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 “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 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 exception’s
__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.
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.
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, it’s recommended to avoid subclassing multiple exception types altogether.
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 exception’s notes which appear in the standard
traceback after the exception string. A TypeError is raised if note
is not a string.
Added 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.
Added 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.
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 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 “from list” 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.
Added 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 (C’s 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 – in that case either leave the operator /
method undefined or, if a subclass, set it to None.
Caution
NotImplementedError and NotImplemented are not
interchangeable. This exception should only be used as described
above; see NotImplemented for details on correct usage of
the built-in constant.
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 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
an operation is blocked during interpreter shutdown also known as
Python finalization.
Examples of operations which can be blocked with a
PythonFinalizationError during the Python finalization:
Creating a new Python thread.
See also the sys.is_finalizing() function.
Added in version 3.13: Previously, a plain RuntimeError was raised.
This exception is derived from RuntimeError. It is raised when the
interpreter detects that the maximum recursion depth (see
sys.getrecursionlimit()) is exceeded.
Added 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 doesn’t 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 exception’s 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.
Added 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 “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)).
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). In 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.
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
C’s 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 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.
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:
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 doesn’t 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: WASI’s ENOTCAPABLE is now mapped to
PermissionError.
Raised when a given process doesn’t exist.
Corresponds to errno ESRCH.
Raised when a system function timed out at the system level.
Corresponds to errno ETIMEDOUT.
Added in version 3.3: All the above OSError subclasses were added.
See also
PEP 3151 - Reworking the OS and IO exception hierarchy
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.
Added 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.
Added 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 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 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.
Added in version 3.13: condition can be any callable which is not a type object.
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(), which
are used in various contexts to break up an exception group. 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 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 ExceptionGroup, any subclass of BaseExceptionGroup which
is also a subclass of Exception can only wrap instances of
Exception.
Added in version 3.11.
The class hierarchy for built-in exceptions is:
BaseException
├── BaseExceptionGroup
├── GeneratorExit
├── KeyboardInterrupt
├── SystemExit
└── Exception
├── ArithmeticError
│ ├── FloatingPointError
│ ├── OverflowError
│ └── ZeroDivisionError
├── AssertionError
├── AttributeError
├── BufferError
├── EOFError
├── ExceptionGroup [BaseExceptionGroup]
├── ImportError
│ └── ModuleNotFoundError
├── LookupError
│ ├── IndexError
│ └── KeyError
├── MemoryError
├── NameError
│ └── UnboundLocalError
├── OSError
│ ├── BlockingIOError
│ ├── ChildProcessError
│ ├── ConnectionError
│ │ ├── BrokenPipeError
│ │ ├── ConnectionAbortedError
│ │ ├── ConnectionRefusedError
│ │ └── ConnectionResetError
│ ├── FileExistsError
│ ├── FileNotFoundError
│ ├── InterruptedError
│ ├── IsADirectoryError
│ ├── NotADirectoryError
│ ├── PermissionError
│ ├── ProcessLookupError
│ └── TimeoutError
├── ReferenceError
├── RuntimeError
│ ├── NotImplementedError
│ ├── PythonFinalizationError
│ └── RecursionError
├── StopAsyncIteration
├── StopIteration
├── SyntaxError
│ └── IndentationError
│ └── TabError
├── SystemError
├── TypeError
├── ValueError
│ └── UnicodeError
│ ├── UnicodeDecodeError
│ ├── UnicodeEncodeError
│ └── UnicodeTranslateError
└── Warning
├── BytesWarning
├── DeprecationWarning
├── EncodingWarning
├── FutureWarning
├── ImportWarning
├── PendingDeprecationWarning
├── ResourceWarning
├── RuntimeWarning
├── SyntaxWarning
├── UnicodeWarning
└── UserWarning