Built-in Exceptions
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.
Base classes
The following exceptions are used mostly as base classes for other exceptions.
-
exception
BaseException
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 is usually used in exception handling code like this:
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.
-
exception
ArithmeticError
The base class for those built-in exceptions that are raised for various arithmetic errors: OverflowError, ZeroDivisionError, FloatingPointError.
-
exception
BufferError
Raised when a buffer related operation cannot be performed.
-
exception
LookupError
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.
-
exception
AssertionError
Raised when an assert statement fails.
-
exception
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.)
-
exception
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.)
-
exception
FloatingPointError
Raised when a floating point operation fails. This exception is always defined, but can only be raised when Python is configured with the
–with-fpectl
option, or theWANT_SIGFPE_HANDLER
symbol is defined in thepyconfig.h
file.
-
exception
GeneratorExit
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.
-
exception
ImportError
Raised when an import statement fails to find the module definition or when a
from ... import
fails to find a name that is to be imported.The
name
andpath
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
andpath
attributes.
-
exception
IndexError
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.)
-
exception
KeyError
Raised when a mapping (dictionary) key is not found in the set of existing keys.
-
exception
KeyboardInterrupt
Raised when the user hits the interrupt key (normally
Control-C
orDelete
). 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.
-
exception
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.
-
exception
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.
-
exception
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.
-
exception
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, andFormatMessage()
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 3.3: 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. Also, the filename2 constructor argument and attribute was added.
-
-
exception
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.
-
exception
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.
-
exception
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.
-
exception
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.
-
exception
StopIteration
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 function defined in the presence of a
from __future__ import generator_stop
directive raises StopIteration, it will be 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.
-
exception
StopAsyncIteration
Must be raised by __anext__() method of an asynchronous iterator object to stop the iteration.
New in version 3.5.
-
exception
SyntaxError
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).
Instances of this class have attributes
filename
,lineno
,offset
andtext
for easier access to the details. str() of the exception instance returns only the message.
-
exception
IndentationError
Base class for syntax errors related to incorrect indentation. This is a subclass of SyntaxError.
-
exception
TabError
Raised when indentation contains an inconsistent use of tabs and spaces. This is a subclass of IndentationError.
-
exception
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.
-
exception
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 isNone
, 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
.)
-
-
exception
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.
-
exception
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.
-
exception
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.
-
-
exception
UnicodeEncodeError
Raised when a Unicode-related error occurs during encoding. It is a subclass of UnicodeError.
-
exception
UnicodeDecodeError
Raised when a Unicode-related error occurs during decoding. It is a subclass of UnicodeError.
-
exception
UnicodeTranslateError
Raised when a Unicode-related error occurs during translating. It is a subclass of UnicodeError.
-
exception
ValueError
Raised when a built-in 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.
-
exception
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.
-
exception
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.
-
exception
BlockingIOError
Raised when an operation would block on an object (e.g. socket) set for non-blocking operation. Corresponds to
errno
EAGAIN
,EALREADY
,EWOULDBLOCK
andEINPROGRESS
.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.
-
-
exception
ChildProcessError
Raised when an operation on a child process failed. Corresponds to
errno
ECHILD
.
-
exception
ConnectionError
A base class for connection-related issues.
Subclasses are BrokenPipeError, ConnectionAbortedError, ConnectionRefusedError and ConnectionResetError.
-
exception
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
andESHUTDOWN
.
-
exception
ConnectionAbortedError
A subclass of ConnectionError, raised when a connection attempt is aborted by the peer. Corresponds to
errno
ECONNABORTED
.
-
exception
ConnectionRefusedError
A subclass of ConnectionError, raised when a connection attempt is refused by the peer. Corresponds to
errno
ECONNREFUSED
.
-
exception
ConnectionResetError
A subclass of ConnectionError, raised when a connection is reset by the peer. Corresponds to
errno
ECONNRESET
.
-
exception
FileExistsError
Raised when trying to create a file or directory which already exists. Corresponds to
errno
EEXIST
.
-
exception
FileNotFoundError
Raised when a file or directory is requested but doesn’t exist. Corresponds to
errno
ENOENT
.
-
exception
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.
-
exception
IsADirectoryError
Raised when a file operation (such as os.remove()) is requested on a directory. Corresponds to
errno
EISDIR
.
-
exception
NotADirectoryError
Raised when a directory operation (such as os.listdir()) is requested on something which is not a directory. Corresponds to
errno
ENOTDIR
.
-
exception
PermissionError
Raised when trying to run an operation without the adequate access rights - for example filesystem permissions. Corresponds to
errno
EACCES
andEPERM
.
-
exception
ProcessLookupError
Raised when a given process doesn’t exist. Corresponds to
errno
ESRCH
.
-
exception
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 warnings module for more information.
-
exception
Warning
Base class for warning categories.
-
exception
UserWarning
Base class for warnings generated by user code.
-
exception
DeprecationWarning
Base class for warnings about deprecated features.
-
exception
PendingDeprecationWarning
Base class for warnings about features which will be deprecated in the future.
-
exception
SyntaxWarning
Base class for warnings about dubious syntax.
-
exception
RuntimeWarning
Base class for warnings about dubious runtime behavior.
-
exception
FutureWarning
Base class for warnings about constructs that will change semantically in the future.
-
exception
ImportWarning
Base class for warnings about probable mistakes in module imports.
-
exception
UnicodeWarning
Base class for warnings related to Unicode.
-
exception
ResourceWarning
Base class for warnings related to resource usage.
New in version 3.2.
Exception hierarchy
The class hierarchy for built-in exceptions is: