6. Built-in Exceptions¶
Exceptions should be class objects. The exceptions are defined in the module
exceptions
. This module never needs to be imported explicitly: the
exceptions are provided in the built-in namespace as well as the
exceptions
module.
For class exceptions, 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
containing several items of information (e.g., an error code and a string
explaining the code). The associated value is the second argument to the
raise
statement. If the exception class is derived from the standard
root class BaseException
, the associated value is present as the
exception instance’s args
attribute.
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.
The following exceptions are only used 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()
orunicode()
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.New in version 2.5.
-
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.
Changed in version 2.5: Changed to inherit from
BaseException
.
-
exception
StandardError
¶ The base class for all built-in exceptions except
StopIteration
,GeneratorExit
,KeyboardInterrupt
andSystemExit
.StandardError
itself is derived fromException
.
-
exception
ArithmeticError
¶ The base class for those built-in exceptions that are raised for various arithmetic errors:
OverflowError
,ZeroDivisionError
,FloatingPointError
.
-
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()
.
-
exception
EnvironmentError
¶ 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’serrno
attribute (it is assumed to be an error number), and the second item is available on thestrerror
attribute (it is usually the associated error message). The tuple itself is also available on theargs
attribute.New in version 1.5.2.
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 thefilename
attribute. However, for backwards compatibility, theargs
attribute contains only a 2-tuple of the first two constructor arguments.The
filename
attribute isNone
when this exception is created with other than 3 arguments. Theerrno
andstrerror
attributes are alsoNone
when the instance was created with other than 2 or 3 arguments. In this last case,args
contains the verbatim constructor arguments as a tuple.
The following exceptions are the exceptions that are actually raised.
-
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 one of the built-in functions (
input()
orraw_input()
) hits an end-of-file condition (EOF) without reading any data. (N.B.: thefile.read()
andfile.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’s
close()
method is called. It directly inherits fromBaseException
instead ofStandardError
since it is technically not an error.New in version 2.5.
Changed in version 2.6: Changed to inherit from
BaseException
.
-
exception
IOError
¶ Raised when an I/O operation (such as a
print
statement, the built-inopen()
function 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.Changed in version 2.6: Changed
socket.error
to use this as a base class.
-
exception
ImportError
¶ Raised when an
import
statement fails to find the module definition or when afrom ... import
fails to find a name that is to be imported.
-
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 a plain 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. Interrupts typed when a built-in function
input()
orraw_input()
is waiting for input also raise this exception. The exception inherits fromBaseException
so as to not be accidentally caught by code that catchesException
and thus prevent the interpreter from exiting.Changed in version 2.5: Changed to inherit from
BaseException
.
-
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.New in version 1.5.2.
-
exception
OSError
¶ 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). Theerrno
attribute is a numeric error code fromerrno
, and thestrerror
attribute is the corresponding string, as would be printed by the C functionperror()
. See the moduleerrno
, which contains names for the error codes defined by the underlying operating system.For exceptions that involve a file system path (such as
chdir()
orunlink()
), the exception instance will contain a third attribute,filename
, which is the file name passed to the function.New in version 1.5.2.
-
exception
OverflowError
¶ Raised when the result of an arithmetic operation is too large to be represented. This cannot occur for long integers (which would rather raise
MemoryError
than give up) and for most operations with plain integers, which return a long integer instead. Because of the lack of standardization of floating point exception handling in C, most floating point operations also aren’t checked.
-
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.New in version 2.2: Previously known as the
weakref.ReferenceError
exception.
-
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 an iterator’s
next()
method to signal that there are no further values. This is derived fromException
rather thanStandardError
, since this is not considered an error in its normal application.New in version 2.2.
-
exception
SyntaxError
¶ Raised when the parser encounters a syntax error. This may occur in an
import
statement, in anexec
statement, in a call to the built-in functioneval()
orinput()
, 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. When it is not handled, the Python interpreter exits; no stack traceback is printed. If the associated value is a plain 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.Instances have an attribute
code
which is set to the proposed exit status or error message (defaulting toNone
). Also, this exception derives directly fromBaseException
and notStandardError
, since it is not technically an error.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()
).The exception inherits from
BaseException
instead ofStandardError
orException
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.Changed in version 2.5: Changed to inherit from
BaseException
.
-
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
.New in version 2.0.
-
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.
New in version 2.0.
-
-
exception
UnicodeEncodeError
¶ Raised when a Unicode-related error occurs during encoding. It is a subclass of
UnicodeError
.New in version 2.3.
-
exception
UnicodeDecodeError
¶ Raised when a Unicode-related error occurs during decoding. It is a subclass of
UnicodeError
.New in version 2.3.
-
exception
UnicodeTranslateError
¶ Raised when a Unicode-related error occurs during translating. It is a subclass of
UnicodeError
.New in version 2.3.
-
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
VMSError
¶ Only available on VMS. Raised when a VMS-specific error occurs.
-
exception
WindowsError
¶ Raised when a Windows-specific error occurs or when the error number does not correspond to an
errno
value. Thewinerror
andstrerror
values are created from the return values of theGetLastError()
andFormatMessage()
functions from the Windows Platform API. Theerrno
value maps thewinerror
value to correspondingerrno.h
values. This is a subclass ofOSError
.New in version 2.0.
Changed in version 2.5: Previous versions put the
GetLastError()
codes intoerrno
.
-
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 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.
New in version 2.5.
-
exception
UnicodeWarning
¶ Base class for warnings related to Unicode.
New in version 2.5.
-
exception
BytesWarning
¶ Base class for warnings related to bytes and bytearray.
New in version 2.6.
6.1. Exception hierarchy¶
The class hierarchy for built-in exceptions is:
BaseException
+-- SystemExit
+-- KeyboardInterrupt
+-- GeneratorExit
+-- Exception
+-- StopIteration
+-- StandardError
| +-- BufferError
| +-- ArithmeticError
| | +-- FloatingPointError
| | +-- OverflowError
| | +-- ZeroDivisionError
| +-- AssertionError
| +-- AttributeError
| +-- EnvironmentError
| | +-- IOError
| | +-- OSError
| | +-- WindowsError (Windows)
| | +-- VMSError (VMS)
| +-- EOFError
| +-- ImportError
| +-- LookupError
| | +-- IndexError
| | +-- KeyError
| +-- MemoryError
| +-- NameError
| | +-- UnboundLocalError
| +-- ReferenceError
| +-- RuntimeError
| | +-- NotImplementedError
| +-- SyntaxError
| | +-- IndentationError
| | +-- TabError
| +-- SystemError
| +-- TypeError
| +-- ValueError
| +-- UnicodeError
| +-- UnicodeDecodeError
| +-- UnicodeEncodeError
| +-- UnicodeTranslateError
+-- Warning
+-- DeprecationWarning
+-- PendingDeprecationWarning
+-- RuntimeWarning
+-- SyntaxWarning
+-- UserWarning
+-- FutureWarning
+-- ImportWarning
+-- UnicodeWarning
+-- BytesWarning