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unified style; etc.
173 lines
7.2 KiB
TeX
173 lines
7.2 KiB
TeX
\section{Built-in Exceptions}
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Exceptions are string objects. Two distinct string objects with the
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same value are different exceptions. This is done to force programmers
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to use exception names rather than their string value when specifying
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exception handlers. The string value of all built-in exceptions is
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their name, but this is not a requirement for user-defined exceptions
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or exceptions defined by library modules.
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The following exceptions can be generated by the interpreter or
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built-in functions. Except where mentioned, they have an `associated
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value' indicating the detailed cause of the error. This may be a
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string or a tuple containing several items of information (e.g., an
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error code and a string explaining the code).
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User code can raise built-in exceptions. This can be used to test an
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exception handler or to report an error condition `just like' the
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situation in which the interpreter raises the same exception; but
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beware that there is nothing to prevent user code from raising an
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inappropriate error.
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\renewcommand{\indexsubitem}{(built-in exception)}
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\begin{excdesc}{AttributeError}
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% xref to attribute reference?
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Raised when an attribute reference or assignment fails. (When an
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object does not support attribute references or attribute assignments
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at all, \code{TypeError} is raised.)
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\end{excdesc}
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\begin{excdesc}{EOFError}
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% XXXJH xrefs here
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Raised when one of the built-in functions (\code{input()} or
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\code{raw_input()}) hits an end-of-file condition (\EOF{}) without
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reading any data.
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% XXXJH xrefs here
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(N.B.: the \code{read()} and \code{readline()} methods of file
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objects return an empty string when they hit \EOF{}.) No associated value.
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\end{excdesc}
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\begin{excdesc}{IOError}
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% XXXJH xrefs here
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Raised when an I/O operation (such as a \code{print} statement, the
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built-in \code{open()} function or a method of a file object) fails
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for an I/O-related reason, e.g., `file not found', `disk full'.
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\end{excdesc}
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\begin{excdesc}{ImportError}
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% XXXJH xref to import statement?
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Raised when an \code{import} statement fails to find the module
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definition or when a \code{from {\rm \ldots} import} fails to find a
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name that is to be imported.
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\end{excdesc}
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\begin{excdesc}{IndexError}
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% XXXJH xref to sequences
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Raised when a sequence subscript is out of range. (Slice indices are
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silently truncated to fall in the allowed range; if an index is not a
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plain integer, \code{TypeError} is raised.)
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\end{excdesc}
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\begin{excdesc}{KeyError}
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% XXXJH xref to mapping objects?
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Raised when a mapping (dictionary) key is not found in the set of
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existing keys.
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\end{excdesc}
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\begin{excdesc}{KeyboardInterrupt}
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Raised when the user hits the interrupt key (normally
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\kbd{Control-C} or
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\key{DEL}). During execution, a check for interrupts is made regularly.
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% XXXJH xrefs here
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Interrupts typed when a built-in function \code{input()} or
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\code{raw_input()}) is waiting for input also raise this exception. No
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associated value.
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\end{excdesc}
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\begin{excdesc}{MemoryError}
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Raised when an operation runs out of memory but the situation may
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still be rescued (by deleting some objects). The associated value is
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a string indicating what kind of (internal) operation ran out of memory.
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Note that because of the underlying memory management architecture
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(\C{}'s \code{malloc()} function), the interpreter may not always be able
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to completely recover from this situation; it nevertheless raises an
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exception so that a stack traceback can be printed, in case a run-away
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program was the cause.
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\end{excdesc}
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\begin{excdesc}{NameError}
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Raised when a local or global name is not found. This applies only
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to unqualified names. The associated value is the name that could
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not be found.
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\end{excdesc}
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\begin{excdesc}{OverflowError}
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% XXXJH reference to long's and/or int's?
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Raised when the result of an arithmetic operation is too large to be
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represented. This cannot occur for long integers (which would rather
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raise \code{MemoryError} than give up). Because of the lack of
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standardization of floating point exception handling in \C{}, most
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floating point operations also aren't checked. For plain integers,
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all operations that can overflow are checked except left shift, where
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typical applications prefer to drop bits than raise an exception.
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\end{excdesc}
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\begin{excdesc}{RuntimeError}
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Raised when an error is detected that doesn't fall in any of the
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other categories. The associated value is a string indicating what
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precisely went wrong. (This exception is a relic from a previous
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version of the interpreter; it is not used any more except by some
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extension modules that haven't been converted to define their own
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exceptions yet.)
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\end{excdesc}
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\begin{excdesc}{SyntaxError}
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% XXXJH xref to these functions?
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Raised when the parser encounters a syntax error. This may occur in
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an \code{import} statement, in an \code{exec} statement, in a call
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to the built-in function \code{eval()} or \code{input()}, or
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when reading the initial script or standard input (also
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interactively).
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\end{excdesc}
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\begin{excdesc}{SystemError}
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Raised when the interpreter finds an internal error, but the
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situation does not look so serious to cause it to abandon all hope.
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The associated value is a string indicating what went wrong (in
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low-level terms).
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You should report this to the author or maintainer of your Python
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interpreter. Be sure to report the version string of the Python
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interpreter (\code{sys.version}; it is also printed at the start of an
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interactive Python session), the exact error message (the exception's
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associated value) and if possible the source of the program that
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triggered the error.
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\end{excdesc}
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\begin{excdesc}{SystemExit}
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% XXXJH xref to module sys?
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This exception is raised by the \code{sys.exit()} function. When it
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is not handled, the Python interpreter exits; no stack traceback is
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printed. If the associated value is a plain integer, it specifies the
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system exit status (passed to \C{}'s \code{exit()} function); if it is
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\code{None}, the exit status is zero; if it has another type (such as
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a string), the object's value is printed and the exit status is one.
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A call to \code{sys.exit} is translated into an exception so that
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clean-up handlers (\code{finally} clauses of \code{try} statements)
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can be executed, and so that a debugger can execute a script without
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running the risk of losing control. The \code{posix._exit()} function
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can be used if it is absolutely positively necessary to exit
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immediately (e.g., after a \code{fork()} in the child process).
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\end{excdesc}
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\begin{excdesc}{TypeError}
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Raised when a built-in operation or function is applied to an object
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of inappropriate type. The associated value is a string giving
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details about the type mismatch.
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\end{excdesc}
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\begin{excdesc}{ValueError}
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Raised when a built-in operation or function receives an argument
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that has the right type but an inappropriate value, and the
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situation is not described by a more precise exception such as
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\code{IndexError}.
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\end{excdesc}
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\begin{excdesc}{ZeroDivisionError}
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Raised when the second argument of a division or modulo operation is
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zero. The associated value is a string indicating the type of the
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operands and the operation.
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\end{excdesc}
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