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.. _bltin-exceptions: Built-in Exceptions =================== .. index:: statement: try 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:: 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 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`. When raising (or re-raising) an exception in an :keyword:`except` or :keyword:`finally` clause :attr:`__context__` is automatically set to the last exception caught; if the new exception is not handled the traceback that is eventually displayed will include the originating exception(s) and the final exception. When raising a new exception (rather than using a bare ``raise`` to re-raise the exception currently being handled), the 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` 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. Base classes ------------ 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 is usually used in exception handling code like this:: try: ... except SomeException: tb = sys.exc_info()[2] raise OtherException(...).with_traceback(tb) .. 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`. Concrete exceptions ------------------- The following exceptions are the exceptions that are usually raised. .. exception:: AssertionError .. index:: 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.) .. exception:: EOFError Raised when the :func:`input` function hits an end-of-file condition (EOF) without reading any data. (N.B.: the :meth:`io.IOBase.read` and :meth:`io.IOBase.readline` methods return an empty string when they hit EOF.) .. 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 ``--with-fpectl`` option, or the :const:`WANT_SIGFPE_HANDLER` symbol is defined in the :file:`pyconfig.h` file. .. 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 :attr:`name` and :attr:`path` attributes can be set using keyword-only arguments to the constructor. When set they represent the name of the module that was attempted to be imported and the path to any file which triggered the exception, respectively. .. 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:: 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. .. 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. .. 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`. .. note:: ``NotImplementedError`` and ``NotImplemented`` are not interchangeable, even though they have similar names and purposes. See :data:`NotImplemented` for details on when to use it. .. exception:: OSError([arg]) OSError(errno, strerror[, filename[, winerror[, filename2]]]) .. index:: 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 filesystem encoding. 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:: 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. 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 function defined in the presence of a ``from __future__ import generator_stop`` directive raises :exc:`StopIteration`, it will be 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. .. exception:: StopAsyncIteration Must be raised by :meth:`__anext__` method of an :term:`asynchronous iterator` object to stop the iteration. .. versionadded:: 3.5 .. exception:: SyntaxError 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:`exec` or :func:`eval`, or when reading the initial script or standard input (also interactively). 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:: 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). 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`. .. attribute:: end The index after the last invalid data in :attr:`object`. .. 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` ``EAGAIN``, ``EALREADY``, ``EWOULDBLOCK`` and ``EINPROGRESS``. In addition to those of :exc:`OSError`, :exc:`BlockingIOError` can have one more attribute: .. attribute:: characters_written An integer containing the number of characters 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` ``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` ``EPIPE`` and ``ESHUTDOWN``. .. exception:: ConnectionAbortedError A subclass of :exc:`ConnectionError`, raised when a connection attempt is aborted by the peer. Corresponds to :c:data:`errno` ``ECONNABORTED``. .. exception:: ConnectionRefusedError A subclass of :exc:`ConnectionError`, raised when a connection attempt is refused by the peer. Corresponds to :c:data:`errno` ``ECONNREFUSED``. .. exception:: ConnectionResetError A subclass of :exc:`ConnectionError`, raised when a connection is reset by the peer. Corresponds to :c:data:`errno` ``ECONNRESET``. .. exception:: FileExistsError Raised when trying to create a file or directory which already exists. Corresponds to :c:data:`errno` ``EEXIST``. .. exception:: FileNotFoundError Raised when a file or directory is requested but doesn't exist. Corresponds to :c:data:`errno` ``ENOENT``. .. exception:: InterruptedError Raised when a system call is interrupted by an incoming signal. Corresponds to :c:data:`errno` :py:data:`~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` ``EISDIR``. .. exception:: NotADirectoryError Raised when a directory operation (such as :func:`os.listdir`) is requested on something which is not a directory. Corresponds to :c:data:`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` ``EACCES`` and ``EPERM``. .. exception:: ProcessLookupError Raised when a given process doesn't exist. Corresponds to :c:data:`errno` ``ESRCH``. .. exception:: TimeoutError Raised when a system function timed out at the system level. Corresponds to :c:data:`errno` ``ETIMEDOUT``. .. versionadded:: 3.3 All the above :exc:`OSError` subclasses were added. .. seealso:: :pep:`3151` - Reworking the OS and IO exception hierarchy Warnings -------- 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. .. exception:: UnicodeWarning Base class for warnings related to Unicode. .. exception:: BytesWarning Base class for warnings related to :class:`bytes` and :class:`bytearray`. .. exception:: ResourceWarning Base class for warnings related to resource usage. .. versionadded:: 3.2 Exception hierarchy ------------------- The class hierarchy for built-in exceptions is: .. literalinclude:: ../../Lib/test/exception_hierarchy.txt