5.1. Base classes¶

The following exceptions are used mostly as base classes for other exceptions.

exceptionBaseException¶

The base class for all built-in exceptions. It is not meant to be directlyinherited by user-defined classes (for that, useException). Ifstr() is called on an instance of this class, the representation ofthe argument(s) to the instance are returned, or the empty string whenthere were no arguments.

args¶

The tuple of arguments given to the exception constructor. Some built-inexceptions (likeOSError) expect a certain number of arguments andassign a special meaning to the elements of this tuple, while others areusually called only with a single string giving an error message.

with_traceback(tb)¶

This method setstb as the new traceback for the exception and returnsthe exception object. It is usually used in exception handling code likethis:

try:...exceptSomeException:tb=sys.exc_info()[2]raiseOtherException(...).with_traceback(tb)

exceptionException¶

All built-in, non-system-exiting exceptions are derived from this class. Alluser-defined exceptions should also be derived from this class.

exceptionArithmeticError¶

The base class for those built-in exceptions that are raised for variousarithmetic errors:OverflowError,ZeroDivisionError,FloatingPointError.

exceptionBufferError¶

Raised when abuffer related operation cannot beperformed.

exceptionLookupError¶

The base class for the exceptions that are raised when a key or index used ona mapping or sequence is invalid:IndexError,KeyError. Thiscan be raised directly bycodecs.lookup().

5.2. Concrete exceptions¶

The following exceptions are the exceptions that are usually raised.

exceptionAssertionError¶

Raised when anassert statement fails.

exceptionAttributeError¶

Raised when an attribute reference (seeAttribute references) orassignment fails. (When an object does not support attribute references orattribute assignments at all,TypeError is raised.)

exceptionEOFError¶

Raised when theinput() 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.)

exceptionFloatingPointError¶

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 isdefined in thepyconfig.h file.

exceptionGeneratorExit¶

Raised when agenerator orcoroutine is closed;seegenerator.close() andcoroutine.close(). Itdirectly inherits fromBaseException instead ofException sinceit is technically not an error.

exceptionImportError¶

Raised when animport statement fails to find the module definitionor when afrom...import fails to find a name that is to be imported.

Thename andpath attributes can be set using keyword-onlyarguments to the constructor. When set they represent the name of the modulethat was attempted to be imported and the path to any file which triggeredthe exception, respectively.

Changed in version 3.3:Added thename andpath attributes.

exceptionIndexError¶

Raised when a sequence subscript is out of range. (Slice indices aresilently truncated to fall in the allowed range; if an index is not aninteger,TypeError is raised.)

exceptionKeyError¶

Raised when a mapping (dictionary) key is not found in the set of existing keys.

exceptionKeyboardInterrupt¶

Raised when the user hits the interrupt key (normallyControl-C orDelete). During execution, a check for interrupts is maderegularly. The exception inherits fromBaseException so as to not beaccidentally caught by code that catchesException and thus preventthe interpreter from exiting.

exceptionMemoryError¶

Raised when an operation runs out of memory but the situation may still berescued (by deleting some objects). The associated value is a string indicatingwhat kind of (internal) operation ran out of memory. Note that because of theunderlying memory management architecture (C’smalloc() function), theinterpreter may not always be able to completely recover from this situation; itnevertheless raises an exception so that a stack traceback can be printed, incase a run-away program was the cause.

exceptionNameError¶

Raised when a local or global name is not found. This applies only tounqualified names. The associated value is an error message that includes thename that could not be found.

exceptionNotImplementedError¶

This exception is derived fromRuntimeError. In user defined baseclasses, abstract methods should raise this exception when they require derivedclasses to override the method.

exceptionOSError([arg])¶

exceptionOSError(errno,strerror[,filename[,winerror[,filename2]]])

This exception is raised when a system function returns a system-relatederror, 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 toNone if notspecified. For backwards compatibility, if three arguments are passed,theargs attribute contains only a 2-tupleof the first two constructor arguments.

The constructor often actually returns a subclass ofOSError, asdescribed inOS exceptions below. The particular subclass depends onthe finalerrno value. This behaviour only occurs whenconstructingOSError directly or via an alias, and is notinherited when subclassing.

errno¶

A numeric error code from the C variableerrno.

winerror¶

Under Windows, this gives you the nativeWindows error code. Theerrno attribute is then an approximatetranslation, in POSIX terms, of that native error code.

Under Windows, if thewinerror constructor argument is an integer,theerrno attribute is determined from the Windows error code,and theerrno argument is ignored. On other platforms, thewinerror argument is ignored, and thewinerror attributedoes not exist.

strerror¶

The corresponding error message, as provided bythe operating system. It is formatted by the Cfunctionsperror() under POSIX, andFormatMessage()under Windows.

filename¶

filename2¶

For exceptions that involve a file system path (such asopen() 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 secondfile name passed to the function.

Changed in version 3.3:EnvironmentError,IOError,WindowsError,socket.error,select.error andmmap.error have been merged intoOSError, and theconstructor may return a subclass.

Changed in version 3.4:Thefilename attribute is now the original file name passed tothe function, instead of the name encoded to or decoded from thefilesystem encoding. Also, thefilename2 constructor argument andattribute was added.

exceptionOverflowError¶

Raised when the result of an arithmetic operation is too large to berepresented. This cannot occur for integers (which would rather raiseMemoryError than give up). However, for historical reasons,OverflowError is sometimes raised for integers that are outside a requiredrange. Because of the lack of standardization of floating point exceptionhandling in C, most floating point operations are not checked.

exceptionRecursionError¶

This exception is derived fromRuntimeError. It is raised when theinterpreter detects that the maximum recursion depth (seesys.getrecursionlimit()) is exceeded.

New in version 3.5:Previously, a plainRuntimeError was raised.

exceptionReferenceError¶

This exception is raised when a weak reference proxy, created by theweakref.proxy() function, is used to access an attribute of the referentafter it has been garbage collected. For more information on weak references,see theweakref module.

exceptionRuntimeError¶

Raised when an error is detected that doesn’t fall in any of the othercategories. The associated value is a string indicating what precisely wentwrong.

exceptionStopIteration¶

Raised by built-in functionnext() and aniterator’s__next__() method to signal that there are no furtheritems produced by the iterator.

The exception object has a single attributevalue, which isgiven as an argument when constructing the exception, and defaultstoNone.

When agenerator orcoroutine functionreturns, a newStopIteration instance israised, and the value returned by the function is used as thevalue parameter to the constructor of the exception.

If a generator function defined in the presence of afrom__future__importgenerator_stop directive raisesStopIteration, it will beconverted into aRuntimeError (retaining theStopIterationas the new exception’s cause).

Changed in version 3.3:Addedvalue attribute and the ability for generator functions touse it to return a value.

Changed in version 3.5:Introduced the RuntimeError transformation.

exceptionStopAsyncIteration¶

Must be raised by__anext__() method of anasynchronous iterator object to stop the iteration.

New in version 3.5.

exceptionSyntaxError¶

Raised when the parser encounters a syntax error. This may occur in animport 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 attributesfilename,lineno,offset andtext for easier access to the details.str()of the exception instance returns only the message.

exceptionIndentationError¶

Base class for syntax errors related to incorrect indentation. This is asubclass ofSyntaxError.

exceptionTabError¶

Raised when indentation contains an inconsistent use of tabs and spaces.This is a subclass ofIndentationError.

exceptionSystemError¶

Raised when the interpreter finds an internal error, but the situation does notlook so serious to cause it to abandon all hope. The associated value is astring 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 isalso printed at the start of an interactive Python session), the exact errormessage (the exception’s associated value) and if possible the source of theprogram that triggered the error.

exceptionSystemExit¶

This exception is raised by thesys.exit() function. It inherits fromBaseException instead ofException so that it is not accidentallycaught by code that catchesException. This allows the exception toproperly propagate up and cause the interpreter to exit. When it is nothandled, the Python interpreter exits; no stack traceback is printed. Theconstructor accepts the same optional argument passed tosys.exit().If the value is an integer, it specifies the system exit status (passed toC’sexit() function); if it isNone, the exit status is zero; ifit has another type (such as a string), the object’s value is printed andthe exit status is one.

A call tosys.exit() is translated into an exception so that clean-uphandlers (finally clauses oftry statements) can beexecuted, and so that a debugger can execute a script without running the riskof losing control. Theos._exit() function can be used if it isabsolutely positively necessary to exit immediately (for example, in the childprocess after a call toos.fork()).

code¶

The exit status or error message that is passed to the constructor.(Defaults toNone.)

exceptionTypeError¶

Raised when an operation or function is applied to an object of inappropriatetype. The associated value is a string giving details about the type mismatch.

exceptionUnboundLocalError¶

Raised when a reference is made to a local variable in a function or method, butno value has been bound to that variable. This is a subclass ofNameError.

exceptionUnicodeError¶

Raised when a Unicode-related encoding or decoding error occurs. It is asubclass ofValueError.

UnicodeError has attributes that describe the encoding or decodingerror. For example,err.object[err.start:err.end] gives the particularinvalid 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 inobject.

end¶

The index after the last invalid data inobject.

exceptionUnicodeEncodeError¶

Raised when a Unicode-related error occurs during encoding. It is a subclass ofUnicodeError.

exceptionUnicodeDecodeError¶

Raised when a Unicode-related error occurs during decoding. It is a subclass ofUnicodeError.

exceptionUnicodeTranslateError¶

Raised when a Unicode-related error occurs during translating. It is a subclassofUnicodeError.

exceptionValueError¶

Raised when a built-in operation or function receives an argument that has theright type but an inappropriate value, and the situation is not described by amore precise exception such asIndexError.

exceptionZeroDivisionError¶

Raised when the second argument of a division or modulo operation is zero. Theassociated value is a string indicating the type of the operands and theoperation.

The following exceptions are kept for compatibility with previous versions;starting from Python 3.3, they are aliases ofOSError.

exceptionEnvironmentError¶

exceptionIOError¶

exceptionWindowsError¶

Only available on Windows.

5.3. Warnings¶

The following exceptions are used as warning categories; see thewarningsmodule for more information.

exceptionWarning¶

Base class for warning categories.

exceptionUserWarning¶

Base class for warnings generated by user code.

exceptionDeprecationWarning¶

Base class for warnings about deprecated features.

exceptionPendingDeprecationWarning¶

Base class for warnings about features which will be deprecated in the future.

exceptionSyntaxWarning¶

Base class for warnings about dubious syntax.

exceptionRuntimeWarning¶

Base class for warnings about dubious runtime behavior.

exceptionFutureWarning¶

Base class for warnings about constructs that will change semantically in thefuture.

exceptionImportWarning¶

Base class for warnings about probable mistakes in module imports.

exceptionUnicodeWarning¶

Base class for warnings related to Unicode.

exceptionBytesWarning¶

Base class for warnings related tobytes andbytearray.

exceptionResourceWarning¶

Base class for warnings related to resource usage.

New in version 3.2.

5.4. Exception hierarchy¶

The class hierarchy for built-in exceptions is:

BaseException+--SystemExit|+--TaskletExit+--KeyboardInterrupt+--GeneratorExit+--Exception+--StopIteration+--StopAsyncIteration+--ArithmeticError|+--FloatingPointError|+--OverflowError|+--ZeroDivisionError+--AssertionError+--AttributeError+--BufferError+--EOFError+--ImportError+--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