486 lines
16 KiB
ReStructuredText
486 lines
16 KiB
ReStructuredText
.. _bltin-exceptions:
|
|
|
|
Built-in Exceptions
|
|
===================
|
|
|
|
.. module:: exceptions
|
|
:synopsis: Standard exception classes.
|
|
|
|
|
|
Exceptions should be class objects. The exceptions are defined in the module
|
|
:mod:`exceptions`. This module never needs to be imported explicitly: the
|
|
exceptions are provided in the built-in namespace as well as the
|
|
:mod:`exceptions` module.
|
|
|
|
.. index::
|
|
statement: try
|
|
statement: except
|
|
|
|
For class exceptions, in a :keyword:`try` statement with an :keyword:`except`
|
|
clause that mentions a particular class, that clause also handles any exception
|
|
classes derived from that class (but not exception classes from which *it* is
|
|
derived). Two exception classes that are not related via subclassing are never
|
|
equivalent, even if they have the same name.
|
|
|
|
.. index:: statement: raise
|
|
|
|
The built-in exceptions listed below can be generated by the interpreter or
|
|
built-in functions. Except where mentioned, they have an "associated value"
|
|
indicating the detailed cause of the error. This may be a string or a tuple
|
|
containing several items of information (e.g., an error code and a string
|
|
explaining the code). The associated value is the second argument to the
|
|
:keyword:`raise` statement. If the exception class is derived from the standard
|
|
root class :exc:`BaseException`, the associated value is present as the
|
|
exception instance's :attr:`args` attribute.
|
|
|
|
User code can raise built-in exceptions. This can be used to test an exception
|
|
handler or to report an error condition "just like" the situation in which the
|
|
interpreter raises the same exception; but beware that there is nothing to
|
|
prevent user code from raising an inappropriate error.
|
|
|
|
The built-in exception classes can be sub-classed to define new exceptions;
|
|
programmers are encouraged to at least derive new exceptions from the
|
|
:exc:`Exception` class and not :exc:`BaseException`. More information on
|
|
defining exceptions is available in the Python Tutorial under
|
|
:ref:`tut-userexceptions`.
|
|
|
|
The following exceptions are only used as base classes for other exceptions.
|
|
|
|
|
|
.. exception:: BaseException
|
|
|
|
The base class for all built-in exceptions. It is not meant to be directly
|
|
inherited by user-defined classes (for that use :exc:`Exception`). If
|
|
:func:`str` or :func:`unicode` is called on an instance of this class, the
|
|
representation of the argument(s) to the instance are returned or the emptry
|
|
string when there were no arguments. All arguments are stored in :attr:`args`
|
|
as a tuple.
|
|
|
|
.. versionadded:: 2.5
|
|
|
|
|
|
.. exception:: Exception
|
|
|
|
All built-in, non-system-exiting exceptions are derived from this class. All
|
|
user-defined exceptions should also be derived from this class.
|
|
|
|
.. versionchanged:: 2.5
|
|
Changed to inherit from :exc:`BaseException`.
|
|
|
|
|
|
.. exception:: StandardError
|
|
|
|
The base class for all built-in exceptions except :exc:`StopIteration`,
|
|
:exc:`GeneratorExit`, :exc:`KeyboardInterrupt` and :exc:`SystemExit`.
|
|
:exc:`StandardError` itself is derived from :exc:`Exception`.
|
|
|
|
|
|
.. exception:: ArithmeticError
|
|
|
|
The base class for those built-in exceptions that are raised for various
|
|
arithmetic errors: :exc:`OverflowError`, :exc:`ZeroDivisionError`,
|
|
:exc:`FloatingPointError`.
|
|
|
|
|
|
.. exception:: LookupError
|
|
|
|
The base class for the exceptions that are raised when a key or index used on a
|
|
mapping or sequence is invalid: :exc:`IndexError`, :exc:`KeyError`. This can be
|
|
raised directly by :func:`sys.setdefaultencoding`.
|
|
|
|
|
|
.. exception:: EnvironmentError
|
|
|
|
The base class for exceptions that can occur outside the Python system:
|
|
:exc:`IOError`, :exc:`OSError`. When exceptions of this type are created with a
|
|
2-tuple, the first item is available on the instance's :attr:`errno` attribute
|
|
(it is assumed to be an error number), and the second item is available on the
|
|
:attr:`strerror` attribute (it is usually the associated error message). The
|
|
tuple itself is also available on the :attr:`args` attribute.
|
|
|
|
.. versionadded:: 1.5.2
|
|
|
|
When an :exc:`EnvironmentError` exception is instantiated with a 3-tuple, the
|
|
first two items are available as above, while the third item is available on the
|
|
:attr:`filename` attribute. However, for backwards compatibility, the
|
|
:attr:`args` attribute contains only a 2-tuple of the first two constructor
|
|
arguments.
|
|
|
|
The :attr:`filename` attribute is ``None`` when this exception is created with
|
|
other than 3 arguments. The :attr:`errno` and :attr:`strerror` attributes are
|
|
also ``None`` when the instance was created with other than 2 or 3 arguments.
|
|
In this last case, :attr:`args` contains the verbatim constructor arguments as a
|
|
tuple.
|
|
|
|
The following exceptions are the exceptions that are actually raised.
|
|
|
|
|
|
.. exception:: AssertionError
|
|
|
|
.. index:: statement: assert
|
|
|
|
Raised when an :keyword:`assert` statement fails.
|
|
|
|
|
|
.. exception:: AttributeError
|
|
|
|
Raised when an attribute reference or assignment fails. (When an object does
|
|
not support attribute references or attribute assignments at all,
|
|
:exc:`TypeError` is raised.)
|
|
|
|
.. % xref to attribute reference?
|
|
|
|
|
|
.. exception:: EOFError
|
|
|
|
Raised when one of the built-in functions (:func:`input` or :func:`raw_input`)
|
|
hits an end-of-file condition (EOF) without reading any data. (N.B.: the
|
|
:meth:`read` and :meth:`readline` methods of file objects return an empty string
|
|
when they hit EOF.)
|
|
|
|
.. % XXXJH xrefs here
|
|
.. % XXXJH xrefs here
|
|
|
|
|
|
.. exception:: FloatingPointError
|
|
|
|
Raised when a floating point operation fails. This exception is always defined,
|
|
but can only be raised when Python is configured with the
|
|
:option:`--with-fpectl` option, or the :const:`WANT_SIGFPE_HANDLER` symbol is
|
|
defined in the :file:`pyconfig.h` file.
|
|
|
|
|
|
.. exception:: GeneratorExit
|
|
|
|
Raise when a generator's :meth:`close` method is called. It directly inherits
|
|
from :exc:`Exception` instead of :exc:`StandardError` since it is technically
|
|
not an error.
|
|
|
|
.. versionadded:: 2.5
|
|
|
|
|
|
.. exception:: IOError
|
|
|
|
Raised when an I/O operation (such as a :keyword:`print` statement, the built-in
|
|
:func:`open` function or a method of a file object) fails for an I/O-related
|
|
reason, e.g., "file not found" or "disk full".
|
|
|
|
.. % XXXJH xrefs here
|
|
|
|
This class is derived from :exc:`EnvironmentError`. See the discussion above
|
|
for more information on exception instance attributes.
|
|
|
|
.. versionchanged:: 2.6
|
|
Changed :exc:`socket.error` to use this as a base class.
|
|
|
|
|
|
.. exception:: ImportError
|
|
|
|
Raised when an :keyword:`import` statement fails to find the module definition
|
|
or when a ``from ... import`` fails to find a name that is to be imported.
|
|
|
|
.. % XXXJH xref to import statement?
|
|
|
|
|
|
.. exception:: IndexError
|
|
|
|
Raised when a sequence subscript is out of range. (Slice indices are silently
|
|
truncated to fall in the allowed range; if an index is not a plain integer,
|
|
:exc:`TypeError` is raised.)
|
|
|
|
.. % XXXJH xref to sequences
|
|
|
|
|
|
.. exception:: KeyError
|
|
|
|
Raised when a mapping (dictionary) key is not found in the set of existing keys.
|
|
|
|
.. % XXXJH xref to mapping objects?
|
|
|
|
|
|
.. exception:: KeyboardInterrupt
|
|
|
|
Raised when the user hits the interrupt key (normally :kbd:`Control-C` or
|
|
:kbd:`Delete`). During execution, a check for interrupts is made regularly.
|
|
Interrupts typed when a built-in function :func:`input` or :func:`raw_input` is
|
|
waiting for input also raise this exception. The exception inherits from
|
|
:exc:`BaseException` so as to not be accidentally caught by code that catches
|
|
:exc:`Exception` and thus prevent the interpreter from exiting.
|
|
|
|
.. % XXX(hylton) xrefs here
|
|
|
|
.. versionchanged:: 2.5
|
|
Changed to inherit from :exc:`BaseException`.
|
|
|
|
|
|
.. exception:: MemoryError
|
|
|
|
Raised when an operation runs out of memory but the situation may still be
|
|
rescued (by deleting some objects). The associated value is a string indicating
|
|
what kind of (internal) operation ran out of memory. Note that because of the
|
|
underlying memory management architecture (C's :cfunc:`malloc` function), the
|
|
interpreter may not always be able to completely recover from this situation; it
|
|
nevertheless raises an exception so that a stack traceback can be printed, in
|
|
case a run-away program was the cause.
|
|
|
|
|
|
.. exception:: NameError
|
|
|
|
Raised when a local or global name is not found. This applies only to
|
|
unqualified names. The associated value is an error message that includes the
|
|
name that could not be found.
|
|
|
|
|
|
.. exception:: NotImplementedError
|
|
|
|
This exception is derived from :exc:`RuntimeError`. In user defined base
|
|
classes, abstract methods should raise this exception when they require derived
|
|
classes to override the method.
|
|
|
|
.. versionadded:: 1.5.2
|
|
|
|
|
|
.. exception:: OSError
|
|
|
|
This class is derived from :exc:`EnvironmentError` and is used primarily as the
|
|
:mod:`os` module's ``os.error`` exception. See :exc:`EnvironmentError` above for
|
|
a description of the possible associated values.
|
|
|
|
.. % xref for os module
|
|
|
|
.. versionadded:: 1.5.2
|
|
|
|
|
|
.. exception:: OverflowError
|
|
|
|
Raised when the result of an arithmetic operation is too large to be
|
|
represented. This cannot occur for long integers (which would rather raise
|
|
:exc:`MemoryError` than give up). Because of the lack of standardization of
|
|
floating point exception handling in C, most floating point operations also
|
|
aren't checked. For plain integers, all operations that can overflow are
|
|
checked except left shift, where typical applications prefer to drop bits than
|
|
raise an exception.
|
|
|
|
.. % XXXJH reference to long's and/or int's?
|
|
|
|
|
|
.. exception:: ReferenceError
|
|
|
|
This exception is raised when a weak reference proxy, created by the
|
|
:func:`weakref.proxy` function, is used to access an attribute of the referent
|
|
after it has been garbage collected. For more information on weak references,
|
|
see the :mod:`weakref` module.
|
|
|
|
.. versionadded:: 2.2
|
|
Previously known as the :exc:`weakref.ReferenceError` exception.
|
|
|
|
|
|
.. exception:: RuntimeError
|
|
|
|
Raised when an error is detected that doesn't fall in any of the other
|
|
categories. The associated value is a string indicating what precisely went
|
|
wrong. (This exception is mostly a relic from a previous version of the
|
|
interpreter; it is not used very much any more.)
|
|
|
|
|
|
.. exception:: StopIteration
|
|
|
|
Raised by an iterator's :meth:`next` method to signal that there are no further
|
|
values. This is derived from :exc:`Exception` rather than :exc:`StandardError`,
|
|
since this is not considered an error in its normal application.
|
|
|
|
.. versionadded:: 2.2
|
|
|
|
|
|
.. exception:: SyntaxError
|
|
|
|
Raised when the parser encounters a syntax error. This may occur in an
|
|
:keyword:`import` statement, in an :keyword:`exec` statement, in a call to the
|
|
built-in function :func:`eval` or :func:`input`, or when reading the initial
|
|
script or standard input (also interactively).
|
|
|
|
.. % XXXJH xref to these functions?
|
|
|
|
Instances of this class have attributes :attr:`filename`, :attr:`lineno`,
|
|
:attr:`offset` and :attr:`text` for easier access to the details. :func:`str`
|
|
of the exception instance returns only the message.
|
|
|
|
|
|
.. exception:: SystemError
|
|
|
|
Raised when the interpreter finds an internal error, but the situation does not
|
|
look so serious to cause it to abandon all hope. The associated value is a
|
|
string indicating what went wrong (in low-level terms).
|
|
|
|
You should report this to the author or maintainer of your Python interpreter.
|
|
Be sure to report the version of the Python interpreter (``sys.version``; it is
|
|
also printed at the start of an interactive Python session), the exact error
|
|
message (the exception's associated value) and if possible the source of the
|
|
program that triggered the error.
|
|
|
|
|
|
.. exception:: SystemExit
|
|
|
|
This exception is raised by the :func:`sys.exit` function. When it is not
|
|
handled, the Python interpreter exits; no stack traceback is printed. If the
|
|
associated value is a plain integer, it specifies the system exit status (passed
|
|
to C's :cfunc:`exit` function); if it is ``None``, the exit status is zero; if
|
|
it has another type (such as a string), the object's value is printed and the
|
|
exit status is one.
|
|
|
|
.. % XXX(hylton) xref to module sys?
|
|
|
|
Instances have an attribute :attr:`code` which is set to the proposed exit
|
|
status or error message (defaulting to ``None``). Also, this exception derives
|
|
directly from :exc:`BaseException` and not :exc:`StandardError`, since it is not
|
|
technically an error.
|
|
|
|
A call to :func:`sys.exit` is translated into an exception so that clean-up
|
|
handlers (:keyword:`finally` clauses of :keyword:`try` statements) can be
|
|
executed, and so that a debugger can execute a script without running the risk
|
|
of losing control. The :func:`os._exit` function can be used if it is
|
|
absolutely positively necessary to exit immediately (for example, in the child
|
|
process after a call to :func:`fork`).
|
|
|
|
The exception inherits from :exc:`BaseException` instead of :exc:`StandardError`
|
|
or :exc:`Exception` so that it is not accidentally caught by code that catches
|
|
:exc:`Exception`. This allows the exception to properly propagate up and cause
|
|
the interpreter to exit.
|
|
|
|
.. versionchanged:: 2.5
|
|
Changed to inherit from :exc:`BaseException`.
|
|
|
|
|
|
.. exception:: TypeError
|
|
|
|
Raised when an operation or function is applied to an object of inappropriate
|
|
type. The associated value is a string giving details about the type mismatch.
|
|
|
|
|
|
.. exception:: UnboundLocalError
|
|
|
|
Raised when a reference is made to a local variable in a function or method, but
|
|
no value has been bound to that variable. This is a subclass of
|
|
:exc:`NameError`.
|
|
|
|
.. versionadded:: 2.0
|
|
|
|
|
|
.. exception:: UnicodeError
|
|
|
|
Raised when a Unicode-related encoding or decoding error occurs. It is a
|
|
subclass of :exc:`ValueError`.
|
|
|
|
.. versionadded:: 2.0
|
|
|
|
|
|
.. exception:: UnicodeEncodeError
|
|
|
|
Raised when a Unicode-related error occurs during encoding. It is a subclass of
|
|
:exc:`UnicodeError`.
|
|
|
|
.. versionadded:: 2.3
|
|
|
|
|
|
.. exception:: UnicodeDecodeError
|
|
|
|
Raised when a Unicode-related error occurs during decoding. It is a subclass of
|
|
:exc:`UnicodeError`.
|
|
|
|
.. versionadded:: 2.3
|
|
|
|
|
|
.. exception:: UnicodeTranslateError
|
|
|
|
Raised when a Unicode-related error occurs during translating. It is a subclass
|
|
of :exc:`UnicodeError`.
|
|
|
|
.. versionadded:: 2.3
|
|
|
|
|
|
.. exception:: ValueError
|
|
|
|
Raised when a built-in operation or function receives an argument that has the
|
|
right type but an inappropriate value, and the situation is not described by a
|
|
more precise exception such as :exc:`IndexError`.
|
|
|
|
|
|
.. exception:: WindowsError
|
|
|
|
Raised when a Windows-specific error occurs or when the error number does not
|
|
correspond to an :cdata:`errno` value. The :attr:`winerror` and
|
|
:attr:`strerror` values are created from the return values of the
|
|
:cfunc:`GetLastError` and :cfunc:`FormatMessage` functions from the Windows
|
|
Platform API. The :attr:`errno` value maps the :attr:`winerror` value to
|
|
corresponding ``errno.h`` values. This is a subclass of :exc:`OSError`.
|
|
|
|
.. versionadded:: 2.0
|
|
|
|
.. versionchanged:: 2.5
|
|
Previous versions put the :cfunc:`GetLastError` codes into :attr:`errno`.
|
|
|
|
|
|
.. exception:: ZeroDivisionError
|
|
|
|
Raised when the second argument of a division or modulo operation is zero. The
|
|
associated value is a string indicating the type of the operands and the
|
|
operation.
|
|
|
|
The following exceptions are used as warning categories; see the :mod:`warnings`
|
|
module for more information.
|
|
|
|
|
|
.. exception:: Warning
|
|
|
|
Base class for warning categories.
|
|
|
|
|
|
.. exception:: UserWarning
|
|
|
|
Base class for warnings generated by user code.
|
|
|
|
|
|
.. exception:: DeprecationWarning
|
|
|
|
Base class for warnings about deprecated features.
|
|
|
|
|
|
.. exception:: PendingDeprecationWarning
|
|
|
|
Base class for warnings about features which will be deprecated in the future.
|
|
|
|
|
|
.. exception:: SyntaxWarning
|
|
|
|
Base class for warnings about dubious syntax
|
|
|
|
|
|
.. exception:: RuntimeWarning
|
|
|
|
Base class for warnings about dubious runtime behavior.
|
|
|
|
|
|
.. exception:: FutureWarning
|
|
|
|
Base class for warnings about constructs that will change semantically in the
|
|
future.
|
|
|
|
|
|
.. exception:: ImportWarning
|
|
|
|
Base class for warnings about probable mistakes in module imports.
|
|
|
|
.. versionadded:: 2.5
|
|
|
|
|
|
.. exception:: UnicodeWarning
|
|
|
|
Base class for warnings related to Unicode.
|
|
|
|
.. versionadded:: 2.5
|
|
|
|
The class hierarchy for built-in exceptions is:
|
|
|
|
|
|
.. literalinclude:: ../../Lib/test/exception_hierarchy.txt
|