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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
). Ifstr()
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.
- 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 bycodecs.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.: theio.IOBase.read()
andio.IOBase.readline()
methods return an empty string when they hit EOF.)
- exception
GeneratorExit
-
Raised when a generator or coroutine is closed; see
generator.close()
andcoroutine.close()
. It directly inherits fromBaseException
instead ofException
since it is technically not an error.
- exception
ImportError
-
Raised when the
import
statement has troubles trying to load a module. Also raised when the “from list” infrom ... import
has a name that cannot be found.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
ModuleNotFoundError
-
A subclass of
ImportError
which is raised byimport
when a module could not be located. It is also raised whenNone
is found insys.modules
.New in version 3.6.
- 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
KeyboardInterrupt
-
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 catchesException
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, 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
andNotImplemented
are not interchangeable, even though they have similar names and purposes. SeeNotImplemented
for details on when to use it.
- 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, theargs
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 finalerrno
value. This behaviour only occurs when constructingOSError
directly or via an alias, and is not inherited when subclassing.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 thewinerror
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()
oros.unlink()
),filename
is the file name passed to the function. For functions that involve two file system paths (such asos.rename()
),filename2
corresponds to the second file name passed to the function.
Changed in version 3.3:
EnvironmentError
,IOError
,WindowsError
,socket.error
,select.error
andmmap.error
have been merged intoOSError
, 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 (seesys.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 theweakref
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 toNone
.When a generator or coroutine function returns, a new
StopIteration
instance is raised, and the value returned by the function is used as thevalue
parameter to the constructor of the exception.If a generator code directly or indirectly raises
StopIteration
, it is converted into aRuntimeError
(retaining theStopIteration
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 aRuntimeError
.
- 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 functionsexec()
oreval()
, 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 fromBaseException
instead ofException
so that it is not accidentally caught by code that catchesException
. 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 tosys.exit()
. If the value is an integer, it specifies the system exit status (passed to C’sexit()
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 oftry
statements) can be executed, and so that a debugger can execute a script without running the risk of losing control. Theos._exit()
function can be used if it is absolutely positively necessary to exit immediately (for example, in the child process after a call toos.fork()
).
- 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.
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 anint
is expected) should result in aTypeError
, but passing arguments with the wrong value (e.g. a number outside expected boundaries) should result in aValueError
.
- 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.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 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
.
- 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
.
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
andConnectionResetError
.
- 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 toerrno
EPIPE
andESHUTDOWN
.
- exception
ConnectionAbortedError
-
A subclass of
ConnectionError
, raised when a connection attempt is aborted by the peer. Corresponds toerrno
ECONNABORTED
.
- exception
ConnectionRefusedError
-
A subclass of
ConnectionError
, raised when a connection attempt is refused by the peer. Corresponds toerrno
ECONNREFUSED
.
- exception
ConnectionResetError
-
A subclass of
ConnectionError
, raised when a connection is reset by the peer. Corresponds toerrno
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 toerrno
EISDIR
.
- exception
NotADirectoryError
-
Raised when a directory operation (such as
os.listdir()
) is requested on something which is not a directory. Corresponds toerrno
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
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.
- exception
DeprecationWarning
-
Base class for warnings about deprecated features when those warnings are intended for other Python developers.
- exception
PendingDeprecationWarning
-
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.
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
+-- ResourceWarning