literal">io</code></a> module. The canonical way to create a file object is by using

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the <a class="reference internal" href="library/functions.html#open" title="open"><code class="xref py py-func docutils literal">open()</code></a> function.

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</dd>

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<dt id="term-file-like-object">file-like object</dt>

290

<dd>A synonym for <a class="reference internal" href="#term-file-object">file object</a>.</dd>

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<dt id="term-finder">finder</dt>

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<dd>An object that tries to find the <a class="reference internal" href="#term-loader">loader</a> for a module. It must

293

implement a method named <code class="xref py py-meth docutils literal">find_module()</code>. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0302">PEP 302</a> for

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details.</dd>

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<dt id="term-floor-division">floor division</dt>

296

<dd>Mathematical division that rounds down to nearest integer. The floor

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division operator is <code class="docutils literal">//</code>. For example, the expression <code class="docutils literal">11 // 4</code>

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evaluates to <code class="docutils literal">2</code> in contrast to the <code class="docutils literal">2.75</code> returned by float true

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division. Note that <code class="docutils literal">(-11) // 4</code> is <code class="docutils literal">-3</code> because that is <code class="docutils literal">-2.75</code>

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rounded downward. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0238">PEP 238</a>.</dd>

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<dt id="term-function">function</dt>

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<dd>A series of statements which returns some value to a caller. It can also

303

be passed zero or more <a class="reference internal" href="#term-argument">arguments</a> which may be used in

304

the execution of the body. See also <a class="reference internal" href="#term-parameter">parameter</a>, <a class="reference internal" href="#term-method">method</a>,

305

and the <a class="reference internal" href="reference/compound_stmts.html#function">Function definitions</a> section.</dd>

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<dt id="term-future">__future__</dt>

307

<dd>A pseudo-module which programmers can use to enable new language features

308

which are not compatible with the current interpreter. For example, the

309

expression <code class="docutils literal">11/4</code> currently evaluates to <code class="docutils literal">2</code>. If the module in which

310

it is executed had enabled true division by executing:

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<div class="highlight-python"><div class="highlight"><pre>from __future__ import division

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</pre></div>

313

</div>

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the expression <code class="docutils literal">11/4</code> would evaluate to <code class="docutils literal">2.75</code>. By importing the

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<a class="reference internal" href="library/__future__.html#module-__future__" title="__future__: Future statement definitions"><code class="xref py py-mod docutils literal">__future__</code></a> module and evaluating its variables, you can see when a

316

new feature was first added to the language and when it will become the

317

default:

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<div class="last highlight-python"><div class="highlight"><pre>>>> import __future__

319

>>> __future__.division

320

_Feature((2, 2, 0, 'alpha', 2), (3, 0, 0, 'alpha', 0), 8192)

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</pre></div>

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</div>

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</dd>

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<dt id="term-garbage-collection">garbage collection</dt>

325

<dd>The process of freeing memory when it is not used anymore. Python

326

performs garbage collection via reference counting and a cyclic garbage

327

collector that is able to detect and break reference cycles.</dd>

328

<dt id="term-generator">generator</dt>

329

<dd>A function which returns an iterator. It looks like a normal function

330

except that it contains <a class="reference internal" href="reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal">yield</code></a> statements for producing a series

331

of values usable in a for-loop or that can be retrieved one at a time with

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the <a class="reference internal" href="library/functions.html#next" title="next"><code class="xref py py-func docutils literal">next()</code></a> function. Each <a class="reference internal" href="reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal">yield</code></a> temporarily suspends

333

processing, remembering the location execution state (including local

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variables and pending try-statements). When the generator resumes, it

335

picks-up where it left-off (in contrast to functions which start fresh on

336

every invocation).</dd>

337

<dt id="term-generator-expression">generator expression</dt>

338

<dd>An expression that returns an iterator. It looks like a normal expression

339

followed by a <a class="reference internal" href="reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal">for</code></a> expression defining a loop variable, range,

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and an optional <a class="reference internal" href="reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal">if</code></a> expression. The combined expression

341

generates values for an enclosing function:

342

<div class="last highlight-python"><div class="highlight"><pre>>>> sum(i*i for i in range(10)) # sum of squares 0, 1, 4, ... 81

343

285

344

</pre></div>

345

</div>

346

</dd>

347

348

<dd>See <a class="reference internal" href="#term-global-interpreter-lock">global interpreter lock</a>.</dd>

349

<dt id="term-global-interpreter-lock">global interpreter lock</dt>

350

<dd>The mechanism used by the <a class="reference internal" href="#term-cpython">CPython</a> interpreter to assure that

351

only one thread executes Python <a class="reference internal" href="#term-bytecode">bytecode</a> at a time.

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This simplifies the CPython implementation by making the object model

353

(including critical built-in types such as <a class="reference internal" href="library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal">dict</code></a>) implicitly

354

safe against concurrent access. Locking the entire interpreter

355

makes it easier for the interpreter to be multi-threaded, at the

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expense of much of the parallelism afforded by multi-processor

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machines.

358

However, some extension modules, either standard or third-party,

359

are designed so as to release the GIL when doing computationally-intensive

360

tasks such as compression or hashing. Also, the GIL is always released

361

when doing I/O.

362

Past efforts to create a “free-threaded” interpreter (one which locks

363

shared data at a much finer granularity) have not been successful

364

because performance suffered in the common single-processor case. It

365

is believed that overcoming this performance issue would make the

366

implementation much more complicated and therefore costlier to maintain.

367

</dd>

368

<dt id="term-hashable">hashable</dt>

369

<dd>An object is hashable if it has a hash value which never changes during

370

its lifetime (it needs a <a class="reference internal" href="reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal">__hash__()</code></a> method), and can be compared to

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other objects (it needs an <a class="reference internal" href="reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal">__eq__()</code></a> or <a class="reference internal" href="reference/datamodel.html#object.__cmp__" title="object.__cmp__"><code class="xref py py-meth docutils literal">__cmp__()</code></a> method).

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Hashable objects which compare equal must have the same hash value.

373

Hashability makes an object usable as a dictionary key and a set member,

374

because these data structures use the hash value internally.

375

All of Python’s immutable built-in objects are hashable, while no mutable

376

containers (such as lists or dictionaries) are. Objects which are

377

instances of user-defined classes are hashable by default; they all

378

compare unequal (except with themselves), and their hash value is derived

379

from their <a class="reference internal" href="library/functions.html#id" title="id"><code class="xref py py-func docutils literal">id()</code></a>.

380

</dd>

381

382

<dd>An Integrated Development Environment for Python. IDLE is a basic editor

383

and interpreter environment which ships with the standard distribution of

384

Python.</dd>

385

<dt id="term-immutable">immutable</dt>

386

<dd>An object with a fixed value. Immutable objects include numbers, strings and

387

tuples. Such an object cannot be altered. A new object has to

388

be created if a different value has to be stored. They play an important

389

role in places where a constant hash value is needed, for example as a key

390

in a dictionary.</dd>

391

<dt id="term-integer-division">integer division</dt>

392

<dd>Mathematical division discarding any remainder. For example, the

393

expression <code class="docutils literal">11/4</code> currently evaluates to <code class="docutils literal">2</code> in contrast to the

394

<code class="docutils literal">2.75</code> returned by float division. Also called floor division.

395

When dividing two integers the outcome will always be another integer

396

(having the floor function applied to it). However, if one of the operands

397

is another numeric type (such as a <a class="reference internal" href="library/functions.html#float" title="float"><code class="xref py py-class docutils literal">float</code></a>), the result will be

398

coerced (see <a class="reference internal" href="#term-coercion">coercion</a>) to a common type. For example, an integer

399

divided by a float will result in a float value, possibly with a decimal

400

fraction. Integer division can be forced by using the <code class="docutils literal">//</code> operator

401

instead of the <code class="docutils literal">/</code> operator. See also <a class="reference internal" href="#term-future">__future__</a>.</dd>

402

<dt id="term-importing">importing</dt>

403

<dd>The process by which Python code in one module is made available to

404

Python code in another module.</dd>

405

<dt id="term-importer">importer</dt>

406

<dd>An object that both finds and loads a module; both a

407

<a class="reference internal" href="#term-finder">finder</a> and <a class="reference internal" href="#term-loader">loader</a> object.</dd>

408

<dt id="term-interactive">interactive</dt>

409

<dd>Python has an interactive interpreter which means you can enter

410

statements and expressions at the interpreter prompt, immediately

411

execute them and see their results. Just launch <code class="docutils literal">python</code> with no

412

arguments (possibly by selecting it from your computer’s main

413

menu). It is a very powerful way to test out new ideas or inspect

414

modules and packages (remember <code class="docutils literal">help(x)</code>).</dd>

415

<dt id="term-interpreted">interpreted</dt>

416

<dd>Python is an interpreted language, as opposed to a compiled one,

417

though the distinction can be blurry because of the presence of the

418

bytecode compiler. This means that source files can be run directly

419

without explicitly creating an executable which is then run.

420

Interpreted languages typically have a shorter development/debug cycle

421

than compiled ones, though their programs generally also run more

422

slowly. See also <a class="reference internal" href="#term-interactive">interactive</a>.</dd>

423

<dt id="term-iterable">iterable</dt>

424

<dd>An object capable of returning its members one at a time. Examples of

425

iterables include all sequence types (such as <a class="reference internal" href="library/functions.html#list" title="list"><code class="xref py py-class docutils literal">list</code></a>, <a class="reference internal" href="library/functions.html#str" title="str"><code class="xref py py-class docutils literal">str</code></a>,

426

and <a class="reference internal" href="library/functions.html#tuple" title="tuple"><code class="xref py py-class docutils literal">tuple</code></a>) and some non-sequence types like <a class="reference internal" href="library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal">dict</code></a>

427

and <a class="reference internal" href="library/functions.html#file" title="file"><code class="xref py py-class docutils literal">file</code></a> and objects of any classes you define

428

with an <a class="reference internal" href="reference/datamodel.html#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal">__iter__()</code></a> or <a class="reference internal" href="reference/datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal">__getitem__()</code></a> method. Iterables can be

429

used in a <a class="reference internal" href="reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal">for</code></a> loop and in many other places where a sequence is

430

needed (<a class="reference internal" href="library/functions.html#zip" title="zip"><code class="xref py py-func docutils literal">zip()</code></a>, <a class="reference internal" href="library/functions.html#map" title="map"><code class="xref py py-func docutils literal">map()</code></a>, ...). When an iterable object is passed

431

as an argument to the built-in function <a class="reference internal" href="library/functions.html#iter" title="iter"><code class="xref py py-func docutils literal">iter()</code></a>, it returns an

432

iterator for the object. This iterator is good for one pass over the set

433

of values. When using iterables, it is usually not necessary to call

434

<a class="reference internal" href="library/functions.html#iter" title="iter"><code class="xref py py-func docutils literal">iter()</code></a> or deal with iterator objects yourself. The <code class="docutils literal">for</code>

435

statement does that automatically for you, creating a temporary unnamed

436

variable to hold the iterator for the duration of the loop. See also

437

<a class="reference internal" href="#term-iterator">iterator</a>, <a class="reference internal" href="#term-sequence">sequence</a>, and <a class="reference internal" href="#term-generator">generator</a>.</dd>

438

<dt id="term-iterator">iterator</dt>

439

<dd>An object representing a stream of data. Repeated calls to the iterator’s

440

441

data are available a <a class="reference internal" href="library/exceptions.html#exceptions.StopIteration" title="exceptions.StopIteration"><code class="xref py py-exc docutils literal">StopIteration</code></a> exception is raised instead. At

442

this point, the iterator object is exhausted and any further calls to its

443

<a class="reference internal" href="reference/expressions.html#generator.next" title="generator.next"><code class="xref py py-meth docutils literal">next()</code></a> method just raise <a class="reference internal" href="library/exceptions.html#exceptions.StopIteration" title="exceptions.StopIteration"><code class="xref py py-exc docutils literal">StopIteration</code></a> again. Iterators are

444

required to have an <a class="reference internal" href="reference/datamodel.html#object.__iter__" title="object.__iter__"><code class="xref py py-meth docutils literal">__iter__()</code></a> method that returns the iterator

445

object itself so every iterator is also iterable and may be used in most

446

places where other iterables are accepted. One notable exception is code

447

which attempts multiple iteration passes. A container object (such as a

448

<a class="reference internal" href="library/functions.html#list" title="list"><code class="xref py py-class docutils literal">list</code></a>) produces a fresh new iterator each time you pass it to the

449

<a class="reference internal" href="library/functions.html#iter" title="iter"><code class="xref py py-func docutils literal">iter()</code></a> function or use it in a <a class="reference internal" href="reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal">for</code></a> loop. Attempting this

450

with an iterator will just return the same exhausted iterator object used

451

in the previous iteration pass, making it appear like an empty container.

452

More information can be found in <a class="reference internal" href="library/stdtypes.html#typeiter">Iterator Types</a>.

453

</dd>

454

<dt id="term-key-function">key function</dt>

455

<dd>A key function or collation function is a callable that returns a value

456

used for sorting or ordering. For example, <a class="reference internal" href="library/locale.html#locale.strxfrm" title="locale.strxfrm"><code class="xref py py-func docutils literal">locale.strxfrm()</code></a> is

457

used to produce a sort key that is aware of locale specific sort

458

conventions.

459

A number of tools in Python accept key functions to control how elements

460

are ordered or grouped. They include <a class="reference internal" href="library/functions.html#min" title="min"><code class="xref py py-func docutils literal">min()</code></a>, <a class="reference internal" href="library/functions.html#max" title="max"><code class="xref py py-func docutils literal">max()</code></a>,

461

<a class="reference internal" href="library/functions.html#sorted" title="sorted"><code class="xref py py-func docutils literal">sorted()</code></a>, <code class="xref py py-meth docutils literal">list.sort()</code>, <a class="reference internal" href="library/heapq.html#heapq.nsmallest" title="heapq.nsmallest"><code class="xref py py-func docutils literal">heapq.nsmallest()</code></a>,

462

<a class="reference internal" href="library/heapq.html#heapq.nlargest" title="heapq.nlargest"><code class="xref py py-func docutils literal">heapq.nlargest()</code></a>, and <a class="reference internal" href="library/itertools.html#itertools.groupby" title="itertools.groupby"><code class="xref py py-func docutils literal">itertools.groupby()</code></a>.

463

There are several ways to create a key function. For example. the

464

<a class="reference internal" href="library/stdtypes.html#str.lower" title="str.lower"><code class="xref py py-meth docutils literal">str.lower()</code></a> method can serve as a key function for case insensitive

465

sorts. Alternatively, an ad-hoc key function can be built from a

466

<a class="reference internal" href="reference/expressions.html#lambda"><code class="xref std std-keyword docutils literal">lambda</code></a> expression such as <code class="docutils literal">lambda r: (r[0], r[2])</code>. Also,

467

the <a class="reference internal" href="library/operator.html#module-operator" title="operator: Functions corresponding to the standard operators."><code class="xref py py-mod docutils literal">operator</code></a> module provides three key function constructors:

468

<a class="reference internal" href="library/operator.html#operator.attrgetter" title="operator.attrgetter"><code class="xref py py-func docutils literal">attrgetter()</code></a>, <a class="reference internal" href="library/operator.html#operator.itemgetter" title="operator.itemgetter"><code class="xref py py-func docutils literal">itemgetter()</code></a>, and

469

<a class="reference internal" href="library/operator.html#operator.methodcaller" title="operator.methodcaller"><code class="xref py py-func docutils literal">methodcaller()</code></a>. See the <a class="reference internal" href="howto/sorting.html#sortinghowto">Sorting HOW TO</a> for examples of how to create and use key functions.

470

</dd>

471

<dt id="term-keyword-argument">keyword argument</dt>

472

<dd>See <a class="reference internal" href="#term-argument">argument</a>.</dd>

473

<dt id="term-lambda">lambda</dt>

474

<dd>An anonymous inline function consisting of a single <a class="reference internal" href="#term-expression">expression</a>

475

which is evaluated when the function is called. The syntax to create

476

a lambda function is <code class="docutils literal">lambda [arguments]: expression</code></dd>

477

478

<dd>Look before you leap. This coding style explicitly tests for

479

pre-conditions before making calls or lookups. This style contrasts with

480

the <a class="reference internal" href="#term-eafp">EAFP</a> approach and is characterized by the presence of many

481

<a class="reference internal" href="reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal">if</code></a> statements.

482

In a multi-threaded environment, the LBYL approach can risk introducing a

483

race condition between “the looking” and “the leaping”. For example, the

484

code, <code class="docutils literal">if key in mapping: return mapping[key]</code> can fail if another

485

thread removes key from mapping after the test, but before the lookup.

486

This issue can be solved with locks or by using the EAFP approach.

487

</dd>

488

489

<dd>A built-in Python <a class="reference internal" href="#term-sequence">sequence</a>. Despite its name it is more akin

490

to an array in other languages than to a linked list since access to

491

elements are O(1).</dd>

492

<dt id="term-list-comprehension">list comprehension</dt>

493

<dd>A compact way to process all or part of the elements in a sequence and

494

return a list with the results. <code class="docutils literal">result = ["0x%02x" % x for x in

495

range(256) if x % 2 == 0]</code> generates a list of strings containing

496

even hex numbers (0x..) in the range from 0 to 255. The <a class="reference internal" href="reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal">if</code></a>

497

clause is optional. If omitted, all elements in <code class="docutils literal">range(256)</code> are

498

processed.</dd>

499

<dt id="term-loader">loader</dt>

500

<dd>An object that loads a module. It must define a method named

501

<code class="xref py py-meth docutils literal">load_module()</code>. A loader is typically returned by a

502

<a class="reference internal" href="#term-finder">finder</a>. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0302">PEP 302</a> for details.</dd>

503

<dt id="term-mapping">mapping</dt>

504

<dd>A container object that supports arbitrary key lookups and implements the

505

methods specified in the <a class="reference internal" href="library/collections.html#collections.Mapping" title="collections.Mapping"><code class="xref py py-class docutils literal">Mapping</code></a> or

506

<a class="reference internal" href="library/collections.html#collections.MutableMapping" title="collections.MutableMapping"><code class="xref py py-class docutils literal">MutableMapping</code></a>

507

<a class="reference internal" href="library/collections.html#collections-abstract-base-classes">abstract base classes</a>. Examples

508

include <a class="reference internal" href="library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal">dict</code></a>, <a class="reference internal" href="library/collections.html#collections.defaultdict" title="collections.defaultdict"><code class="xref py py-class docutils literal">collections.defaultdict</code></a>,

509

<a class="reference internal" href="library/collections.html#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal">collections.OrderedDict</code></a> and <a class="reference internal" href="library/collections.html#collections.Counter" title="collections.Counter"><code class="xref py py-class docutils literal">collections.Counter</code></a>.</dd>

510

<dt id="term-metaclass">metaclass</dt>

511

<dd>The class of a class. Class definitions create a class name, a class

512

dictionary, and a list of base classes. The metaclass is responsible for

513

taking those three arguments and creating the class. Most object oriented

514

programming languages provide a default implementation. What makes Python

515

special is that it is possible to create custom metaclasses. Most users

516

never need this tool, but when the need arises, metaclasses can provide

517

powerful, elegant solutions. They have been used for logging attribute

518

access, adding thread-safety, tracking object creation, implementing

519

singletons, and many other tasks.

520

More information can be found in <a class="reference internal" href="reference/datamodel.html#metaclasses">Customizing class creation</a>.

521

</dd>

522

<dt id="term-method">method</dt>

523

<dd>A function which is defined inside a class body. If called as an attribute

524

of an instance of that class, the method will get the instance object as

525

its first <a class="reference internal" href="#term-argument">argument</a> (which is usually called <code class="docutils literal">self</code>).

526

See <a class="reference internal" href="#term-function">function</a> and <a class="reference internal" href="#term-nested-scope">nested scope</a>.</dd>

527

<dt id="term-method-resolution-order">method resolution order</dt>

528

<dd>Method Resolution Order is the order in which base classes are searched

529

for a member during lookup. See <a class="reference external" href="https://www.python.org/download/releases/2.3/mro/">The Python 2.3 Method Resolution Order</a> for details of the

530

algorithm used by the Python interpreter since the 2.3 release.</dd>

531

<dt id="term-module">module</dt>

532

<dd>An object that serves as an organizational unit of Python code. Modules

533

have a namespace containing arbitrary Python objects. Modules are loaded

534

into Python by the process of <a class="reference internal" href="#term-importing">importing</a>.

535

See also <a class="reference internal" href="#term-package">package</a>.

536

</dd>

537

538

<dd>See <a class="reference internal" href="#term-method-resolution-order">method resolution order</a>.</dd>

539

<dt id="term-mutable">mutable</dt>

540

<dd>Mutable objects can change their value but keep their <a class="reference internal" href="library/functions.html#id" title="id"><code class="xref py py-func docutils literal">id()</code></a>. See

541

also <a class="reference internal" href="#term-immutable">immutable</a>.</dd>

542

<dt id="term-named-tuple">named tuple</dt>

543

<dd>Any tuple-like class whose indexable elements are also accessible using

544

named attributes (for example, <a class="reference internal" href="library/time.html#time.localtime" title="time.localtime"><code class="xref py py-func docutils literal">time.localtime()</code></a> returns a

545

tuple-like object where the year is accessible either with an

546

index such as <code class="docutils literal">t[0]</code> or with a named attribute like <code class="docutils literal">t.tm_year</code>).

547

A named tuple can be a built-in type such as <a class="reference internal" href="library/time.html#time.struct_time" title="time.struct_time"><code class="xref py py-class docutils literal">time.struct_time</code></a>,

548

or it can be created with a regular class definition. A full featured

549

named tuple can also be created with the factory function

550

<a class="reference internal" href="library/collections.html#collections.namedtuple" title="collections.namedtuple"><code class="xref py py-func docutils literal">collections.namedtuple()</code></a>. The latter approach automatically

551

provides extra features such as a self-documenting representation like

552

<code class="docutils literal">Employee(name='jones', title='programmer')</code>.

553

</dd>

554

<dt id="term-namespace">namespace</dt>

555

<dd>The place where a variable is stored. Namespaces are implemented as

556

dictionaries. There are the local, global and built-in namespaces as well

557

as nested namespaces in objects (in methods). Namespaces support

558

modularity by preventing naming conflicts. For instance, the functions

559

<code class="xref py py-func docutils literal">__builtin__.open()</code> and <a class="reference internal" href="library/os.html#os.open" title="os.open"><code class="xref py py-func docutils literal">os.open()</code></a> are distinguished by their

560

namespaces. Namespaces also aid readability and maintainability by making

561

it clear which module implements a function. For instance, writing

562

<a class="reference internal" href="library/random.html#random.seed" title="random.seed"><code class="xref py py-func docutils literal">random.seed()</code></a> or <a class="reference internal" href="library/itertools.html#itertools.izip" title="itertools.izip"><code class="xref py py-func docutils literal">itertools.izip()</code></a> makes it clear that those

563

functions are implemented by the <a class="reference internal" href="library/random.html#module-random" title="random: Generate pseudo-random numbers with various common distributions."><code class="xref py py-mod docutils literal">random</code></a> and <a class="reference internal" href="library/itertools.html#module-itertools" title="itertools: Functions creating iterators for efficient looping."><code class="xref py py-mod docutils literal">itertools</code></a>

564

modules, respectively.</dd>

565

<dt id="term-nested-scope">nested scope</dt>

566

<dd>The ability to refer to a variable in an enclosing definition. For

567

instance, a function defined inside another function can refer to

568

variables in the outer function. Note that nested scopes work only for

569

reference and not for assignment which will always write to the innermost

570

scope. In contrast, local variables both read and write in the innermost

571

scope. Likewise, global variables read and write to the global namespace.</dd>

572

<dt id="term-new-style-class">new-style class</dt>

573

<dd>Any class which inherits from <a class="reference internal" href="library/functions.html#object" title="object"><code class="xref py py-class docutils literal">object</code></a>. This includes all built-in

574

types like <a class="reference internal" href="library/functions.html#list" title="list"><code class="xref py py-class docutils literal">list</code></a> and <a class="reference internal" href="library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal">dict</code></a>. Only new-style classes can

575

use Python’s newer, versatile features like <code class="xref py py-attr docutils literal">__slots__</code>,

576

descriptors, properties, and <a class="reference internal" href="reference/datamodel.html#object.__getattribute__" title="object.__getattribute__"><code class="xref py py-meth docutils literal">__getattribute__()</code></a>.

577

More information can be found in <a class="reference internal" href="reference/datamodel.html#newstyle">New-style and classic classes</a>.

578

</dd>

579

<dt id="term-object">object</dt>

580

<dd>Any data with state (attributes or value) and defined behavior

581

(methods). Also the ultimate base class of any <a class="reference internal" href="#term-new-style-class">new-style

582

class</a>.</dd>

583

<dt id="term-package">package</dt>

584

<dd>A Python <a class="reference internal" href="#term-module">module</a> which can contain submodules or recursively,

585

subpackages. Technically, a package is a Python module with an

586

<code class="docutils literal">__path__</code> attribute.</dd>

587

<dt id="term-parameter">parameter</dt>

588

<dd>A named entity in a <a class="reference internal" href="#term-function">function</a> (or method) definition that

589

specifies an <a class="reference internal" href="#term-argument">argument</a> (or in some cases, arguments) that the

590

function can accept. There are four types of parameters:

591

<ul>

592

<li>positional-or-keyword: specifies an argument that can be passed

593

either <a class="reference internal" href="#term-argument">positionally</a> or as a <a class="reference internal" href="#term-argument">keyword argument</a>. This is the default kind of parameter, for example foo

594

and bar in the following:

595

<div class="highlight-python"><div class="highlight"><pre>def func(foo, bar=None): ...

596

</pre></div>

597

</div>

598

</li>

599

<li>positional-only: specifies an argument that can be supplied only

600

by position. Python has no syntax for defining positional-only

601

parameters. However, some built-in functions have positional-only

602

parameters (e.g. <a class="reference internal" href="library/functions.html#abs" title="abs"><code class="xref py py-func docutils literal">abs()</code></a>).

603

</li>

604

<li>var-positional: specifies that an arbitrary sequence of

605

positional arguments can be provided (in addition to any positional

606

arguments already accepted by other parameters). Such a parameter can

607

be defined by prepending the parameter name with <code class="docutils literal">*</code>, for example

608

args in the following:

609

<div class="highlight-python"><div class="highlight"><pre>def func(*args, **kwargs): ...

610

</pre></div>

611

</div>

612

</li>

613

<li>var-keyword: specifies that arbitrarily many keyword arguments

614

can be provided (in addition to any keyword arguments already accepted

615

by other parameters). Such a parameter can be defined by prepending

616

the parameter name with <code class="docutils literal">**</code>, for example kwargs in the example

617

above.

618

</li>

619

</ul>

620

Parameters can specify both optional and required arguments, as well as

621

default values for some optional arguments.

622

See also the <a class="reference internal" href="#term-argument">argument</a> glossary entry, the FAQ question on

623

<a class="reference internal" href="faq/programming.html#faq-argument-vs-parameter">the difference between arguments and parameters</a>, and the <a class="reference internal" href="reference/compound_stmts.html#function">Function definitions</a> section.

624

</dd>

625

<dt id="term-positional-argument">positional argument</dt>

626

<dd>See <a class="reference internal" href="#term-argument">argument</a>.</dd>

627

<dt id="term-python-3000">Python 3000</dt>

628

<dd>Nickname for the Python 3.x release line (coined long ago when the release

629

of version 3 was something in the distant future.) This is also

630

abbreviated “Py3k”.</dd>

631

<dt id="term-pythonic">Pythonic</dt>

632

<dd>An idea or piece of code which closely follows the most common idioms

633

of the Python language, rather than implementing code using concepts

634

common to other languages. For example, a common idiom in Python is

635

to loop over all elements of an iterable using a <a class="reference internal" href="reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal">for</code></a>

636

statement. Many other languages don’t have this type of construct, so

637

people unfamiliar with Python sometimes use a numerical counter instead:

638

<div class="highlight-python"><div class="highlight"><pre>for i in range(len(food)):

639

print food[i]

640

</pre></div>

641

</div>

642

As opposed to the cleaner, Pythonic method:

643

<div class="last highlight-python"><div class="highlight"><pre>for piece in food:

644

print piece

645

</pre></div>

646

</div>

647

</dd>

648

<dt id="term-reference-count">reference count</dt>

649

<dd>The number of references to an object. When the reference count of an

650

object drops to zero, it is deallocated. Reference counting is

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generally not visible to Python code, but it is a key element of the

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<a class="reference internal" href="#term-cpython">CPython</a> implementation. The <a class="reference internal" href="library/sys.html#module-sys" title="sys: Access system-specific parameters and functions."><code class="xref py py-mod docutils literal">sys</code></a> module defines a

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<a class="reference internal" href="library/sys.html#sys.getrefcount" title="sys.getrefcount"><code class="xref py py-func docutils literal">getrefcount()</code></a> function that programmers can call to return the

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reference count for a particular object.</dd>

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<dt id="term-slots">__slots__</dt>

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<dd>A declaration inside a <a class="reference internal" href="#term-new-style-class">new-style class</a> that saves memory by

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pre-declaring space for instance attributes and eliminating instance

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dictionaries. Though popular, the technique is somewhat tricky to get

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right and is best reserved for rare cases where there are large numbers of

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instances in a memory-critical application.</dd>

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<dt id="term-sequence">sequence</dt>

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<dd>An <a class="reference internal" href="#term-iterable">iterable</a> which supports efficient element access using integer

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indices via the <a class="reference internal" href="reference/datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal">__getitem__()</code></a> special method and defines a

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<a class="reference internal" href="library/functions.html#len" title="len"><code class="xref py py-meth docutils literal">len()</code></a> method that returns the length of the sequence.

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Some built-in sequence types are <a class="reference internal" href="library/functions.html#list" title="list"><code class="xref py py-class docutils literal">list</code></a>, <a class="reference internal" href="library/functions.html#str" title="str"><code class="xref py py-class docutils literal">str</code></a>,

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<a class="reference internal" href="library/functions.html#tuple" title="tuple"><code class="xref py py-class docutils literal">tuple</code></a>, and <a class="reference internal" href="library/functions.html#unicode" title="unicode"><code class="xref py py-class docutils literal">unicode</code></a>. Note that <a class="reference internal" href="library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal">dict</code></a> also

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supports <a class="reference internal" href="reference/datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal">__getitem__()</code></a> and <a class="reference internal" href="reference/datamodel.html#object.__len__" title="object.__len__"><code class="xref py py-meth docutils literal">__len__()</code></a>, but is considered a

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mapping rather than a sequence because the lookups use arbitrary

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<a class="reference internal" href="#term-immutable">immutable</a> keys rather than integers.</dd>

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<dt id="term-slice">slice</dt>

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<dd>An object usually containing a portion of a <a class="reference internal" href="#term-sequence">sequence</a>. A slice is

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created using the subscript notation, <code class="docutils literal">[]</code> with colons between numbers

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when several are given, such as in <code class="docutils literal">variable_name[1:3:5]</code>. The bracket

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(subscript) notation uses <a class="reference internal" href="library/functions.html#slice" title="slice"><code class="xref py py-class docutils literal">slice</code></a> objects internally (or in older

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versions, <a class="reference internal" href="reference/datamodel.html#object.__getslice__" title="object.__getslice__"><code class="xref py py-meth docutils literal">__getslice__()</code></a> and <a class="reference internal" href="reference/datamodel.html#object.__setslice__" title="object.__setslice__"><code class="xref py py-meth docutils literal">__setslice__()</code></a>).</dd>

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<dt id="term-special-method">special method</dt>

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<dd>A method that is called implicitly by Python to execute a certain

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operation on a type, such as addition. Such methods have names starting

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and ending with double underscores. Special methods are documented in

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<a class="reference internal" href="reference/datamodel.html#specialnames">Special method names</a>.</dd>

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<dt id="term-statement">statement</dt>

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<dd>A statement is part of a suite (a “block” of code). A statement is either

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an <a class="reference internal" href="#term-expression">expression</a> or one of several constructs with a keyword, such

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as <a class="reference internal" href="reference/compound_stmts.html#if"><code class="xref std std-keyword docutils literal">if</code></a>, <a class="reference internal" href="reference/compound_stmts.html#while"><code class="xref std std-keyword docutils literal">while</code></a> or <a class="reference internal" href="reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal">for</code></a>.</dd>

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<dt id="term-struct-sequence">struct sequence</dt>

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<dd>A tuple with named elements. Struct sequences expose an interface similiar

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to <a class="reference internal" href="#term-named-tuple">named tuple</a> in that elements can either be accessed either by

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index or as an attribute. However, they do not have any of the named tuple

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methods like <a class="reference internal" href="library/collections.html#collections.somenamedtuple._make" title="collections.somenamedtuple._make"><code class="xref py py-meth docutils literal">_make()</code></a> or

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<a class="reference internal" href="library/collections.html#collections.somenamedtuple._asdict" title="collections.somenamedtuple._asdict"><code class="xref py py-meth docutils literal">_asdict()</code></a>. Examples of struct sequences

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include <a class="reference internal" href="library/sys.html#sys.float_info" title="sys.float_info"><code class="xref py py-data docutils literal">sys.float_info</code></a> and the return value of <a class="reference internal" href="library/os.html#os.stat" title="os.stat"><code class="xref py py-func docutils literal">os.stat()</code></a>.</dd>

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<dt id="term-triple-quoted-string">triple-quoted string</dt>

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<dd>A string which is bound by three instances of either a quotation mark

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(”) or an apostrophe (‘). While they don’t provide any functionality

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not available with single-quoted strings, they are useful for a number

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of reasons. They allow you to include unescaped single and double

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quotes within a string and they can span multiple lines without the

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use of the continuation character, making them especially useful when

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writing docstrings.</dd>

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<dd>The type of a Python object determines what kind of object it is; every

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object has a type. An object’s type is accessible as its

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<a class="reference internal" href="library/stdtypes.html#instance.__class__" title="instance.__class__"><code class="xref py py-attr docutils literal">__class__</code></a> attribute or can be retrieved with

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<dt id="term-universal-newlines">universal newlines</dt>

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<dd>A manner of interpreting text streams in which all of the following are

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recognized as ending a line: the Unix end-of-line convention <code class="docutils literal">'\n'</code>,

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the Windows convention <code class="docutils literal">'\r\n'</code>, and the old Macintosh convention

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<code class="docutils literal">'\r'</code>. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0278">PEP 278</a> and <a class="pep reference external" href="https://www.python.org/dev/peps/pep-3116">PEP 3116</a>, as well as

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<a class="reference internal" href="library/stdtypes.html#str.splitlines" title="str.splitlines"><code class="xref py py-func docutils literal">str.splitlines()</code></a> for an additional use.</dd>

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<dt id="term-virtual-environment">virtual environment</dt>

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<dd>A cooperatively isolated runtime environment that allows Python users

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and applications to install and upgrade Python distribution packages

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without interfering with the behaviour of other Python applications

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running on the same system.</dd>

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<dt id="term-virtual-machine">virtual machine</dt>

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<dd>A computer defined entirely in software. Python’s virtual machine

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executes the <a class="reference internal" href="#term-bytecode">bytecode</a> emitted by the bytecode compiler.</dd>

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<dt id="term-zen-of-python">Zen of Python</dt>

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<dd>Listing of Python design principles and philosophies that are helpful in

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understanding and using the language. The listing can be found by typing

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“<code class="docutils literal">import this</code>” at the interactive prompt.</dd>

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</dl>

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</div>

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