7
:synopsis: Standard exception classes.
10
Exceptions should be class objects. The exceptions are defined in the module
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:mod:`exceptions`. This module never needs to be imported explicitly: the
12
exceptions are provided in the built-in namespace as well as the
13
:mod:`exceptions` module.
19
For class exceptions, in a :keyword:`try` statement with an :keyword:`except`
20
clause that mentions a particular class, that clause also handles any exception
21
classes derived from that class (but not exception classes from which *it* is
22
derived). Two exception classes that are not related via subclassing are never
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equivalent, even if they have the same name.
25
.. index:: statement: raise
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The built-in exceptions listed below can be generated by the interpreter or
28
built-in functions. Except where mentioned, they have an "associated value"
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indicating the detailed cause of the error. This may be a string or a tuple
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containing several items of information (e.g., an error code and a string
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explaining the code). The associated value is the second argument to the
32
:keyword:`raise` statement. If the exception class is derived from the standard
33
root class :exc:`BaseException`, the associated value is present as the
34
exception instance's :attr:`args` attribute.
36
User code can raise built-in exceptions. This can be used to test an exception
37
handler or to report an error condition "just like" the situation in which the
38
interpreter raises the same exception; but beware that there is nothing to
39
prevent user code from raising an inappropriate error.
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The built-in exception classes can be subclassed to define new exceptions;
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programmers are encouraged to derive new exceptions from the :exc:`Exception`
43
class or one of its subclasses, and not from :exc:`BaseException`. More
44
information on defining exceptions is available in the Python Tutorial under
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:ref:`tut-userexceptions`.
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The following exceptions are only used as base classes for other exceptions.
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.. exception:: BaseException
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The base class for all built-in exceptions. It is not meant to be directly
52
inherited by user-defined classes (for that, use :exc:`Exception`). If
53
:func:`str` or :func:`unicode` is called on an instance of this class, the
54
representation of the argument(s) to the instance are returned, or the empty
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string when there were no arguments.
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The tuple of arguments given to the exception constructor. Some built-in
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exceptions (like :exc:`IOError`) expect a certain number of arguments and
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assign a special meaning to the elements of this tuple, while others are
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usually called only with a single string giving an error message.
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.. exception:: Exception
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All built-in, non-system-exiting exceptions are derived from this class. All
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user-defined exceptions should also be derived from this class.
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.. versionchanged:: 2.5
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Changed to inherit from :exc:`BaseException`.
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.. exception:: StandardError
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The base class for all built-in exceptions except :exc:`StopIteration`,
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:exc:`GeneratorExit`, :exc:`KeyboardInterrupt` and :exc:`SystemExit`.
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:exc:`StandardError` itself is derived from :exc:`Exception`.
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.. exception:: ArithmeticError
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The base class for those built-in exceptions that are raised for various
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arithmetic errors: :exc:`OverflowError`, :exc:`ZeroDivisionError`,
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:exc:`FloatingPointError`.
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.. exception:: BufferError
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Raised when a :ref:`buffer <bufferobjects>` related operation cannot be
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.. exception:: LookupError
98
The base class for the exceptions that are raised when a key or index used on
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a mapping or sequence is invalid: :exc:`IndexError`, :exc:`KeyError`. This
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can be raised directly by :func:`codecs.lookup`.
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.. exception:: EnvironmentError
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The base class for exceptions that can occur outside the Python system:
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:exc:`IOError`, :exc:`OSError`. When exceptions of this type are created with a
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2-tuple, the first item is available on the instance's :attr:`errno` attribute
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(it is assumed to be an error number), and the second item is available on the
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:attr:`strerror` attribute (it is usually the associated error message). The
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tuple itself is also available on the :attr:`args` attribute.
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.. versionadded:: 1.5.2
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When an :exc:`EnvironmentError` exception is instantiated with a 3-tuple, the
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first two items are available as above, while the third item is available on the
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:attr:`filename` attribute. However, for backwards compatibility, the
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:attr:`args` attribute contains only a 2-tuple of the first two constructor
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The :attr:`filename` attribute is ``None`` when this exception is created with
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other than 3 arguments. The :attr:`errno` and :attr:`strerror` attributes are
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also ``None`` when the instance was created with other than 2 or 3 arguments.
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In this last case, :attr:`args` contains the verbatim constructor arguments as a
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The following exceptions are the exceptions that are actually raised.
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.. exception:: AssertionError
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.. index:: statement: assert
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Raised when an :keyword:`assert` statement fails.
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.. exception:: AttributeError
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Raised when an attribute reference (see :ref:`attribute-references`) or
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assignment fails. (When an object does not support attribute references or
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attribute assignments at all, :exc:`TypeError` is raised.)
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.. exception:: EOFError
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Raised when one of the built-in functions (:func:`input` or :func:`raw_input`)
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hits an end-of-file condition (EOF) without reading any data. (N.B.: the
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:meth:`file.read` and :meth:`file.readline` methods return an empty string
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.. exception:: FloatingPointError
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Raised when a floating point operation fails. This exception is always defined,
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but can only be raised when Python is configured with the
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``--with-fpectl`` option, or the :const:`WANT_SIGFPE_HANDLER` symbol is
156
defined in the :file:`pyconfig.h` file.
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.. exception:: GeneratorExit
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Raised when a :term:`generator`\'s :meth:`close` method is called. It
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directly inherits from :exc:`BaseException` instead of :exc:`StandardError`
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since it is technically not an error.
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.. versionadded:: 2.5
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.. versionchanged:: 2.6
168
Changed to inherit from :exc:`BaseException`.
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.. exception:: IOError
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Raised when an I/O operation (such as a :keyword:`print` statement, the built-in
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:func:`open` function or a method of a file object) fails for an I/O-related
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reason, e.g., "file not found" or "disk full".
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This class is derived from :exc:`EnvironmentError`. See the discussion above
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for more information on exception instance attributes.
179
.. versionchanged:: 2.6
180
Changed :exc:`socket.error` to use this as a base class.
183
.. exception:: ImportError
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Raised when an :keyword:`import` statement fails to find the module definition
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or when a ``from ... import`` fails to find a name that is to be imported.
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.. exception:: IndexError
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Raised when a sequence subscript is out of range. (Slice indices are silently
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truncated to fall in the allowed range; if an index is not a plain integer,
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:exc:`TypeError` is raised.)
195
.. XXX xref to sequences
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.. exception:: KeyError
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Raised when a mapping (dictionary) key is not found in the set of existing keys.
202
.. XXX xref to mapping objects?
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.. exception:: KeyboardInterrupt
207
Raised when the user hits the interrupt key (normally :kbd:`Control-C` or
208
:kbd:`Delete`). During execution, a check for interrupts is made regularly.
209
Interrupts typed when a built-in function :func:`input` or :func:`raw_input` is
210
waiting for input also raise this exception. The exception inherits from
211
:exc:`BaseException` so as to not be accidentally caught by code that catches
212
:exc:`Exception` and thus prevent the interpreter from exiting.
214
.. versionchanged:: 2.5
215
Changed to inherit from :exc:`BaseException`.
218
.. exception:: MemoryError
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Raised when an operation runs out of memory but the situation may still be
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rescued (by deleting some objects). The associated value is a string indicating
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what kind of (internal) operation ran out of memory. Note that because of the
223
underlying memory management architecture (C's :c:func:`malloc` function), the
224
interpreter may not always be able to completely recover from this situation; it
225
nevertheless raises an exception so that a stack traceback can be printed, in
226
case a run-away program was the cause.
229
.. exception:: NameError
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Raised when a local or global name is not found. This applies only to
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unqualified names. The associated value is an error message that includes the
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name that could not be found.
236
.. exception:: NotImplementedError
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This exception is derived from :exc:`RuntimeError`. In user defined base
239
classes, abstract methods should raise this exception when they require derived
240
classes to override the method.
242
.. versionadded:: 1.5.2
245
.. exception:: OSError
247
.. index:: module: errno
249
This exception is derived from :exc:`EnvironmentError`. It is raised when a
250
function returns a system-related error (not for illegal argument types or
251
other incidental errors). The :attr:`errno` attribute is a numeric error
252
code from :c:data:`errno`, and the :attr:`strerror` attribute is the
253
corresponding string, as would be printed by the C function :c:func:`perror`.
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See the module :mod:`errno`, which contains names for the error codes defined
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by the underlying operating system.
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For exceptions that involve a file system path (such as :func:`chdir` or
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:func:`unlink`), the exception instance will contain a third attribute,
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:attr:`filename`, which is the file name passed to the function.
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.. versionadded:: 1.5.2
264
.. exception:: OverflowError
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Raised when the result of an arithmetic operation is too large to be
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represented. This cannot occur for long integers (which would rather raise
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:exc:`MemoryError` than give up) and for most operations with plain integers,
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which return a long integer instead. Because of the lack of standardization
270
of floating point exception handling in C, most floating point operations
274
.. exception:: ReferenceError
276
This exception is raised when a weak reference proxy, created by the
277
:func:`weakref.proxy` function, is used to access an attribute of the referent
278
after it has been garbage collected. For more information on weak references,
279
see the :mod:`weakref` module.
281
.. versionadded:: 2.2
282
Previously known as the :exc:`weakref.ReferenceError` exception.
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.. exception:: RuntimeError
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Raised when an error is detected that doesn't fall in any of the other
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categories. The associated value is a string indicating what precisely went
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.. exception:: StopIteration
294
Raised by an :term:`iterator`\'s :meth:`~iterator.next` method to signal that
295
there are no further values. This is derived from :exc:`Exception` rather
296
than :exc:`StandardError`, since this is not considered an error in its
299
.. versionadded:: 2.2
302
.. exception:: SyntaxError
304
Raised when the parser encounters a syntax error. This may occur in an
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:keyword:`import` statement, in an :keyword:`exec` statement, in a call to the
306
built-in function :func:`eval` or :func:`input`, or when reading the initial
307
script or standard input (also interactively).
309
Instances of this class have attributes :attr:`filename`, :attr:`lineno`,
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:attr:`offset` and :attr:`text` for easier access to the details. :func:`str`
311
of the exception instance returns only the message.
314
.. exception:: IndentationError
316
Base class for syntax errors related to incorrect indentation. This is a
317
subclass of :exc:`SyntaxError`.
320
.. exception:: TabError
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Raised when indentation contains an inconsistent use of tabs and spaces.
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This is a subclass of :exc:`IndentationError`.
326
.. exception:: SystemError
328
Raised when the interpreter finds an internal error, but the situation does not
329
look so serious to cause it to abandon all hope. The associated value is a
330
string indicating what went wrong (in low-level terms).
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You should report this to the author or maintainer of your Python interpreter.
333
Be sure to report the version of the Python interpreter (``sys.version``; it is
334
also printed at the start of an interactive Python session), the exact error
335
message (the exception's associated value) and if possible the source of the
336
program that triggered the error.
339
.. exception:: SystemExit
341
This exception is raised by the :func:`sys.exit` function. When it is not
342
handled, the Python interpreter exits; no stack traceback is printed. If the
343
associated value is a plain integer, it specifies the system exit status (passed
344
to C's :c:func:`exit` function); if it is ``None``, the exit status is zero; if
345
it has another type (such as a string), the object's value is printed and the
348
Instances have an attribute :attr:`!code` which is set to the proposed exit
349
status or error message (defaulting to ``None``). Also, this exception derives
350
directly from :exc:`BaseException` and not :exc:`StandardError`, since it is not
351
technically an error.
353
A call to :func:`sys.exit` is translated into an exception so that clean-up
354
handlers (:keyword:`finally` clauses of :keyword:`try` statements) can be
355
executed, and so that a debugger can execute a script without running the risk
356
of losing control. The :func:`os._exit` function can be used if it is
357
absolutely positively necessary to exit immediately (for example, in the child
358
process after a call to :func:`os.fork`).
360
The exception inherits from :exc:`BaseException` instead of :exc:`StandardError`
361
or :exc:`Exception` so that it is not accidentally caught by code that catches
362
:exc:`Exception`. This allows the exception to properly propagate up and cause
363
the interpreter to exit.
365
.. versionchanged:: 2.5
366
Changed to inherit from :exc:`BaseException`.
369
.. exception:: TypeError
371
Raised when an operation or function is applied to an object of inappropriate
372
type. The associated value is a string giving details about the type mismatch.
375
.. exception:: UnboundLocalError
377
Raised when a reference is made to a local variable in a function or method, but
378
no value has been bound to that variable. This is a subclass of
381
.. versionadded:: 2.0
384
.. exception:: UnicodeError
386
Raised when a Unicode-related encoding or decoding error occurs. It is a
387
subclass of :exc:`ValueError`.
389
:exc:`UnicodeError` has attributes that describe the encoding or decoding
390
error. For example, ``err.object[err.start:err.end]`` gives the particular
391
invalid input that the codec failed on.
393
.. attribute:: encoding
395
The name of the encoding that raised the error.
397
.. attribute:: reason
399
A string describing the specific codec error.
401
.. attribute:: object
403
The object the codec was attempting to encode or decode.
407
The first index of invalid data in :attr:`object`.
411
The index after the last invalid data in :attr:`object`.
413
.. versionadded:: 2.0
416
.. exception:: UnicodeEncodeError
418
Raised when a Unicode-related error occurs during encoding. It is a subclass of
421
.. versionadded:: 2.3
424
.. exception:: UnicodeDecodeError
426
Raised when a Unicode-related error occurs during decoding. It is a subclass of
429
.. versionadded:: 2.3
432
.. exception:: UnicodeTranslateError
434
Raised when a Unicode-related error occurs during translating. It is a subclass
435
of :exc:`UnicodeError`.
437
.. versionadded:: 2.3
440
.. exception:: ValueError
442
Raised when a built-in operation or function receives an argument that has the
443
right type but an inappropriate value, and the situation is not described by a
444
more precise exception such as :exc:`IndexError`.
447
.. exception:: VMSError
449
Only available on VMS. Raised when a VMS-specific error occurs.
452
.. exception:: WindowsError
454
Raised when a Windows-specific error occurs or when the error number does not
455
correspond to an :c:data:`errno` value. The :attr:`winerror` and
456
:attr:`strerror` values are created from the return values of the
457
:c:func:`GetLastError` and :c:func:`FormatMessage` functions from the Windows
458
Platform API. The :attr:`errno` value maps the :attr:`winerror` value to
459
corresponding ``errno.h`` values. This is a subclass of :exc:`OSError`.
461
.. versionadded:: 2.0
463
.. versionchanged:: 2.5
464
Previous versions put the :c:func:`GetLastError` codes into :attr:`errno`.
467
.. exception:: ZeroDivisionError
469
Raised when the second argument of a division or modulo operation is zero. The
470
associated value is a string indicating the type of the operands and the
473
The following exceptions are used as warning categories; see the :mod:`warnings`
474
module for more information.
477
.. exception:: Warning
479
Base class for warning categories.
482
.. exception:: UserWarning
484
Base class for warnings generated by user code.
487
.. exception:: DeprecationWarning
489
Base class for warnings about deprecated features.
492
.. exception:: PendingDeprecationWarning
494
Base class for warnings about features which will be deprecated in the future.
497
.. exception:: SyntaxWarning
499
Base class for warnings about dubious syntax.
502
.. exception:: RuntimeWarning
504
Base class for warnings about dubious runtime behavior.
507
.. exception:: FutureWarning
509
Base class for warnings about constructs that will change semantically in the
513
.. exception:: ImportWarning
515
Base class for warnings about probable mistakes in module imports.
517
.. versionadded:: 2.5
520
.. exception:: UnicodeWarning
522
Base class for warnings related to Unicode.
524
.. versionadded:: 2.5
530
The class hierarchy for built-in exceptions is:
532
.. literalinclude:: ../../Lib/test/exception_hierarchy.txt
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python-2.7.11-docs-html/_sources/library/exceptions.txt
