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.
When raising (or re-raising) an exception in an
__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
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
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
__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.
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
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 is usually used in exception handling code like this:
try: ... except SomeException: tb = sys.exc_info() raise OtherException(...).with_traceback(tb)
All built-in, non-system-exiting exceptions are derived from this class. All user-defined exceptions should also be derived from this class.
Raised when a buffer related operation cannot be performed.
The following exceptions are the exceptions that are usually raised.
Raised when an
assert statement fails.
Not currently used.
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.
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
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.
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.
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.
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
NotImplemented are not interchangeable, even though they have similar names and purposes. See
NotImplemented for details on when to use it.
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.
A numeric error code from the C variable
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
filename is the file name passed to the function. For functions that involve two file system paths (such as
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. 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.
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
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.
The exception object has a single attribute
value, which is given as an argument when constructing the exception, and defaults to
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.
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
eval(), or when reading the initial script or standard input (also interactively).
Instances of this class have attributes
text for easier access to the details.
str() of the exception instance returns only the message.
Base class for syntax errors related to incorrect indentation. This is a subclass of
Raised when indentation contains an inconsistent use of tabs and spaces. This is a subclass of
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.
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
The exit status or error message that is passed to the constructor. (Defaults to
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
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
Raised when a Unicode-related encoding or decoding error occurs. It is a subclass of
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
The index after the last invalid data in
Raised when a Unicode-related error occurs during encoding. It is a subclass of
Raised when a Unicode-related error occurs during decoding. It is a subclass of
Raised when a Unicode-related error occurs during translating. It is a subclass of
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
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
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
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
Raised when an operation on a child process failed. Corresponds to
A base class for connection-related issues.
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
A subclass of
ConnectionError, raised when a connection attempt is aborted by the peer. Corresponds to
A subclass of
ConnectionError, raised when a connection attempt is refused by the peer. Corresponds to
A subclass of
ConnectionError, raised when a connection is reset by the peer. Corresponds to
Raised when trying to create a file or directory which already exists. Corresponds to
Raised when a file or directory is requested but doesn’t exist. Corresponds to
Raised when a system call is interrupted by an incoming signal. Corresponds to
Raised when a file operation (such as
os.remove()) is requested on a directory. Corresponds to
Raised when a directory operation (such as
os.listdir()) is requested on something which is not a directory. Corresponds to
Raised when trying to run an operation without the adequate access rights - for example filesystem permissions. Corresponds to
Raised when a given process doesn’t exist. Corresponds to
Raised when a system function timed out at the system level. Corresponds to
New in version 3.3: All the above
OSError subclasses were added.
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.
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.
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.
Base class for warnings related to Unicode.
Base class for warnings related to resource usage. Ignored by the default warning filters.
New in version 3.2.
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 +-- ResourceWarning
© 2001–2020 Python Software Foundation
Licensed under the PSF License.