7 :synopsis: Standard exception classes.
10 Exceptions should be class objects. The exceptions are defined in the module
11 :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
23 equivalent, even if they have the same name.
25 .. index:: statement: raise
27 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"
29 indicating the detailed cause of the error. This may be a string or a tuple
30 containing several items of information (e.g., an error code and a string
31 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.
41 The built-in exception classes can be sub-classed to define new exceptions;
42 programmers are encouraged to at least derive new exceptions from the
43 :exc:`Exception` class and not :exc:`BaseException`. More information on
44 defining exceptions is available in the Python Tutorial under
45 :ref:`tut-userexceptions`.
47 The following exceptions are only used as base classes for other exceptions.
50 .. exception:: BaseException
52 The base class for all built-in exceptions. It is not meant to be directly
53 inherited by user-defined classes (for that use :exc:`Exception`). If
54 :func:`str` or :func:`unicode` is called on an instance of this class, the
55 representation of the argument(s) to the instance are returned or the empty
56 string when there were no arguments. All arguments are stored in :attr:`args`
62 .. exception:: Exception
64 All built-in, non-system-exiting exceptions are derived from this class. All
65 user-defined exceptions should also be derived from this class.
67 .. versionchanged:: 2.5
68 Changed to inherit from :exc:`BaseException`.
71 .. exception:: StandardError
73 The base class for all built-in exceptions except :exc:`StopIteration`,
74 :exc:`GeneratorExit`, :exc:`KeyboardInterrupt` and :exc:`SystemExit`.
75 :exc:`StandardError` itself is derived from :exc:`Exception`.
78 .. exception:: ArithmeticError
80 The base class for those built-in exceptions that are raised for various
81 arithmetic errors: :exc:`OverflowError`, :exc:`ZeroDivisionError`,
82 :exc:`FloatingPointError`.
85 .. exception:: LookupError
87 The base class for the exceptions that are raised when a key or index used on a
88 mapping or sequence is invalid: :exc:`IndexError`, :exc:`KeyError`. This can be
89 raised directly by :func:`sys.setdefaultencoding`.
92 .. exception:: EnvironmentError
94 The base class for exceptions that can occur outside the Python system:
95 :exc:`IOError`, :exc:`OSError`. When exceptions of this type are created with a
96 2-tuple, the first item is available on the instance's :attr:`errno` attribute
97 (it is assumed to be an error number), and the second item is available on the
98 :attr:`strerror` attribute (it is usually the associated error message). The
99 tuple itself is also available on the :attr:`args` attribute.
101 .. versionadded:: 1.5.2
103 When an :exc:`EnvironmentError` exception is instantiated with a 3-tuple, the
104 first two items are available as above, while the third item is available on the
105 :attr:`filename` attribute. However, for backwards compatibility, the
106 :attr:`args` attribute contains only a 2-tuple of the first two constructor
109 The :attr:`filename` attribute is ``None`` when this exception is created with
110 other than 3 arguments. The :attr:`errno` and :attr:`strerror` attributes are
111 also ``None`` when the instance was created with other than 2 or 3 arguments.
112 In this last case, :attr:`args` contains the verbatim constructor arguments as a
115 The following exceptions are the exceptions that are actually raised.
118 .. exception:: AssertionError
120 .. index:: statement: assert
122 Raised when an :keyword:`assert` statement fails.
125 .. exception:: AttributeError
127 Raised when an attribute reference (see :ref:`attribute-references`) or
128 assignment fails. (When an object does not support attribute references or
129 attribute assignments at all, :exc:`TypeError` is raised.)
132 .. exception:: EOFError
134 Raised when one of the built-in functions (:func:`input` or :func:`raw_input`)
135 hits an end-of-file condition (EOF) without reading any data. (N.B.: the
136 :meth:`file.read` and :meth:`file.readline` methods return an empty string
140 .. exception:: FloatingPointError
142 Raised when a floating point operation fails. This exception is always defined,
143 but can only be raised when Python is configured with the
144 :option:`--with-fpectl` option, or the :const:`WANT_SIGFPE_HANDLER` symbol is
145 defined in the :file:`pyconfig.h` file.
148 .. exception:: GeneratorExit
150 Raise when a :term:`generator`\'s :meth:`close` method is called. It
151 directly inherits from :exc:`BaseException` instead of :exc:`StandardError` since
152 it is technically not an error.
154 .. versionadded:: 2.5
156 .. versionchanged:: 2.6
157 Changed to inherit from :exc:`BaseException`.
159 .. exception:: IOError
161 Raised when an I/O operation (such as a :keyword:`print` statement, the built-in
162 :func:`open` function or a method of a file object) fails for an I/O-related
163 reason, e.g., "file not found" or "disk full".
165 This class is derived from :exc:`EnvironmentError`. See the discussion above
166 for more information on exception instance attributes.
168 .. versionchanged:: 2.6
169 Changed :exc:`socket.error` to use this as a base class.
172 .. exception:: ImportError
174 Raised when an :keyword:`import` statement fails to find the module definition
175 or when a ``from ... import`` fails to find a name that is to be imported.
178 .. exception:: IndexError
180 Raised when a sequence subscript is out of range. (Slice indices are silently
181 truncated to fall in the allowed range; if an index is not a plain integer,
182 :exc:`TypeError` is raised.)
184 .. XXX xref to sequences
187 .. exception:: KeyError
189 Raised when a mapping (dictionary) key is not found in the set of existing keys.
191 .. XXX xref to mapping objects?
194 .. exception:: KeyboardInterrupt
196 Raised when the user hits the interrupt key (normally :kbd:`Control-C` or
197 :kbd:`Delete`). During execution, a check for interrupts is made regularly.
198 Interrupts typed when a built-in function :func:`input` or :func:`raw_input` is
199 waiting for input also raise this exception. The exception inherits from
200 :exc:`BaseException` so as to not be accidentally caught by code that catches
201 :exc:`Exception` and thus prevent the interpreter from exiting.
203 .. versionchanged:: 2.5
204 Changed to inherit from :exc:`BaseException`.
207 .. exception:: MemoryError
209 Raised when an operation runs out of memory but the situation may still be
210 rescued (by deleting some objects). The associated value is a string indicating
211 what kind of (internal) operation ran out of memory. Note that because of the
212 underlying memory management architecture (C's :cfunc:`malloc` function), the
213 interpreter may not always be able to completely recover from this situation; it
214 nevertheless raises an exception so that a stack traceback can be printed, in
215 case a run-away program was the cause.
218 .. exception:: NameError
220 Raised when a local or global name is not found. This applies only to
221 unqualified names. The associated value is an error message that includes the
222 name that could not be found.
225 .. exception:: NotImplementedError
227 This exception is derived from :exc:`RuntimeError`. In user defined base
228 classes, abstract methods should raise this exception when they require derived
229 classes to override the method.
231 .. versionadded:: 1.5.2
234 .. exception:: OSError
236 .. index:: module: errno
238 This exception is derived from :exc:`EnvironmentError`. It is raised when a
239 function returns a system-related error (not for illegal argument types or
240 other incidental errors). The :attr:`errno` attribute is a numeric error
241 code from :cdata:`errno`, and the :attr:`strerror` attribute is the
242 corresponding string, as would be printed by the C function :cfunc:`perror`.
243 See the module :mod:`errno`, which contains names for the error codes defined
244 by the underlying operating system.
246 For exceptions that involve a file system path (such as :func:`chdir` or
247 :func:`unlink`), the exception instance will contain a third attribute,
248 :attr:`filename`, which is the file name passed to the function.
250 .. versionadded:: 1.5.2
253 .. exception:: OverflowError
255 Raised when the result of an arithmetic operation is too large to be
256 represented. This cannot occur for long integers (which would rather raise
257 :exc:`MemoryError` than give up) and for most operations with plain integers,
258 which return a long integer instead. Because of the lack of standardization
259 of floating point exception handling in C, most floating point operations
263 .. exception:: ReferenceError
265 This exception is raised when a weak reference proxy, created by the
266 :func:`weakref.proxy` function, is used to access an attribute of the referent
267 after it has been garbage collected. For more information on weak references,
268 see the :mod:`weakref` module.
270 .. versionadded:: 2.2
271 Previously known as the :exc:`weakref.ReferenceError` exception.
274 .. exception:: RuntimeError
276 Raised when an error is detected that doesn't fall in any of the other
277 categories. The associated value is a string indicating what precisely went
278 wrong. (This exception is mostly a relic from a previous version of the
279 interpreter; it is not used very much any more.)
282 .. exception:: StopIteration
284 Raised by an :term:`iterator`\'s :meth:`next` method to signal that there are
285 no further values. This is derived from :exc:`Exception` rather than
286 :exc:`StandardError`, since this is not considered an error in its normal
289 .. versionadded:: 2.2
292 .. exception:: SyntaxError
294 Raised when the parser encounters a syntax error. This may occur in an
295 :keyword:`import` statement, in an :keyword:`exec` statement, in a call to the
296 built-in function :func:`eval` or :func:`input`, or when reading the initial
297 script or standard input (also interactively).
299 Instances of this class have attributes :attr:`filename`, :attr:`lineno`,
300 :attr:`offset` and :attr:`text` for easier access to the details. :func:`str`
301 of the exception instance returns only the message.
304 .. exception:: SystemError
306 Raised when the interpreter finds an internal error, but the situation does not
307 look so serious to cause it to abandon all hope. The associated value is a
308 string indicating what went wrong (in low-level terms).
310 You should report this to the author or maintainer of your Python interpreter.
311 Be sure to report the version of the Python interpreter (``sys.version``; it is
312 also printed at the start of an interactive Python session), the exact error
313 message (the exception's associated value) and if possible the source of the
314 program that triggered the error.
317 .. exception:: SystemExit
319 This exception is raised by the :func:`sys.exit` function. When it is not
320 handled, the Python interpreter exits; no stack traceback is printed. If the
321 associated value is a plain integer, it specifies the system exit status (passed
322 to C's :cfunc:`exit` function); if it is ``None``, the exit status is zero; if
323 it has another type (such as a string), the object's value is printed and the
326 Instances have an attribute :attr:`code` which is set to the proposed exit
327 status or error message (defaulting to ``None``). Also, this exception derives
328 directly from :exc:`BaseException` and not :exc:`StandardError`, since it is not
329 technically an error.
331 A call to :func:`sys.exit` is translated into an exception so that clean-up
332 handlers (:keyword:`finally` clauses of :keyword:`try` statements) can be
333 executed, and so that a debugger can execute a script without running the risk
334 of losing control. The :func:`os._exit` function can be used if it is
335 absolutely positively necessary to exit immediately (for example, in the child
336 process after a call to :func:`fork`).
338 The exception inherits from :exc:`BaseException` instead of :exc:`StandardError`
339 or :exc:`Exception` so that it is not accidentally caught by code that catches
340 :exc:`Exception`. This allows the exception to properly propagate up and cause
341 the interpreter to exit.
343 .. versionchanged:: 2.5
344 Changed to inherit from :exc:`BaseException`.
347 .. exception:: TypeError
349 Raised when an operation or function is applied to an object of inappropriate
350 type. The associated value is a string giving details about the type mismatch.
353 .. exception:: UnboundLocalError
355 Raised when a reference is made to a local variable in a function or method, but
356 no value has been bound to that variable. This is a subclass of
359 .. versionadded:: 2.0
362 .. exception:: UnicodeError
364 Raised when a Unicode-related encoding or decoding error occurs. It is a
365 subclass of :exc:`ValueError`.
367 .. versionadded:: 2.0
370 .. exception:: UnicodeEncodeError
372 Raised when a Unicode-related error occurs during encoding. It is a subclass of
375 .. versionadded:: 2.3
378 .. exception:: UnicodeDecodeError
380 Raised when a Unicode-related error occurs during decoding. It is a subclass of
383 .. versionadded:: 2.3
386 .. exception:: UnicodeTranslateError
388 Raised when a Unicode-related error occurs during translating. It is a subclass
389 of :exc:`UnicodeError`.
391 .. versionadded:: 2.3
394 .. exception:: ValueError
396 Raised when a built-in operation or function receives an argument that has the
397 right type but an inappropriate value, and the situation is not described by a
398 more precise exception such as :exc:`IndexError`.
401 .. exception:: VMSError
403 Only available on VMS. Raised when a VMS-specific error occurs.
406 .. exception:: WindowsError
408 Raised when a Windows-specific error occurs or when the error number does not
409 correspond to an :cdata:`errno` value. The :attr:`winerror` and
410 :attr:`strerror` values are created from the return values of the
411 :cfunc:`GetLastError` and :cfunc:`FormatMessage` functions from the Windows
412 Platform API. The :attr:`errno` value maps the :attr:`winerror` value to
413 corresponding ``errno.h`` values. This is a subclass of :exc:`OSError`.
415 .. versionadded:: 2.0
417 .. versionchanged:: 2.5
418 Previous versions put the :cfunc:`GetLastError` codes into :attr:`errno`.
421 .. exception:: ZeroDivisionError
423 Raised when the second argument of a division or modulo operation is zero. The
424 associated value is a string indicating the type of the operands and the
427 The following exceptions are used as warning categories; see the :mod:`warnings`
428 module for more information.
431 .. exception:: Warning
433 Base class for warning categories.
436 .. exception:: UserWarning
438 Base class for warnings generated by user code.
441 .. exception:: DeprecationWarning
443 Base class for warnings about deprecated features.
446 .. exception:: PendingDeprecationWarning
448 Base class for warnings about features which will be deprecated in the future.
451 .. exception:: SyntaxWarning
453 Base class for warnings about dubious syntax
456 .. exception:: RuntimeWarning
458 Base class for warnings about dubious runtime behavior.
461 .. exception:: FutureWarning
463 Base class for warnings about constructs that will change semantically in the
467 .. exception:: ImportWarning
469 Base class for warnings about probable mistakes in module imports.
471 .. versionadded:: 2.5
474 .. exception:: UnicodeWarning
476 Base class for warnings related to Unicode.
478 .. versionadded:: 2.5
480 The class hierarchy for built-in exceptions is:
483 .. literalinclude:: ../../Lib/test/exception_hierarchy.txt