Doc/library/exceptions.rst
.. _bltin-exceptions:
.. index:: pair: statement; try pair: statement; except
In Python, all exceptions must be instances of a class that derives from
:class:BaseException. 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:: pair: statement; raise
The built-in exceptions listed in this chapter 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 of several items of information (e.g., an error code and a string explaining the code). The associated value is usually passed as arguments to the exception class's constructor.
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 subclassed to define new exceptions;
programmers are encouraged to derive new exceptions from the :exc:Exception
class or one of its subclasses, and not from :exc:BaseException. More
information on defining exceptions is available in the Python Tutorial under
:ref:tut-userexceptions.
.. index:: pair: exception; chaining cause (exception attribute) context (exception attribute) suppress_context (exception attribute)
Three attributes on exception objects provide information about the context in which the exception was raised:
.. attribute:: BaseException.context BaseException.cause BaseException.suppress_context
When raising a new exception while another exception
is already being handled, the new exception's
:attr:!__context__ attribute is automatically set to the handled
exception. An exception may be handled when an :keyword:except or
:keyword:finally clause, or a :keyword:with statement, is used.
This implicit exception context can be
supplemented with an explicit cause by using :keyword:!from with
:keyword:raise::
raise new_exc from original_exc
The expression following :keyword:from<raise> must be an exception or None. It
will be set as :attr:!__cause__ on the raised exception. Setting
:attr:!__cause__ also implicitly sets the :attr:!__suppress_context__
attribute to True, so that using raise new_exc from None
effectively replaces the old exception with the new one for display
purposes (e.g. converting :exc:KeyError to :exc:AttributeError), while
leaving the old exception available in :attr:!__context__ for introspection
when debugging.
The default traceback display code shows these chained exceptions in
addition to the traceback for the exception itself. An explicitly chained
exception in :attr:!__cause__ is always shown when present. An implicitly
chained exception in :attr:!__context__ is shown only if :attr:!__cause__
is :const:None and :attr:!__suppress_context__ is false.
In either case, the exception itself is always shown after any chained exceptions so that the final line of the traceback always shows the last exception that was raised.
User code can create subclasses that inherit from an exception type.
It's recommended to only subclass one exception type at a time to avoid
any possible conflicts between how the bases handle the args
attribute, as well as due to possible memory layout incompatibilities.
.. impl-detail::
Most built-in exceptions are implemented in C for efficiency, see:
:source:Objects/exceptions.c. Some have custom memory layouts
which makes it impossible to create a subclass that inherits from
multiple exception types. The memory layout of a type is an implementation
detail and might change between Python versions, leading to new
conflicts in the future. Therefore, it's recommended to avoid
subclassing multiple exception types altogether.
The following exceptions are used mostly 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 is called on an instance of this class, the representation of
the argument(s) to the instance are returned, or the empty string when
there were no arguments.
.. attribute:: args
The tuple of arguments given to the exception constructor. Some built-in
exceptions (like :exc:`OSError`) expect a certain number of arguments and
assign a special meaning to the elements of this tuple, while others are
usually called only with a single string giving an error message.
.. method:: with_traceback(tb)
This method sets *tb* as the new traceback for the exception and returns
the exception object. It was more commonly used before the exception
chaining features of :pep:`3134` became available. The following example
shows how we can convert an instance of ``SomeException`` into an
instance of ``OtherException`` while preserving the traceback. Once
raised, the current frame is pushed onto the traceback of the
``OtherException``, as would have happened to the traceback of the
original ``SomeException`` had we allowed it to propagate to the caller. ::
try:
...
except SomeException:
tb = sys.exception().__traceback__
raise OtherException(...).with_traceback(tb)
.. attribute:: traceback
A writable field that holds the
:ref:`traceback object <traceback-objects>` associated with this
exception. See also: :ref:`raise`.
.. method:: add_note(note)
Add the string ``note`` to the exception's notes which appear in the standard
traceback after the exception string. A :exc:`TypeError` is raised if ``note``
is not a string.
.. versionadded:: 3.11
.. attribute:: notes
A list of the notes of this exception, which were added with :meth:`add_note`.
This attribute is created when :meth:`add_note` is called.
.. versionadded:: 3.11
.. 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.
.. exception:: ArithmeticError
The base class for those built-in exceptions that are raised for various
arithmetic errors: :exc:OverflowError, :exc:ZeroDivisionError,
:exc:FloatingPointError.
.. exception:: BufferError
Raised when a :ref:buffer <bufferobjects> related operation cannot be
performed.
.. 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:codecs.lookup.
The following exceptions are the exceptions that are usually raised.
.. exception:: AssertionError
.. index:: pair: statement; assert
Raised when an :keyword:assert statement fails.
.. exception:: AttributeError
Raised when an attribute reference (see :ref:attribute-references) or
assignment fails. (When an object does not support attribute references or
attribute assignments at all, :exc:TypeError is raised.)
The optional name and obj keyword-only arguments set the corresponding attributes:
.. attribute:: name
The name of the attribute that was attempted to be accessed.
.. attribute:: obj
The object that was accessed for the named attribute.
.. versionchanged:: 3.10
Added the :attr:name and :attr:obj attributes.
.. exception:: EOFError
Raised when the :func:input function hits an end-of-file condition (EOF)
without reading any data. (Note: the :meth:io.TextIOBase.read and
:meth:io.IOBase.readline methods return an empty string when they hit EOF.)
.. exception:: FloatingPointError
Not currently used.
.. exception:: GeneratorExit
Raised when a :term:generator or :term:coroutine is closed;
see :meth:generator.close and :meth:coroutine.close. It
directly inherits from :exc:BaseException instead of :exc:Exception since
it is technically not an error.
.. exception:: ImportError
Raised when the :keyword:import statement has troubles trying to
load a module. Also raised when the "from list" in from ... import
has a name that cannot be found.
The optional name and path keyword-only arguments set the corresponding attributes:
.. attribute:: name
The name of the module that was attempted to be imported.
.. attribute:: path
The path to any file which triggered the exception.
.. versionchanged:: 3.3
Added the :attr:name and :attr:path attributes.
.. exception:: ModuleNotFoundError
A subclass of :exc:ImportError which is raised by :keyword:import
when a module could not be located. It is also raised when None
is found in :data:sys.modules.
.. versionadded:: 3.6
.. exception:: ImportCycleError
A subclass of :exc:ImportError which is raised when a lazy import fails
because it (directly or indirectly) tries to import itself.
.. versionadded:: 3.15
.. 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 an
integer, :exc:TypeError is raised.)
.. XXX xref to sequences
.. exception:: KeyError
Raised when a mapping (dictionary) key is not found in the set of existing keys.
.. XXX 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. 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.
.. note::
Catching a :exc:`KeyboardInterrupt` requires special consideration.
Because it can be raised at unpredictable points, it may, in some
circumstances, leave the running program in an inconsistent state. It is
generally best to allow :exc:`KeyboardInterrupt` to end the program as
quickly as possible or avoid raising it entirely. (See
:ref:`handlers-and-exceptions`.)
.. 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 :c:func: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.
The optional name keyword-only argument sets the attribute:
.. attribute:: name
The name of the variable that was attempted to be accessed.
.. versionchanged:: 3.10
Added the :attr:name attribute.
.. 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, or while the class is being
developed to indicate that the real implementation still needs to be added.
.. note::
It should not be used to indicate that an operator or method is not
meant to be supported at all -- in that case either leave the operator /
method undefined or, if a subclass, set it to :data:`None`.
.. caution::
:exc:`!NotImplementedError` and :data:`!NotImplemented` are not
interchangeable. This exception should only be used as described
above; see :data:`NotImplemented` for details on correct usage of
the built-in constant.
.. exception:: OSError([arg]) OSError(errno, strerror[, filename[, winerror[, filename2]]])
.. index:: pair: module; errno
This exception is raised when a system function returns a system-related error, including I/O failures such as "file not found" or "disk full" (not for illegal argument types or other incidental errors).
The second form of the constructor sets the corresponding attributes,
described below. The attributes default to :const:None if not
specified. For backwards compatibility, if three arguments are passed,
the :attr:~BaseException.args attribute contains only a 2-tuple
of the first two constructor arguments.
The constructor often actually returns a subclass of :exc:OSError, as
described in OS exceptions_ below. The particular subclass depends on
the final :attr:.errno value. This behaviour only occurs when
constructing :exc:OSError directly or via an alias, and is not
inherited when subclassing.
.. attribute:: errno
A numeric error code from the C variable :c:data:`errno`.
.. attribute:: winerror
Under Windows, this gives you the native
Windows error code. The :attr:`.errno` attribute is then an approximate
translation, in POSIX terms, of that native error code.
Under Windows, if the *winerror* constructor argument is an integer,
the :attr:`.errno` attribute is determined from the Windows error code,
and the *errno* argument is ignored. On other platforms, the
*winerror* argument is ignored, and the :attr:`winerror` attribute
does not exist.
.. attribute:: strerror
The corresponding error message, as provided by
the operating system. It is formatted by the C
functions :c:func:`!perror` under POSIX, and :c:func:`!FormatMessage`
under Windows.
.. attribute:: filename filename2
For exceptions that involve a file system path (such as :func:`open` or
:func:`os.unlink`), :attr:`filename` is the file name passed to the function.
For functions that involve two file system paths (such as
:func:`os.rename`), :attr:`filename2` corresponds to the second
file name passed to the function.
.. versionchanged:: 3.3
:exc:EnvironmentError, :exc:IOError, :exc:WindowsError,
:exc:socket.error, :exc:select.error and
:exc:!mmap.error have been merged into :exc:OSError, and the
constructor may return a subclass.
.. versionchanged:: 3.4
The :attr:filename attribute is now the original file name passed to
the function, instead of the name encoded to or decoded from the
:term:filesystem encoding and error handler. Also, the filename2
constructor argument and attribute was added.
.. exception:: OverflowError
Raised when the result of an arithmetic operation is too large to be
represented. This cannot occur for integers (which would rather raise
:exc:MemoryError than give up). However, for historical reasons,
OverflowError is sometimes raised for integers that are outside a required
range. Because of the lack of standardization of floating-point exception
handling in C, most floating-point operations are not checked.
.. exception:: PythonFinalizationError
This exception is derived from :exc:RuntimeError. It is raised when
an operation is blocked during interpreter shutdown also known as
:term:Python finalization <interpreter shutdown>.
Examples of operations which can be blocked with a
:exc:PythonFinalizationError during the Python finalization:
Joining <threading.Thread.join> a running daemon thread.os.fork,threading.Lock, when it is known that
the operation would otherwise deadlock.See also the :func:sys.is_finalizing function.
.. versionadded:: 3.13
Previously, a plain :exc:RuntimeError was raised.
.. versionchanged:: 3.14
:meth:`threading.Thread.join` can now raise this exception.
.. versionchanged:: 3.15
This exception may be raised when acquiring :meth:`threading.Lock`
or :meth:`threading.RLock`.
.. exception:: RecursionError
This exception is derived from :exc:RuntimeError. It is raised when the
interpreter detects that the maximum recursion depth (see
:func:sys.getrecursionlimit) is exceeded.
.. versionadded:: 3.5
Previously, a plain :exc:RuntimeError was raised.
.. 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.
.. 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.
.. exception:: StopIteration
Raised by built-in function :func:next and an :term:iterator's
:meth:~iterator.__next__ method to signal that there are no further
items produced by the iterator.
.. attribute:: StopIteration.value
The exception object has a single attribute :attr:`!value`, which is
given as an argument when constructing the exception, and defaults
to :const:`None`.
When a :term:generator or :term:coroutine function
returns, a new :exc:StopIteration instance is
raised, and the value returned by the function is used as the
:attr:value parameter to the constructor of the exception.
If a generator code directly or indirectly raises :exc:StopIteration,
it is converted into a :exc:RuntimeError (retaining the
:exc:StopIteration as the new exception's cause).
.. versionchanged:: 3.3
Added value attribute and the ability for generator functions to
use it to return a value.
.. versionchanged:: 3.5
Introduced the RuntimeError transformation via
from __future__ import generator_stop, see :pep:479.
.. versionchanged:: 3.7
Enable :pep:479 for all code by default: a :exc:StopIteration
error raised in a generator is transformed into a :exc:RuntimeError.
.. exception:: StopAsyncIteration
Must be raised by :meth:~object.__anext__ method of an
:term:asynchronous iterator object to stop the iteration.
.. versionadded:: 3.5
.. exception:: SyntaxError(message, details)
Raised when the parser encounters a syntax error. This may occur in an
:keyword:import statement, in a call to the built-in functions
:func:compile, :func:exec,
or :func:eval, or when reading the initial script or standard input
(also interactively).
The :func:str of the exception instance returns only the error message.
Details is a tuple whose members are also available as separate attributes.
.. attribute:: filename
The name of the file the syntax error occurred in.
.. attribute:: lineno
Which line number in the file the error occurred in. This is
1-indexed: the first line in the file has a ``lineno`` of 1.
.. attribute:: offset
The column in the line where the error occurred. This is
1-indexed: the first character in the line has an ``offset`` of 1.
.. attribute:: text
The source code text involved in the error.
.. attribute:: end_lineno
Which line number in the file the error occurred ends in. This is
1-indexed: the first line in the file has a ``lineno`` of 1.
.. attribute:: end_offset
The column in the end line where the error occurred finishes. This is
1-indexed: the first character in the line has an ``offset`` of 1.
For errors in f-string fields, the message is prefixed by "f-string: " and the offsets are offsets in a text constructed from the replacement expression. For example, compiling f'Bad {a b} field' results in this args attribute: ('f-string: ...', ('', 1, 2, '(a b)\n', 1, 5)).
.. versionchanged:: 3.10
Added the :attr:end_lineno and :attr:end_offset attributes.
.. exception:: IndentationError
Base class for syntax errors related to incorrect indentation. This is a
subclass of :exc:SyntaxError.
.. exception:: TabError
Raised when indentation contains an inconsistent use of tabs and spaces.
This is a subclass of :exc:IndentationError.
.. 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). In :term:CPython,
this could be raised by incorrectly using Python's C API, such as returning
a NULL value without an exception set.
If you're confident that this exception wasn't your fault, or the fault of
a package you're using, 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. It inherits from
:exc:BaseException instead of :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. When it is not
handled, the Python interpreter exits; no stack traceback is printed. The
constructor accepts the same optional argument passed to :func:sys.exit.
If the value is an integer, it specifies the system exit status (passed to
C's :c:func:!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.
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:os.fork).
.. attribute:: code
The exit status or error message that is passed to the constructor.
(Defaults to ``None``.)
.. 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.
This exception may be raised by user code to indicate that an attempted
operation on an object is not supported, and is not meant to be. If an object
is meant to support a given operation but has not yet provided an
implementation, :exc:NotImplementedError is the proper exception to raise.
Passing arguments of the wrong type (e.g. passing a :class:list when an
:class:int is expected) should result in a :exc:TypeError, but passing
arguments with the wrong value (e.g. a number outside expected boundaries)
should result in a :exc:ValueError.
.. 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.
.. exception:: UnicodeError
Raised when a Unicode-related encoding or decoding error occurs. It is a
subclass of :exc:ValueError.
:exc:UnicodeError has attributes that describe the encoding or decoding
error. For example, err.object[err.start:err.end] gives the particular
invalid input that the codec failed on.
.. attribute:: encoding
The name of the encoding that raised the error.
.. attribute:: reason
A string describing the specific codec error.
.. attribute:: object
The object the codec was attempting to encode or decode.
.. attribute:: start
The first index of invalid data in :attr:`object`.
This value should not be negative as it is interpreted as an
absolute offset but this constraint is not enforced at runtime.
.. attribute:: end
The index after the last invalid data in :attr:`object`.
This value should not be negative as it is interpreted as an
absolute offset but this constraint is not enforced at runtime.
.. exception:: UnicodeEncodeError
Raised when a Unicode-related error occurs during encoding. It is a subclass of
:exc:UnicodeError.
.. exception:: UnicodeDecodeError
Raised when a Unicode-related error occurs during decoding. It is a subclass of
:exc:UnicodeError.
.. exception:: UnicodeTranslateError
Raised when a Unicode-related error occurs during translating. It is a subclass
of :exc:UnicodeError.
.. exception:: ValueError
Raised when an 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:: 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 kept for compatibility with previous versions;
starting from Python 3.3, they are aliases of :exc:OSError.
.. exception:: EnvironmentError
.. exception:: IOError
.. exception:: WindowsError
Only available on Windows.
OS exceptions ^^^^^^^^^^^^^
The following exceptions are subclasses of :exc:OSError, they get raised
depending on the system error code.
.. exception:: BlockingIOError
Raised when an operation would block on an object (e.g. socket) set
for non-blocking operation.
Corresponds to :c:data:errno :py:const:~errno.EAGAIN, :py:const:~errno.EALREADY,
:py:const:~errno.EWOULDBLOCK and :py:const:~errno.EINPROGRESS.
In addition to those of :exc:OSError, :exc:BlockingIOError can have
one more attribute:
.. attribute:: characters_written
An integer containing the number of **bytes** written to the stream
before it blocked. This attribute is available when using the
buffered I/O classes from the :mod:`io` module.
.. exception:: ChildProcessError
Raised when an operation on a child process failed.
Corresponds to :c:data:errno :py:const:~errno.ECHILD.
.. exception:: ConnectionError
A base class for connection-related issues.
Subclasses are :exc:BrokenPipeError, :exc:ConnectionAbortedError,
:exc:ConnectionRefusedError and :exc:ConnectionResetError.
.. exception:: BrokenPipeError
A subclass of :exc:ConnectionError, raised when trying to write on a
pipe while the other end has been closed, or trying to write on a socket
which has been shutdown for writing.
Corresponds to :c:data:errno :py:const:~errno.EPIPE and :py:const:~errno.ESHUTDOWN.
.. exception:: ConnectionAbortedError
A subclass of :exc:ConnectionError, raised when a connection attempt
is aborted by the peer.
Corresponds to :c:data:errno :py:const:~errno.ECONNABORTED.
.. exception:: ConnectionRefusedError
A subclass of :exc:ConnectionError, raised when a connection attempt
is refused by the peer.
Corresponds to :c:data:errno :py:const:~errno.ECONNREFUSED.
.. exception:: ConnectionResetError
A subclass of :exc:ConnectionError, raised when a connection is
reset by the peer.
Corresponds to :c:data:errno :py:const:~errno.ECONNRESET.
.. exception:: FileExistsError
Raised when trying to create a file or directory which already exists.
Corresponds to :c:data:errno :py:const:~errno.EEXIST.
.. exception:: FileNotFoundError
Raised when a file or directory is requested but doesn't exist.
Corresponds to :c:data:errno :py:const:~errno.ENOENT.
.. exception:: InterruptedError
Raised when a system call is interrupted by an incoming signal.
Corresponds to :c:data:errno :py:const:~errno.EINTR.
.. versionchanged:: 3.5
Python now retries system calls when a syscall is interrupted by a
signal, except if the signal handler raises an exception (see :pep:475
for the rationale), instead of raising :exc:InterruptedError.
.. exception:: IsADirectoryError
Raised when a file operation (such as :func:os.remove) is requested
on a directory.
Corresponds to :c:data:errno :py:const:~errno.EISDIR.
.. exception:: NotADirectoryError
Raised when a directory operation (such as :func:os.listdir) is requested on
something which is not a directory. On most POSIX platforms, it may also be
raised if an operation attempts to open or traverse a non-directory file as if
it were a directory.
Corresponds to :c:data:errno :py:const:~errno.ENOTDIR.
.. exception:: PermissionError
Raised when trying to run an operation without the adequate access
rights - for example filesystem permissions.
Corresponds to :c:data:errno :py:const:~errno.EACCES,
:py:const:~errno.EPERM, and :py:const:~errno.ENOTCAPABLE.
.. versionchanged:: 3.11.1
WASI's :py:const:~errno.ENOTCAPABLE is now mapped to
:exc:PermissionError.
.. exception:: ProcessLookupError
Raised when a given process doesn't exist.
Corresponds to :c:data:errno :py:const:~errno.ESRCH.
.. exception:: TimeoutError
Raised when a system function timed out at the system level.
Corresponds to :c:data:errno :py:const:~errno.ETIMEDOUT.
.. versionadded:: 3.3
All the above :exc:OSError subclasses were added.
.. seealso::
:pep:3151 - Reworking the OS and IO exception hierarchy
.. _warning-categories-as-exceptions:
The following exceptions are used as warning categories; see the
:ref:warning-categories documentation for more details.
.. 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 when those warnings are intended for other Python developers.
Ignored by the default warning filters, except in the __main__ module
(:pep:565). Enabling the :ref:Python Development Mode <devmode> shows
this warning.
The deprecation policy is described in :pep:387.
.. exception:: PendingDeprecationWarning
Base class for warnings about features which are obsolete and expected to be deprecated in the future, but are not deprecated at the moment.
This class is rarely used as emitting a warning about a possible
upcoming deprecation is unusual, and :exc:DeprecationWarning
is preferred for already active deprecations.
Ignored by the default warning filters. Enabling the :ref:Python Development Mode <devmode> shows this warning.
The deprecation policy is described in :pep:387.
.. exception:: SyntaxWarning
Base class for warnings about dubious syntax.
This warning is typically emitted when compiling Python source code, and usually won't be reported when running already compiled code.
.. exception:: RuntimeWarning
Base class for warnings about dubious runtime behavior.
.. exception:: FutureWarning
Base class for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python.
.. exception:: ImportWarning
Base class for warnings about probable mistakes in module imports.
Ignored by the default warning filters. Enabling the :ref:Python Development Mode <devmode> shows this warning.
.. exception:: UnicodeWarning
Base class for warnings related to Unicode.
.. exception:: EncodingWarning
Base class for warnings related to encodings.
See :ref:io-encoding-warning for details.
.. versionadded:: 3.10
.. exception:: BytesWarning
Base class for warnings related to :class:bytes and :class:bytearray.
.. exception:: ResourceWarning
Base class for warnings related to resource usage.
Ignored by the default warning filters. Enabling the :ref:Python Development Mode <devmode> shows this warning.
.. versionadded:: 3.2
.. _lib-exception-groups:
The following are used when it is necessary to raise multiple unrelated
exceptions. They are part of the exception hierarchy so they can be
handled with :keyword:except like all other exceptions. In addition,
they are recognised by :keyword:except*<except_star>, which matches
their subgroups based on the types of the contained exceptions.
.. exception:: ExceptionGroup(msg, excs) .. exception:: BaseExceptionGroup(msg, excs)
Both of these exception types wrap the exceptions in the sequence excs.
The msg parameter must be a string. The difference between the two
classes is that :exc:BaseExceptionGroup extends :exc:BaseException and
it can wrap any exception, while :exc:ExceptionGroup extends :exc:Exception
and it can only wrap subclasses of :exc:Exception. This design is so that
except Exception catches an :exc:ExceptionGroup but not
:exc:BaseExceptionGroup.
The :exc:BaseExceptionGroup constructor returns an :exc:ExceptionGroup
rather than a :exc:BaseExceptionGroup if all contained exceptions are
:exc:Exception instances, so it can be used to make the selection
automatic. The :exc:ExceptionGroup constructor, on the other hand,
raises a :exc:TypeError if any contained exception is not an
:exc:Exception subclass.
.. impl-detail::
The ``excs`` parameter may be any sequence, but lists and tuples are
specifically processed more efficiently here. For optimal performance,
pass a tuple as ``excs``.
.. attribute:: message
The ``msg`` argument to the constructor. This is a read-only attribute.
.. attribute:: exceptions
A tuple of the exceptions in the ``excs`` sequence given to the
constructor. This is a read-only attribute.
.. method:: subgroup(condition)
Returns an exception group that contains only the exceptions from the
current group that match *condition*, or ``None`` if the result is empty.
The condition can be an exception type or tuple of exception types, in which
case each exception is checked for a match using the same check that is used
in an ``except`` clause. The condition can also be a callable (other than
a type object) that accepts an exception as its single argument and returns
true for the exceptions that should be in the subgroup.
The nesting structure of the current exception is preserved in the result,
as are the values of its :attr:`message`,
:attr:`~BaseException.__traceback__`, :attr:`~BaseException.__cause__`,
:attr:`~BaseException.__context__` and
:attr:`~BaseException.__notes__` fields.
Empty nested groups are omitted from the result.
The condition is checked for all exceptions in the nested exception group,
including the top-level and any nested exception groups. If the condition is
true for such an exception group, it is included in the result in full.
.. versionadded:: 3.13
``condition`` can be any callable which is not a type object.
.. method:: split(condition)
Like :meth:`subgroup`, but returns the pair ``(match, rest)`` where ``match``
is ``subgroup(condition)`` and ``rest`` is the remaining non-matching
part.
.. method:: derive(excs)
Returns an exception group with the same :attr:`message`, but which
wraps the exceptions in ``excs``.
This method is used by :meth:`subgroup` and :meth:`split`, which
are used in various contexts to break up an exception group. A
subclass needs to override it in order to make :meth:`subgroup`
and :meth:`split` return instances of the subclass rather
than :exc:`ExceptionGroup`.
:meth:`subgroup` and :meth:`split` copy the
:attr:`~BaseException.__traceback__`,
:attr:`~BaseException.__cause__`, :attr:`~BaseException.__context__` and
:attr:`~BaseException.__notes__` fields from
the original exception group to the one returned by :meth:`derive`, so
these fields do not need to be updated by :meth:`derive`.
.. doctest::
>>> class MyGroup(ExceptionGroup):
... def derive(self, excs):
... return MyGroup(self.message, excs)
...
>>> e = MyGroup("eg", [ValueError(1), TypeError(2)])
>>> e.add_note("a note")
>>> e.__context__ = Exception("context")
>>> e.__cause__ = Exception("cause")
>>> try:
... raise e
... except Exception as e:
... exc = e
...
>>> match, rest = exc.split(ValueError)
>>> exc, exc.__context__, exc.__cause__, exc.__notes__
(MyGroup('eg', [ValueError(1), TypeError(2)]), Exception('context'), Exception('cause'), ['a note'])
>>> match, match.__context__, match.__cause__, match.__notes__
(MyGroup('eg', [ValueError(1)]), Exception('context'), Exception('cause'), ['a note'])
>>> rest, rest.__context__, rest.__cause__, rest.__notes__
(MyGroup('eg', [TypeError(2)]), Exception('context'), Exception('cause'), ['a note'])
>>> exc.__traceback__ is match.__traceback__ is rest.__traceback__
True
Note that :exc:BaseExceptionGroup defines :meth:~object.__new__, so
subclasses that need a different constructor signature need to
override that rather than :meth:~object.__init__. For example, the following
defines an exception group subclass which accepts an exit_code and
constructs the group's message from it. ::
class Errors(ExceptionGroup):
def __new__(cls, errors, exit_code):
self = super().__new__(Errors, f"exit code: {exit_code}", errors)
self.exit_code = exit_code
return self
def derive(self, excs):
return Errors(excs, self.exit_code)
Like :exc:ExceptionGroup, any subclass of :exc:BaseExceptionGroup which
is also a subclass of :exc:Exception can only wrap instances of
:exc:Exception.
.. versionadded:: 3.11
The class hierarchy for built-in exceptions is:
.. literalinclude:: ../../Lib/test/exception_hierarchy.txt :language: text