10
The functions described in this chapter will let you handle and raise Python
11
exceptions. It is important to understand some of the basics of Python
12
exception handling. It works somewhat like the Unix :cdata:`errno` variable:
13
there is a global indicator (per thread) of the last error that occurred. Most
14
functions don't clear this on success, but will set it to indicate the cause of
15
the error on failure. Most functions also return an error indicator, usually
16
*NULL* if they are supposed to return a pointer, or ``-1`` if they return an
17
integer (exception: the :cfunc:`PyArg_\*` functions return ``1`` for success and
20
When a function must fail because some function it called failed, it generally
21
doesn't set the error indicator; the function it called already set it. It is
22
responsible for either handling the error and clearing the exception or
23
returning after cleaning up any resources it holds (such as object references or
24
memory allocations); it should *not* continue normally if it is not prepared to
25
handle the error. If returning due to an error, it is important to indicate to
26
the caller that an error has been set. If the error is not handled or carefully
27
propagated, additional calls into the Python/C API may not behave as intended
28
and may fail in mysterious ways.
31
single: exc_type (in module sys)
32
single: exc_value (in module sys)
33
single: exc_traceback (in module sys)
35
The error indicator consists of three Python objects corresponding to the
36
Python variables ``sys.exc_type``, ``sys.exc_value`` and ``sys.exc_traceback``.
37
API functions exist to interact with the error indicator in various ways. There
38
is a separate error indicator for each thread.
40
.. XXX Order of these should be more thoughtful.
41
Either alphabetical or some kind of structure.
44
.. cfunction:: void PyErr_Print()
46
Print a standard traceback to ``sys.stderr`` and clear the error indicator.
47
Call this function only when the error indicator is set. (Otherwise it will
51
.. cfunction:: PyObject* PyErr_Occurred()
53
Test whether the error indicator is set. If set, return the exception *type*
54
(the first argument to the last call to one of the :cfunc:`PyErr_Set\*`
55
functions or to :cfunc:`PyErr_Restore`). If not set, return *NULL*. You do not
56
own a reference to the return value, so you do not need to :cfunc:`Py_DECREF`
61
Do not compare the return value to a specific exception; use
62
:cfunc:`PyErr_ExceptionMatches` instead, shown below. (The comparison could
63
easily fail since the exception may be an instance instead of a class, in the
64
case of a class exception, or it may the a subclass of the expected exception.)
67
.. cfunction:: int PyErr_ExceptionMatches(PyObject *exc)
69
Equivalent to ``PyErr_GivenExceptionMatches(PyErr_Occurred(), exc)``. This
70
should only be called when an exception is actually set; a memory access
71
violation will occur if no exception has been raised.
74
.. cfunction:: int PyErr_GivenExceptionMatches(PyObject *given, PyObject *exc)
76
Return true if the *given* exception matches the exception in *exc*. If *exc*
77
is a class object, this also returns true when *given* is an instance of a
78
subclass. If *exc* is a tuple, all exceptions in the tuple (and recursively in
79
subtuples) are searched for a match. If *given* is *NULL*, a memory access
83
.. cfunction:: void PyErr_NormalizeException(PyObject**exc, PyObject**val, PyObject**tb)
85
Under certain circumstances, the values returned by :cfunc:`PyErr_Fetch` below
86
can be "unnormalized", meaning that ``*exc`` is a class object but ``*val`` is
87
not an instance of the same class. This function can be used to instantiate
88
the class in that case. If the values are already normalized, nothing happens.
89
The delayed normalization is implemented to improve performance.
92
.. cfunction:: void PyErr_Clear()
94
Clear the error indicator. If the error indicator is not set, there is no
98
.. cfunction:: void PyErr_Fetch(PyObject **ptype, PyObject **pvalue, PyObject **ptraceback)
100
Retrieve the error indicator into three variables whose addresses are passed.
101
If the error indicator is not set, set all three variables to *NULL*. If it is
102
set, it will be cleared and you own a reference to each object retrieved. The
103
value and traceback object may be *NULL* even when the type object is not.
107
This function is normally only used by code that needs to handle exceptions or
108
by code that needs to save and restore the error indicator temporarily.
111
.. cfunction:: void PyErr_Restore(PyObject *type, PyObject *value, PyObject *traceback)
113
Set the error indicator from the three objects. If the error indicator is
114
already set, it is cleared first. If the objects are *NULL*, the error
115
indicator is cleared. Do not pass a *NULL* type and non-*NULL* value or
116
traceback. The exception type should be a class. Do not pass an invalid
117
exception type or value. (Violating these rules will cause subtle problems
118
later.) This call takes away a reference to each object: you must own a
119
reference to each object before the call and after the call you no longer own
120
these references. (If you don't understand this, don't use this function. I
125
This function is normally only used by code that needs to save and restore the
126
error indicator temporarily; use :cfunc:`PyErr_Fetch` to save the current
130
.. cfunction:: void PyErr_SetString(PyObject *type, const char *message)
132
This is the most common way to set the error indicator. The first argument
133
specifies the exception type; it is normally one of the standard exceptions,
134
e.g. :cdata:`PyExc_RuntimeError`. You need not increment its reference count.
135
The second argument is an error message; it is converted to a string object.
138
.. cfunction:: void PyErr_SetObject(PyObject *type, PyObject *value)
140
This function is similar to :cfunc:`PyErr_SetString` but lets you specify an
141
arbitrary Python object for the "value" of the exception.
144
.. cfunction:: PyObject* PyErr_Format(PyObject *exception, const char *format, ...)
146
This function sets the error indicator and returns *NULL*. *exception* should be
147
a Python exception (class, not an instance). *format* should be a string,
148
containing format codes, similar to :cfunc:`printf`. The ``width.precision``
149
before a format code is parsed, but the width part is ignored.
151
.. % This should be exactly the same as the table in PyString_FromFormat.
152
.. % One should just refer to the other.
153
.. % The descriptions for %zd and %zu are wrong, but the truth is complicated
154
.. % because not all compilers support the %z width modifier -- we fake it
155
.. % when necessary via interpolating PY_FORMAT_SIZE_T.
156
.. % %u, %lu, %zu should have "new in Python 2.5" blurbs.
158
+-------------------+---------------+--------------------------------+
159
| Format Characters | Type | Comment |
160
+===================+===============+================================+
161
| :attr:`%%` | *n/a* | The literal % character. |
162
+-------------------+---------------+--------------------------------+
163
| :attr:`%c` | int | A single character, |
164
| | | represented as an C int. |
165
+-------------------+---------------+--------------------------------+
166
| :attr:`%d` | int | Exactly equivalent to |
167
| | | ``printf("%d")``. |
168
+-------------------+---------------+--------------------------------+
169
| :attr:`%u` | unsigned int | Exactly equivalent to |
170
| | | ``printf("%u")``. |
171
+-------------------+---------------+--------------------------------+
172
| :attr:`%ld` | long | Exactly equivalent to |
173
| | | ``printf("%ld")``. |
174
+-------------------+---------------+--------------------------------+
175
| :attr:`%lu` | unsigned long | Exactly equivalent to |
176
| | | ``printf("%lu")``. |
177
+-------------------+---------------+--------------------------------+
178
| :attr:`%zd` | Py_ssize_t | Exactly equivalent to |
179
| | | ``printf("%zd")``. |
180
+-------------------+---------------+--------------------------------+
181
| :attr:`%zu` | size_t | Exactly equivalent to |
182
| | | ``printf("%zu")``. |
183
+-------------------+---------------+--------------------------------+
184
| :attr:`%i` | int | Exactly equivalent to |
185
| | | ``printf("%i")``. |
186
+-------------------+---------------+--------------------------------+
187
| :attr:`%x` | int | Exactly equivalent to |
188
| | | ``printf("%x")``. |
189
+-------------------+---------------+--------------------------------+
190
| :attr:`%s` | char\* | A null-terminated C character |
192
+-------------------+---------------+--------------------------------+
193
| :attr:`%p` | void\* | The hex representation of a C |
194
| | | pointer. Mostly equivalent to |
195
| | | ``printf("%p")`` except that |
196
| | | it is guaranteed to start with |
197
| | | the literal ``0x`` regardless |
198
| | | of what the platform's |
199
| | | ``printf`` yields. |
200
+-------------------+---------------+--------------------------------+
202
An unrecognized format character causes all the rest of the format string to be
203
copied as-is to the result string, and any extra arguments discarded.
206
.. cfunction:: void PyErr_SetNone(PyObject *type)
208
This is a shorthand for ``PyErr_SetObject(type, Py_None)``.
211
.. cfunction:: int PyErr_BadArgument()
213
This is a shorthand for ``PyErr_SetString(PyExc_TypeError, message)``, where
214
*message* indicates that a built-in operation was invoked with an illegal
215
argument. It is mostly for internal use.
218
.. cfunction:: PyObject* PyErr_NoMemory()
220
This is a shorthand for ``PyErr_SetNone(PyExc_MemoryError)``; it returns *NULL*
221
so an object allocation function can write ``return PyErr_NoMemory();`` when it
225
.. cfunction:: PyObject* PyErr_SetFromErrno(PyObject *type)
227
.. index:: single: strerror()
229
This is a convenience function to raise an exception when a C library function
230
has returned an error and set the C variable :cdata:`errno`. It constructs a
231
tuple object whose first item is the integer :cdata:`errno` value and whose
232
second item is the corresponding error message (gotten from :cfunc:`strerror`),
233
and then calls ``PyErr_SetObject(type, object)``. On Unix, when the
234
:cdata:`errno` value is :const:`EINTR`, indicating an interrupted system call,
235
this calls :cfunc:`PyErr_CheckSignals`, and if that set the error indicator,
236
leaves it set to that. The function always returns *NULL*, so a wrapper
237
function around a system call can write ``return PyErr_SetFromErrno(type);``
238
when the system call returns an error.
241
.. cfunction:: PyObject* PyErr_SetFromErrnoWithFilename(PyObject *type, const char *filename)
243
Similar to :cfunc:`PyErr_SetFromErrno`, with the additional behavior that if
244
*filename* is not *NULL*, it is passed to the constructor of *type* as a third
245
parameter. In the case of exceptions such as :exc:`IOError` and :exc:`OSError`,
246
this is used to define the :attr:`filename` attribute of the exception instance.
249
.. cfunction:: PyObject* PyErr_SetFromWindowsErr(int ierr)
251
This is a convenience function to raise :exc:`WindowsError`. If called with
252
*ierr* of :cdata:`0`, the error code returned by a call to :cfunc:`GetLastError`
253
is used instead. It calls the Win32 function :cfunc:`FormatMessage` to retrieve
254
the Windows description of error code given by *ierr* or :cfunc:`GetLastError`,
255
then it constructs a tuple object whose first item is the *ierr* value and whose
256
second item is the corresponding error message (gotten from
257
:cfunc:`FormatMessage`), and then calls ``PyErr_SetObject(PyExc_WindowsError,
258
object)``. This function always returns *NULL*. Availability: Windows.
261
.. cfunction:: PyObject* PyErr_SetExcFromWindowsErr(PyObject *type, int ierr)
263
Similar to :cfunc:`PyErr_SetFromWindowsErr`, with an additional parameter
264
specifying the exception type to be raised. Availability: Windows.
266
.. versionadded:: 2.3
269
.. cfunction:: PyObject* PyErr_SetFromWindowsErrWithFilename(int ierr, const char *filename)
271
Similar to :cfunc:`PyErr_SetFromWindowsErr`, with the additional behavior that
272
if *filename* is not *NULL*, it is passed to the constructor of
273
:exc:`WindowsError` as a third parameter. Availability: Windows.
276
.. cfunction:: PyObject* PyErr_SetExcFromWindowsErrWithFilename(PyObject *type, int ierr, char *filename)
278
Similar to :cfunc:`PyErr_SetFromWindowsErrWithFilename`, with an additional
279
parameter specifying the exception type to be raised. Availability: Windows.
281
.. versionadded:: 2.3
284
.. cfunction:: void PyErr_BadInternalCall()
286
This is a shorthand for ``PyErr_SetString(PyExc_TypeError, message)``, where
287
*message* indicates that an internal operation (e.g. a Python/C API function)
288
was invoked with an illegal argument. It is mostly for internal use.
291
.. cfunction:: int PyErr_WarnEx(PyObject *category, char *message, int stacklevel)
293
Issue a warning message. The *category* argument is a warning category (see
294
below) or *NULL*; the *message* argument is a message string. *stacklevel* is a
295
positive number giving a number of stack frames; the warning will be issued from
296
the currently executing line of code in that stack frame. A *stacklevel* of 1
297
is the function calling :cfunc:`PyErr_WarnEx`, 2 is the function above that,
300
This function normally prints a warning message to *sys.stderr*; however, it is
301
also possible that the user has specified that warnings are to be turned into
302
errors, and in that case this will raise an exception. It is also possible that
303
the function raises an exception because of a problem with the warning machinery
304
(the implementation imports the :mod:`warnings` module to do the heavy lifting).
305
The return value is ``0`` if no exception is raised, or ``-1`` if an exception
306
is raised. (It is not possible to determine whether a warning message is
307
actually printed, nor what the reason is for the exception; this is
308
intentional.) If an exception is raised, the caller should do its normal
309
exception handling (for example, :cfunc:`Py_DECREF` owned references and return
312
Warning categories must be subclasses of :cdata:`Warning`; the default warning
313
category is :cdata:`RuntimeWarning`. The standard Python warning categories are
314
available as global variables whose names are ``PyExc_`` followed by the Python
315
exception name. These have the type :ctype:`PyObject\*`; they are all class
316
objects. Their names are :cdata:`PyExc_Warning`, :cdata:`PyExc_UserWarning`,
317
:cdata:`PyExc_UnicodeWarning`, :cdata:`PyExc_DeprecationWarning`,
318
:cdata:`PyExc_SyntaxWarning`, :cdata:`PyExc_RuntimeWarning`, and
319
:cdata:`PyExc_FutureWarning`. :cdata:`PyExc_Warning` is a subclass of
320
:cdata:`PyExc_Exception`; the other warning categories are subclasses of
321
:cdata:`PyExc_Warning`.
323
For information about warning control, see the documentation for the
324
:mod:`warnings` module and the :option:`-W` option in the command line
325
documentation. There is no C API for warning control.
328
.. cfunction:: int PyErr_Warn(PyObject *category, char *message)
330
Issue a warning message. The *category* argument is a warning category (see
331
below) or *NULL*; the *message* argument is a message string. The warning will
332
appear to be issued from the function calling :cfunc:`PyErr_Warn`, equivalent to
333
calling :cfunc:`PyErr_WarnEx` with a *stacklevel* of 1.
335
Deprecated; use :cfunc:`PyErr_WarnEx` instead.
338
.. cfunction:: int PyErr_WarnExplicit(PyObject *category, const char *message, const char *filename, int lineno, const char *module, PyObject *registry)
340
Issue a warning message with explicit control over all warning attributes. This
341
is a straightforward wrapper around the Python function
342
:func:`warnings.warn_explicit`, see there for more information. The *module*
343
and *registry* arguments may be set to *NULL* to get the default effect
347
.. cfunction:: int PyErr_WarnPy3k(char *message, int stacklevel)
349
Issue a :exc:`DeprecationWarning` with the given *message* and *stacklevel*
350
if the :cdata:`Py_Py3kWarningFlag` flag is enabled.
352
.. versionadded:: 2.6
355
.. cfunction:: int PyErr_CheckSignals()
360
single: KeyboardInterrupt (built-in exception)
362
This function interacts with Python's signal handling. It checks whether a
363
signal has been sent to the processes and if so, invokes the corresponding
364
signal handler. If the :mod:`signal` module is supported, this can invoke a
365
signal handler written in Python. In all cases, the default effect for
366
:const:`SIGINT` is to raise the :exc:`KeyboardInterrupt` exception. If an
367
exception is raised the error indicator is set and the function returns ``-1``;
368
otherwise the function returns ``0``. The error indicator may or may not be
369
cleared if it was previously set.
372
.. cfunction:: void PyErr_SetInterrupt()
376
single: KeyboardInterrupt (built-in exception)
378
This function simulates the effect of a :const:`SIGINT` signal arriving --- the
379
next time :cfunc:`PyErr_CheckSignals` is called, :exc:`KeyboardInterrupt` will
380
be raised. It may be called without holding the interpreter lock.
382
.. % XXX This was described as obsolete, but is used in
383
.. % thread.interrupt_main() (used from IDLE), so it's still needed.
386
.. cfunction:: int PySignal_SetWakeupFd(int fd)
388
This utility function specifies a file descriptor to which a ``'\0'`` byte will
389
be written whenever a signal is received. It returns the previous such file
390
descriptor. The value ``-1`` disables the feature; this is the initial state.
391
This is equivalent to :func:`signal.set_wakeup_fd` in Python, but without any
392
error checking. *fd* should be a valid file descriptor. The function should
393
only be called from the main thread.
396
.. cfunction:: PyObject* PyErr_NewException(char *name, PyObject *base, PyObject *dict)
398
This utility function creates and returns a new exception object. The *name*
399
argument must be the name of the new exception, a C string of the form
400
``module.class``. The *base* and *dict* arguments are normally *NULL*. This
401
creates a class object derived from :exc:`Exception` (accessible in C as
402
:cdata:`PyExc_Exception`).
404
The :attr:`__module__` attribute of the new class is set to the first part (up
405
to the last dot) of the *name* argument, and the class name is set to the last
406
part (after the last dot). The *base* argument can be used to specify alternate
407
base classes; it can either be only one class or a tuple of classes. The *dict*
408
argument can be used to specify a dictionary of class variables and methods.
411
.. cfunction:: void PyErr_WriteUnraisable(PyObject *obj)
413
This utility function prints a warning message to ``sys.stderr`` when an
414
exception has been set but it is impossible for the interpreter to actually
415
raise the exception. It is used, for example, when an exception occurs in an
416
:meth:`__del__` method.
418
The function is called with a single argument *obj* that identifies the context
419
in which the unraisable exception occurred. The repr of *obj* will be printed in
423
.. _standardexceptions:
428
All standard Python exceptions are available as global variables whose names are
429
``PyExc_`` followed by the Python exception name. These have the type
430
:ctype:`PyObject\*`; they are all class objects. For completeness, here are all
433
+------------------------------------+----------------------------+----------+
434
| C Name | Python Name | Notes |
435
+====================================+============================+==========+
436
| :cdata:`PyExc_BaseException` | :exc:`BaseException` | (1), (4) |
437
+------------------------------------+----------------------------+----------+
438
| :cdata:`PyExc_Exception` | :exc:`Exception` | \(1) |
439
+------------------------------------+----------------------------+----------+
440
| :cdata:`PyExc_StandardError` | :exc:`StandardError` | \(1) |
441
+------------------------------------+----------------------------+----------+
442
| :cdata:`PyExc_ArithmeticError` | :exc:`ArithmeticError` | \(1) |
443
+------------------------------------+----------------------------+----------+
444
| :cdata:`PyExc_LookupError` | :exc:`LookupError` | \(1) |
445
+------------------------------------+----------------------------+----------+
446
| :cdata:`PyExc_AssertionError` | :exc:`AssertionError` | |
447
+------------------------------------+----------------------------+----------+
448
| :cdata:`PyExc_AttributeError` | :exc:`AttributeError` | |
449
+------------------------------------+----------------------------+----------+
450
| :cdata:`PyExc_EOFError` | :exc:`EOFError` | |
451
+------------------------------------+----------------------------+----------+
452
| :cdata:`PyExc_EnvironmentError` | :exc:`EnvironmentError` | \(1) |
453
+------------------------------------+----------------------------+----------+
454
| :cdata:`PyExc_FloatingPointError` | :exc:`FloatingPointError` | |
455
+------------------------------------+----------------------------+----------+
456
| :cdata:`PyExc_IOError` | :exc:`IOError` | |
457
+------------------------------------+----------------------------+----------+
458
| :cdata:`PyExc_ImportError` | :exc:`ImportError` | |
459
+------------------------------------+----------------------------+----------+
460
| :cdata:`PyExc_IndexError` | :exc:`IndexError` | |
461
+------------------------------------+----------------------------+----------+
462
| :cdata:`PyExc_KeyError` | :exc:`KeyError` | |
463
+------------------------------------+----------------------------+----------+
464
| :cdata:`PyExc_KeyboardInterrupt` | :exc:`KeyboardInterrupt` | |
465
+------------------------------------+----------------------------+----------+
466
| :cdata:`PyExc_MemoryError` | :exc:`MemoryError` | |
467
+------------------------------------+----------------------------+----------+
468
| :cdata:`PyExc_NameError` | :exc:`NameError` | |
469
+------------------------------------+----------------------------+----------+
470
| :cdata:`PyExc_NotImplementedError` | :exc:`NotImplementedError` | |
471
+------------------------------------+----------------------------+----------+
472
| :cdata:`PyExc_OSError` | :exc:`OSError` | |
473
+------------------------------------+----------------------------+----------+
474
| :cdata:`PyExc_OverflowError` | :exc:`OverflowError` | |
475
+------------------------------------+----------------------------+----------+
476
| :cdata:`PyExc_ReferenceError` | :exc:`ReferenceError` | \(2) |
477
+------------------------------------+----------------------------+----------+
478
| :cdata:`PyExc_RuntimeError` | :exc:`RuntimeError` | |
479
+------------------------------------+----------------------------+----------+
480
| :cdata:`PyExc_SyntaxError` | :exc:`SyntaxError` | |
481
+------------------------------------+----------------------------+----------+
482
| :cdata:`PyExc_SystemError` | :exc:`SystemError` | |
483
+------------------------------------+----------------------------+----------+
484
| :cdata:`PyExc_SystemExit` | :exc:`SystemExit` | |
485
+------------------------------------+----------------------------+----------+
486
| :cdata:`PyExc_TypeError` | :exc:`TypeError` | |
487
+------------------------------------+----------------------------+----------+
488
| :cdata:`PyExc_ValueError` | :exc:`ValueError` | |
489
+------------------------------------+----------------------------+----------+
490
| :cdata:`PyExc_WindowsError` | :exc:`WindowsError` | \(3) |
491
+------------------------------------+----------------------------+----------+
492
| :cdata:`PyExc_ZeroDivisionError` | :exc:`ZeroDivisionError` | |
493
+------------------------------------+----------------------------+----------+
496
single: PyExc_BaseException
497
single: PyExc_Exception
498
single: PyExc_StandardError
499
single: PyExc_ArithmeticError
500
single: PyExc_LookupError
501
single: PyExc_AssertionError
502
single: PyExc_AttributeError
503
single: PyExc_EOFError
504
single: PyExc_EnvironmentError
505
single: PyExc_FloatingPointError
506
single: PyExc_IOError
507
single: PyExc_ImportError
508
single: PyExc_IndexError
509
single: PyExc_KeyError
510
single: PyExc_KeyboardInterrupt
511
single: PyExc_MemoryError
512
single: PyExc_NameError
513
single: PyExc_NotImplementedError
514
single: PyExc_OSError
515
single: PyExc_OverflowError
516
single: PyExc_ReferenceError
517
single: PyExc_RuntimeError
518
single: PyExc_SyntaxError
519
single: PyExc_SystemError
520
single: PyExc_SystemExit
521
single: PyExc_TypeError
522
single: PyExc_ValueError
523
single: PyExc_WindowsError
524
single: PyExc_ZeroDivisionError
529
This is a base class for other standard exceptions.
532
This is the same as :exc:`weakref.ReferenceError`.
535
Only defined on Windows; protect code that uses this by testing that the
536
preprocessor macro ``MS_WINDOWS`` is defined.
539
.. versionadded:: 2.5
542
Deprecation of String Exceptions
543
================================
545
.. index:: single: BaseException (built-in exception)
547
All exceptions built into Python or provided in the standard library are derived
548
from :exc:`BaseException`.
550
String exceptions are still supported in the interpreter to allow existing code
551
to run unmodified, but this will also change in a future release.
added
removed
Doc/c-api/exceptions.rst
