6. 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
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 sub-classed to define new exceptions;
programmers are encouraged to at least derive new exceptions from the
Exception class and not BaseException. More information on
defining exceptions is available in the Python Tutorial under
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
bytes() or 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 IOError) 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
tb = sys.exc_info()
- All built-in, non-system-exiting exceptions are derived from this class. All
user-defined exceptions should also be derived from this class.
- The base class for those built-in exceptions that are raised for various
arithmetic errors: OverflowError, ZeroDivisionError,
- Raised when a buffer related operation cannot be
- 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().
The base class for exceptions that can occur outside the Python system:
IOError, OSError. When exceptions of this type are created with a
2-tuple, the first item is available on the instance’s errno attribute
(it is assumed to be an error number), and the second item is available on the
strerror attribute (it is usually the associated error message). The
tuple itself is also available on the args attribute.
When an EnvironmentError exception is instantiated with a 3-tuple, the
first two items are available as above, while the third item is available on the
filename attribute. However, for backwards compatibility, the
args attribute contains only a 2-tuple of the first two constructor
The filename attribute is None when this exception is created with
other than 3 arguments. The errno and strerror attributes are
also None when the instance was created with other than 2 or 3 arguments.
In this last case, args contains the verbatim constructor arguments as a
The following exceptions are the exceptions that are usually raised.
Raised when an assert statement fails.
- 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.)
- Raised when one of the built-in functions (input() or raw_input())
hits an end-of-file condition (EOF) without reading any data. (N.B.: the
file.read() and file.readline() methods return an empty string
when they hit EOF.)
- 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.
- Raise when a generator‘s close() method is called. It
directly inherits from BaseException instead of Exception since
it is technically not an error.
Raised when an I/O operation (such as the built-in print() or
open() functions or a method of a file object) fails for an
I/O-related reason, e.g., “file not found” or “disk full”.
This class is derived from EnvironmentError. See the discussion above
for more information on exception instance attributes.
- 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.
- 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.
This exception is derived from EnvironmentError. It is raised when a
function returns a system-related error (not for illegal argument types or
other incidental errors). The errno attribute is a numeric error
code from errno, and the strerror attribute is the
corresponding string, as would be printed by the C function perror().
See the module errno, which contains names for the error codes defined
by the underlying operating system.
For exceptions that involve a file system path (such as chdir() or
unlink()), the exception instance will contain a third attribute,
filename, which is the file name passed to the function.
- 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). Because of the lack of standardization of
floating point exception handling in C, most floating point operations also
- 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.
- 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. (This exception is mostly a relic from a previous version of the
interpreter; it is not used very much any more.)
- Raised by builtin next() and an iterator‘s __next__()
method to signal that there are no further values.
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
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.
- Base class for syntax errors related to incorrect indentation. This is a
subclass of SyntaxError.
- Raised when indentation contains an inconsistent use of tabs and spaces.
This is a subclass of IndentationError.
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. When it is not
handled, the Python interpreter exits; no stack traceback is printed. If the
associated 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.
Instances have an attribute code which is set to the proposed exit
status or error message (defaulting to None). Also, this exception derives
directly from BaseException and not Exception, since it is not
technically an error.
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 fork()).
The exception 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.
- 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.
- 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 ValueError.
- 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
- 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.
- Only available on VMS. Raised when a VMS-specific error occurs.
- Raised when a Windows-specific error occurs or when the error number does not
correspond to an errno value. The winerror and
strerror values are created from the return values of the
GetLastError() and FormatMessage() functions from the Windows
Platform API. The errno value maps the winerror value to
corresponding errno.h values. This is a subclass of OSError.
- 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
The following exceptions are used as warning categories; see the warnings
module for more information.
- Base class for warning categories.
- Base class for warnings generated by user code.
- Base class for warnings about deprecated features.
- Base class for warnings about features which will be deprecated in the future.
- Base class for warnings about dubious syntax
- Base class for warnings about dubious runtime behavior.
- Base class for warnings about constructs that will change semantically in the
- Base class for warnings about probable mistakes in module imports.
- Base class for warnings related to Unicode.
- Base class for warnings related to bytes and buffer.
6.1. Exception hierarchy
The class hierarchy for built-in exceptions is:
| +-- FloatingPointError
| +-- OverflowError
| +-- ZeroDivisionError
| +-- IOError
| +-- OSError
| +-- WindowsError (Windows)
| +-- VMSError (VMS)
| +-- IndexError
| +-- KeyError
| +-- UnboundLocalError
| +-- NotImplementedError
| +-- IndentationError
| +-- TabError
| +-- UnicodeError
| +-- UnicodeDecodeError
| +-- UnicodeEncodeError
| +-- UnicodeTranslateError