Python 3.6.5 Documentation >  Built-in Exceptions

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 the "WANT_SIGFPE_HANDLER"
symbol is defined in the "pyconfig.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 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.

exception 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.

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"
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.

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" and "NotImplemented" are not
interchangeable, even though they have similar names and
purposes. See "NotImplemented" 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, 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 3.3: "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" and "text" 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 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".)

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 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".

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" 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.

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" and "ESHUTDOWN".

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" and "EPERM".

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 BytesWarning

Base class for warnings related to "bytes" and "bytearray".

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:

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