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 except or finally clause
__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 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.
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 is usually used in exception handling code like this:
try: ... except SomeException: tb = sys.exc_info() raise OtherException(...).with_traceback(tb)
The following exceptions are the exceptions that are usually raised.
io.IOBase.read()and io.IOBase.readline() methods return an empty string when they hit EOF.)
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.
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.
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 None.
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 “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 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. Also, the filename2 constructor argument and attribute was added.
New in version 3.5: Previously, a plain RuntimeError was raised.
The exception object has a single attribute
value, which is given as an argument when constructing the exception, and defaults to None.
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.
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 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.
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.
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 os.fork()).
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 ValueError.
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.
The following exceptions are kept for compatibility with previous versions; starting from Python 3.3, they are aliases of OSError.
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 io module.
A base class for connection-related issues.
Raised when a system call is interrupted by an incoming signal. 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 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 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