5. Built-in Exceptions

Python 3.4

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5. 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 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 User-defined Exceptions.

When raising (or re-raising) an exception in an except 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.

5.1. 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 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.

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().

5.2. 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: Raise when a generator‘s close() method is called. It directly inherits from BaseException instead of Exception since it is technically not an error.

exception ImportError

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.

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 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.

exception OSError

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). Often a subclass of OSError will actually be raised as described in OS exceptions below. The errno attribute is a numeric error code from the C variable errno.

Under Windows, the winerror attribute gives you the native Windows error code. The errno attribute is then an approximate translation, in POSIX terms, of that native error code.

Under all platforms, the strerror attribute is the corresponding error message as provided by the operating system (as formatted by the C functions perror() under POSIX, and FormatMessage() Windows).

For exceptions that involve a file system path (such as open() or os.unlink()), the exception instance will contain an additional attribute, filename, which is the file name passed to the function. For functions that involve two file system paths (such as os.rename()), the exception instance will contain a second filename2 attribute corresponding to the second file name passed to the function.

Changed in version 3.3: EnvironmentError, IOError, WindowsError, VMSError, socket.error, select.error and mmap.error have been merged into OSError.

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 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). 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 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 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.

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

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. 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 os.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.

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.

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 VMSError: Only available on VMS.

exception WindowsError: Only available on Windows.

5.2.1. 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.

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.

5.3. 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.

5.4. Exception hierarchy

The class hierarchy for built-in exceptions is:

BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- GeneratorExit
 +-- Exception
      +-- StopIteration
      +-- 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
      +-- SyntaxError
      |    +-- IndentationError
      |         +-- TabError
      +-- SystemError
      +-- TypeError
      +-- ValueError
      |    +-- UnicodeError
      |         +-- UnicodeDecodeError
      |         +-- UnicodeEncodeError
      |         +-- UnicodeTranslateError
      +-- Warning
           +-- DeprecationWarning
           +-- PendingDeprecationWarning
           +-- RuntimeWarning
           +-- SyntaxWarning
           +-- UserWarning
           +-- FutureWarning
           +-- ImportWarning
           +-- UnicodeWarning
           +-- BytesWarning
           +-- ResourceWarning

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