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 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 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.
Exception context¶
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.
Inheriting from built-in exceptions¶
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.
Base classes¶
The following exceptions are used mostly as base classes for other exceptions.
exceptionBaseException
¶
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.
args
¶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.
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.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.
exceptionArithmeticError
¶The base class for those built-in exceptions that are raised for various arithmetic errors: OverflowError
,
ZeroDivisionError
, FloatingPointError
.
BufferError
¶Raised when a buffer related operation cannot be performed.
exceptionLookupError
¶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[]
.
Concrete exceptions¶
The following exceptions are the exceptions that are usually raised.
exceptionAssertionError
¶Raised when an
assert
statement fails.
AttributeError
¶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.
EOFError
¶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.]
FloatingPointError
¶Not currently used.
exceptionGeneratorExit
¶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.
ImportError
¶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 name
and 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.
Changed in version 3.3: Added the name
and path
attributes.
ModuleNotFoundError
¶
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.
exceptionIndexError
¶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.]
KeyError
¶Raised when a mapping [dictionary] key is not found in the set of existing keys.
exceptionKeyboardInterrupt
¶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.]
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 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.
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 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.
NotImplementedError
¶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
.
Note
NotImplementedError
and NotImplemented
are not interchangeable, even though they have
similar names and purposes. See NotImplemented
for details on when to use it.
OSError
[[arg]]¶ exception
OSError
[errno, strerror[, filename[, winerror[, filename2]]]]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.
errno
¶A numeric error code from the C variable errno
.
winerror
¶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.
strerror
¶The corresponding error message, as provided by the operating system. It is formatted by the C functions perror[]
under POSIX, and FormatMessage[]
under Windows.
filename
¶ filename2
¶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 3.4: 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.
OverflowError
¶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.
RecursionError
¶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.
ReferenceError
¶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.
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.
exceptionStopIteration
¶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
.
StopAsyncIteration
¶Must be raised by __anext__[]
method of an asynchronous
iterator object to stop the iteration.
New in version 3.5.
exceptionSyntaxError
[message, details]¶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.
filename
¶The name of the file the syntax error occurred in.
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.
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.
text
¶The source code text involved in the error.
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.
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]].
exceptionIndentationError
¶Base class for syntax errors related to incorrect indentation. This is a subclass of
SyntaxError
.
TabError
¶Raised when indentation contains an inconsistent use of tabs and spaces. This is a subclass of
IndentationError
.
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.
SystemExit
¶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[]
].
code
¶The exit status or error message that is passed to the constructor. [Defaults to None
.]
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,
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
.
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 NameError
.
UnicodeError
¶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.
encoding
¶The name of the encoding that raised the error.
reason
¶A string describing the specific codec error.
object
¶The object the codec was attempting to encode or decode.
start
¶The first index of invalid data in object
.
end
¶The index after the last invalid data in object
.
UnicodeEncodeError
¶Raised when a Unicode-related error occurs during encoding. It is a subclass of UnicodeError
.
UnicodeDecodeError
¶Raised when a Unicode-related error occurs during decoding. It is a subclass of UnicodeError
.
UnicodeTranslateError
¶Raised when a Unicode-related error occurs during translating. It is a subclass of UnicodeError
.
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 IndexError
.
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
OSError
.
EnvironmentError
¶ exception
IOError
¶ exception WindowsError
¶Only available on Windows.
OS exceptions¶
The following exceptions are subclasses of OSError
, they get raised depending on the system error code.
BlockingIOError
¶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:
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 io
module.
ChildProcessError
¶Raised when an operation on a child process failed. Corresponds to errno
ECHILD
.
ConnectionError
¶A base class for connection-related issues.
Subclasses
are BrokenPipeError
, ConnectionAbortedError
, ConnectionRefusedError
and
ConnectionResetError
.
BrokenPipeError
¶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
.
ConnectionAbortedError
¶A subclass of ConnectionError
, raised when a connection attempt is aborted by the peer. Corresponds to errno
ECONNABORTED
.
ConnectionRefusedError
¶A subclass of
ConnectionError
, raised when a connection attempt is refused by the peer. Corresponds to errno
ECONNREFUSED
.
ConnectionResetError
¶A subclass of ConnectionError
, raised when a connection is reset by the peer. Corresponds to errno
ECONNRESET
.
FileExistsError
¶Raised when trying to create a file or directory which already exists. Corresponds to errno
EEXIST
.
FileNotFoundError
¶Raised when a file or directory is requested but doesn’t exist. Corresponds to errno
ENOENT
.
InterruptedError
¶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
.
IsADirectoryError
¶Raised when a file operation [such as os.remove[]
]
is requested on a directory. Corresponds to errno
EISDIR
.
NotADirectoryError
¶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
.
PermissionError
¶Raised when trying to run an operation without the adequate access rights - for example filesystem permissions. Corresponds to errno
EACCES
and
EPERM
.
ProcessLookupError
¶Raised when a given process doesn’t exist. Corresponds to errno
ESRCH
.
TimeoutError
¶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.
exceptionWarning
¶Base class for warning categories.
exceptionUserWarning
¶Base class for warnings generated by user code.
exceptionDeprecationWarning
¶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.
exceptionPendingDeprecationWarning
¶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.
exceptionSyntaxWarning
¶Base class for warnings about dubious syntax.
exceptionRuntimeWarning
¶Base class for warnings about dubious runtime behavior.
exceptionFutureWarning
¶Base class for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python.
exceptionImportWarning
¶Base class for warnings about probable mistakes in module imports.
Ignored by the default warning filters. Enabling the Python Development Mode shows this warning.
exceptionUnicodeWarning
¶Base class for warnings related to Unicode.
exceptionEncodingWarning
¶Base class for warnings related to encodings.
See Opt-in EncodingWarning for details.
New in version 3.10.
exceptionBytesWarning
¶Base class for warnings related to bytes
and bytearray
.
ResourceWarning
¶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.
Exception hierarchy¶
The class hierarchy for built-in exceptions is:
BaseException +-- SystemExit +-- KeyboardInterrupt +-- GeneratorExit +-- Exception +-- StopIteration +-- StopAsyncIteration +-- ArithmeticError | +-- FloatingPointError | +-- OverflowError | +-- ZeroDivisionError +-- AssertionError +-- AttributeError +-- BufferError +-- EOFError +-- 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 | +-- RecursionError +-- SyntaxError | +-- IndentationError | +-- TabError +-- SystemError +-- TypeError +-- ValueError | +-- UnicodeError | +-- UnicodeDecodeError | +-- UnicodeEncodeError | +-- UnicodeTranslateError +-- Warning +-- DeprecationWarning +-- PendingDeprecationWarning +-- RuntimeWarning +-- SyntaxWarning +-- UserWarning +-- FutureWarning +-- ImportWarning +-- UnicodeWarning +-- BytesWarning +-- EncodingWarning +-- ResourceWarning