In Python, all exceptions must be instances of a class that derives from BaseException. In a try statement with an 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.
The built-in exceptions listed in this chapter can be generated by the interpreter or built-in functions. Except where mentioned, they have an âassociated valueâ 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 classâs constructor.
User code can raise built-in exceptions. This can be used to test an exception handler or to report an error condition âjust likeâ 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 Exception class or one of its subclasses, and not from BaseException. More information on defining exceptions is available in the Python Tutorial under User-defined Exceptions.
Three attributes on exception objects provide information about the context in which the exception was raised:
When raising a new exception while another exception is already being handled, the new exceptionâs __context__ attribute is automatically set to the handled exception. An exception may be handled when an except or finally clause, or a with statement, is used.
This implicit exception context can be supplemented with an explicit cause by using from with raise:
raise new_exc from original_exc
The expression following from must be an exception or None. It will be set as __cause__ on the raised exception. Setting __cause__ also implicitly sets the __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 KeyError to AttributeError), while leaving the old exception available in __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 __cause__ is always shown when present. An implicitly chained exception in __context__ is shown only if __cause__ is None and __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. Itâs 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.
CPython implementation detail: Most built-in exceptions are implemented in C for efficiency, see: 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, itâs recommended to avoid subclassing multiple exception types altogether.
Base classes¶The following exceptions are used mostly as base classes for other exceptions.
The base class for all built-in exceptions. It is not meant to be directly inherited by user-defined classes (for that, use Exception). If 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.
The tuple of arguments given to the exception constructor. Some built-in exceptions (like 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.
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)
A writable field that holds the traceback object associated with this exception. See also: The raise statement.
Add the string note to the exceptionâs notes which appear in the standard traceback after the exception string. A TypeError is raised if note is not a string.
New in version 3.11.
A list of the notes of this exception, which were added with add_note(). This attribute is created when add_note() is called.
New in version 3.11.
All built-in, non-system-exiting exceptions are derived from this class. All user-defined exceptions should also be derived from this class.
The base class for those built-in exceptions that are raised for various arithmetic errors: OverflowError, ZeroDivisionError, FloatingPointError.
Raised when a buffer related operation cannot be performed.
The base class for the exceptions that are raised when a key or index used on a mapping or sequence is invalid: IndexError, KeyError. This can be raised directly by codecs.lookup().
The following exceptions are the exceptions that are usually raised.
Raised when an assert statement fails.
Raised when an attribute reference (see Attribute references) or assignment fails. (When an object does not support attribute references or attribute assignments at all, TypeError is raised.)
The name and obj attributes can be set using keyword-only arguments to the constructor. When set they represent the name of the attribute that was attempted to be accessed and the object that was accessed for said attribute, respectively.
Changed in version 3.10: Added the name and obj attributes.
Raised when the input() function hits an end-of-file condition (EOF) without reading any data. (N.B.: the io.IOBase.read() and io.IOBase.readline() methods return an empty string when they hit EOF.)
Not currently used.
Raised when a generator or coroutine is closed; see generator.close() and coroutine.close(). It directly inherits from BaseException instead of Exception since it is technically not an error.
Raised when the import statement has troubles trying to load a module. Also raised when the âfrom listâ in from ... import has a name that cannot be found.
The optional name and path keyword-only arguments set the corresponding attributes:
The name of the module that was attempted to be imported.
The path to any file which triggered the exception.
A subclass of ImportError which is raised by import when a module could not be located. It is also raised when None is found in sys.modules.
New in version 3.6.
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, TypeError is raised.)
Raised when a mapping (dictionary) key is not found in the set of existing keys.
Raised when the user hits the interrupt key (normally Control-C or Delete). During execution, a check for interrupts is made regularly. The exception inherits from BaseException so as to not be accidentally caught by code that catches Exception and thus prevent the interpreter from exiting.
Note
Catching a 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 KeyboardInterrupt to end the program as quickly as possible or avoid raising it entirely. (See Note on Signal Handlers and Exceptions.)
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 (Câs 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.
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 name attribute can be set using a keyword-only argument to the constructor. When set it represent the name of the variable that was attempted to be accessed.
Changed in version 3.10: Added the name attribute.
This exception is derived from 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 â in that case either leave the operator / method undefined or, if a subclass, set it to None.
Note
NotImplementedError and NotImplemented are not interchangeable, even though they have similar names and purposes. See NotImplemented for details on when to use it.
This exception is raised when a system function returns a system-related error, including I/O failures such as âfile not foundâ or âdisk fullâ (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 None if not specified. For backwards compatibility, if three arguments are passed, the args attribute contains only a 2-tuple of the first two constructor arguments.
The constructor often actually returns a subclass of OSError, as described in OS exceptions below. The particular subclass depends on the final errno value. This behaviour only occurs when constructing OSError directly or via an alias, and is not inherited when subclassing.
A numeric error code from the C variable errno.
Under Windows, this gives you the native Windows error code. The 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 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 winerror attribute does not exist.
The corresponding error message, as provided by the operating system. It is formatted by the C functions perror() under POSIX, and FormatMessage() under Windows.
For exceptions that involve a file system path (such as open() or os.unlink()), filename is the file name passed to the function. For functions that involve two file system paths (such as os.rename()), filename2 corresponds to the second file name passed to the function.
Changed in version 3.3: EnvironmentError, IOError, WindowsError, socket.error, select.error and mmap.error have been merged into OSError, and the constructor may return a subclass.
Changed in version 3.4: The filename attribute is now the original file name passed to the function, instead of the name encoded to or decoded from the filesystem encoding and error handler. Also, the filename2 constructor argument and attribute was added.
Raised when the result of an arithmetic operation is too large to be represented. This cannot occur for integers (which would rather raise 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.
This exception is derived from RuntimeError. It is raised when the interpreter detects that the maximum recursion depth (see sys.getrecursionlimit()) is exceeded.
New in version 3.5: Previously, a plain RuntimeError was raised.
This exception is raised when a weak reference proxy, created by the 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 weakref module.
Raised when an error is detected that doesnât fall in any of the other categories. The associated value is a string indicating what precisely went wrong.
Raised by built-in function next() and an iterator's __next__() method to signal that there are no further items produced by the iterator.
The exception object has a single attribute value, which is given as an argument when constructing the exception, and defaults to None.
When a generator or coroutine function returns, a new StopIteration instance is raised, and the value returned by the function is used as the value parameter to the constructor of the exception.
If a generator code directly or indirectly raises StopIteration, it is converted into a RuntimeError (retaining the StopIteration as the new exceptionâs cause).
Changed in version 3.3: Added value attribute and the ability for generator functions to use it to return a value.
Changed in version 3.5: Introduced the RuntimeError transformation via from __future__ import generator_stop, see PEP 479.
Changed in version 3.7: Enable PEP 479 for all code by default: a StopIteration error raised in a generator is transformed into a RuntimeError.
Must be raised by __anext__() method of an asynchronous iterator object to stop the iteration.
New in version 3.5.
Raised when the parser encounters a syntax error. This may occur in an import statement, in a call to the built-in functions compile(), exec(), or eval(), or when reading the initial script or standard input (also interactively).
The str() of the exception instance returns only the error message. Details is a tuple whose members are also available as separate attributes.
The name of the file the syntax error occurred in.
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.
The column in the line where the error occurred. This is 1-indexed: the first character in the line has an offset of 1.
The source code text involved in the error.
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.
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 âf-string: â and the offsets are offsets in a text constructed from the replacement expression. For example, compiling fâBad {a b} fieldâ results in this args attribute: (âf-string: â¦â, (ââ, 1, 2, â(a b)nâ, 1, 5)).
Changed in version 3.10: Added the end_lineno and end_offset attributes.
Base class for syntax errors related to incorrect indentation. This is a subclass of SyntaxError.
Raised when indentation contains an inconsistent use of tabs and spaces. This is a subclass of IndentationError.
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 exceptionâs associated value) and if possible the source of the program that triggered the error.
This exception is raised by the sys.exit() function. It inherits from BaseException instead of Exception so that it is not accidentally caught by code that catches 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 sys.exit(). If the value is an integer, it specifies the system exit status (passed to Câs exit() function); if it is None, the exit status is zero; if it has another type (such as a string), the objectâs value is printed and the exit status is one.
A call to sys.exit() is translated into an exception so that clean-up handlers (finally clauses of try statements) can be executed, and so that a debugger can execute a script without running the risk of losing control. The 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 os.fork()).
The exit status or error message that is passed to the constructor. (Defaults to None.)
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, NotImplementedError is the proper exception to raise.
Passing arguments of the wrong type (e.g. passing a list when an int is expected) should result in a TypeError, but passing arguments with the wrong value (e.g. a number outside expected boundaries) should result in a ValueError.
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 NameError.
Raised when a Unicode-related encoding or decoding error occurs. It is a subclass of ValueError.
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.
The name of the encoding that raised the error.
A string describing the specific codec error.
The object the codec was attempting to encode or decode.
The first index of invalid data in object.
The index after the last invalid data in object.
Raised when a Unicode-related error occurs during encoding. It is a subclass of UnicodeError.
Raised when a Unicode-related error occurs during decoding. It is a subclass of UnicodeError.
Raised when a Unicode-related error occurs during translating. It is a subclass of UnicodeError.
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 IndexError.
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 OSError.
Only available on Windows.
The following exceptions are subclasses of OSError, they get raised depending on the system error code.
Raised when an operation would block on an object (e.g. socket) set for non-blocking operation. Corresponds to errno EAGAIN, EALREADY, EWOULDBLOCK and EINPROGRESS.
In addition to those of OSError, BlockingIOError can have one more attribute:
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 io module.
Raised when an operation on a child process failed. Corresponds to errno ECHILD.
A base class for connection-related issues.
Subclasses are BrokenPipeError, ConnectionAbortedError, ConnectionRefusedError and ConnectionResetError.
A subclass of 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 errno EPIPE and ESHUTDOWN.
A subclass of ConnectionError, raised when a connection attempt is aborted by the peer. Corresponds to errno ECONNABORTED.
A subclass of ConnectionError, raised when a connection attempt is refused by the peer. Corresponds to errno ECONNREFUSED.
A subclass of ConnectionError, raised when a connection is reset by the peer. Corresponds to errno ECONNRESET.
Raised when trying to create a file or directory which already exists. Corresponds to errno EEXIST.
Raised when a file or directory is requested but doesnât exist. Corresponds to errno ENOENT.
Raised when a system call is interrupted by an incoming signal. Corresponds to errno EINTR.
Changed in version 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 InterruptedError.
Raised when a file operation (such as os.remove()) is requested on a directory. Corresponds to errno EISDIR.
Raised when a directory operation (such as 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 errno ENOTDIR.
Raised when trying to run an operation without the adequate access rights - for example filesystem permissions. Corresponds to errno EACCES, EPERM, and ENOTCAPABLE.
Changed in version 3.11.1: WASIâs ENOTCAPABLE is now mapped to PermissionError.
Raised when a given process doesnât exist. Corresponds to errno ESRCH.
Raised when a system function timed out at the system level. Corresponds to errno ETIMEDOUT.
New in version 3.3: All the above OSError subclasses were added.
See also
PEP 3151 - Reworking the OS and IO exception hierarchy
Warnings¶The following exceptions are used as warning categories; see the Warning Categories documentation for more details.
Base class for warning categories.
Base class for warnings generated by user code.
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 Python Development Mode shows this warning.
The deprecation policy is described in PEP 387.
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 DeprecationWarning is preferred for already active deprecations.
Ignored by the default warning filters. Enabling the Python Development Mode shows this warning.
The deprecation policy is described in PEP 387.
Base class for warnings about dubious syntax.
Base class for warnings about dubious runtime behavior.
Base class for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python.
Base class for warnings about probable mistakes in module imports.
Ignored by the default warning filters. Enabling the Python Development Mode shows this warning.
Base class for warnings related to Unicode.
Base class for warnings related to encodings.
See Opt-in EncodingWarning for details.
New in version 3.10.
Base class for warnings related to resource usage.
Ignored by the default warning filters. Enabling the Python Development Mode shows this warning.
New in version 3.2.
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 except like all other exceptions. In addition, they are recognised by except*, which matches their subgroups based on the types of the contained exceptions.
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 BaseExceptionGroup extends BaseException and it can wrap any exception, while ExceptionGroup extends Exception and it can only wrap subclasses of Exception. This design is so that except Exception catches an ExceptionGroup but not BaseExceptionGroup.
The BaseExceptionGroup constructor returns an ExceptionGroup rather than a BaseExceptionGroup if all contained exceptions are Exception instances, so it can be used to make the selection automatic. The ExceptionGroup constructor, on the other hand, raises a TypeError if any contained exception is not an Exception subclass.
The msg argument to the constructor. This is a read-only attribute.
A tuple of the exceptions in the excs sequence given to the constructor. This is a read-only attribute.
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 either a function that accepts an exception and returns true for those that should be in the subgroup, or it can be an exception type or a tuple of exception types, which is used to check for a match using the same check that is used in an except clause.
The nesting structure of the current exception is preserved in the result, as are the values of its message, __traceback__, __cause__, __context__ and __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.
Like subgroup(), but returns the pair (match, rest) where match is subgroup(condition) and rest is the remaining non-matching part.
Returns an exception group with the same message, but which wraps the exceptions in excs.
This method is used by subgroup() and split(). A subclass needs to override it in order to make subgroup() and split() return instances of the subclass rather than ExceptionGroup.
subgroup() and split() copy the __traceback__, __cause__, __context__ and __notes__ fields from the original exception group to the one returned by derive(), so these fields do not need to be updated by derive().
>>> 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 BaseExceptionGroup defines __new__(), so subclasses that need a different constructor signature need to override that rather than __init__(). For example, the following defines an exception group subclass which accepts an exit_code and and constructs the groupâs 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 ExceptionGroup, any subclass of BaseExceptionGroup which is also a subclass of Exception can only wrap instances of Exception.
New in version 3.11.
The class hierarchy for built-in exceptions is:
BaseException
âââ BaseExceptionGroup
âââ GeneratorExit
âââ KeyboardInterrupt
âââ SystemExit
âââ Exception
âââ ArithmeticError
â âââ FloatingPointError
â âââ OverflowError
â âââ ZeroDivisionError
âââ AssertionError
âââ AttributeError
âââ BufferError
âââ EOFError
âââ ExceptionGroup [BaseExceptionGroup]
âââ ImportError
â âââ ModuleNotFoundError
âââ LookupError
â âââ IndexError
â âââ KeyError
âââ MemoryError
âââ NameError
â âââ UnboundLocalError
âââ OSError
â âââ BlockingIOError
â âââ ChildProcessError
â âââ ConnectionError
â â âââ BrokenPipeError
â â âââ ConnectionAbortedError
â â âââ ConnectionRefusedError
â â âââ ConnectionResetError
â âââ FileExistsError
â âââ FileNotFoundError
â âââ InterruptedError
â âââ IsADirectoryError
â âââ NotADirectoryError
â âââ PermissionError
â âââ ProcessLookupError
â âââ TimeoutError
âââ ReferenceError
âââ RuntimeError
â âââ NotImplementedError
â âââ RecursionError
âââ StopAsyncIteration
âââ StopIteration
âââ SyntaxError
â âââ IndentationError
â âââ TabError
âââ SystemError
âââ TypeError
âââ ValueError
â âââ UnicodeError
â âââ UnicodeDecodeError
â âââ UnicodeEncodeError
â âââ UnicodeTranslateError
âââ Warning
âââ BytesWarning
âââ DeprecationWarning
âââ EncodingWarning
âââ FutureWarning
âââ ImportWarning
âââ PendingDeprecationWarning
âââ ResourceWarning
âââ RuntimeWarning
âââ SyntaxWarning
âââ UnicodeWarning
âââ UserWarning
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