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authorGeorg Brandl <georg@python.org>2007-08-15 14:26:55 (GMT)
committerGeorg Brandl <georg@python.org>2007-08-15 14:26:55 (GMT)
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-\chapter{Exception Handling \label{exceptionHandling}}
-
-The functions described in this chapter will let you handle and raise Python
-exceptions. It is important to understand some of the basics of
-Python exception handling. It works somewhat like the
-\UNIX{} \cdata{errno} variable: there is a global indicator (per
-thread) of the last error that occurred. Most functions don't clear
-this on success, but will set it to indicate the cause of the error on
-failure. Most functions also return an error indicator, usually
-\NULL{} if they are supposed to return a pointer, or \code{-1} if they
-return an integer (exception: the \cfunction{PyArg_*()} functions
-return \code{1} for success and \code{0} for failure).
-
-When a function must fail because some function it called failed, it
-generally doesn't set the error indicator; the function it called
-already set it. It is responsible for either handling the error and
-clearing the exception or returning after cleaning up any resources it
-holds (such as object references or memory allocations); it should
-\emph{not} continue normally if it is not prepared to handle the
-error. If returning due to an error, it is important to indicate to
-the caller that an error has been set. If the error is not handled or
-carefully propagated, additional calls into the Python/C API may not
-behave as intended and may fail in mysterious ways.
-
-The error indicator consists of three Python objects corresponding to
-\withsubitem{(in module sys)}{
- \ttindex{exc_type}\ttindex{exc_value}\ttindex{exc_traceback}}
-the Python variables \code{sys.exc_type}, \code{sys.exc_value} and
-\code{sys.exc_traceback}. API functions exist to interact with the
-error indicator in various ways. There is a separate error indicator
-for each thread.
-
-% XXX Order of these should be more thoughtful.
-% Either alphabetical or some kind of structure.
-
-\begin{cfuncdesc}{void}{PyErr_Print}{}
- Print a standard traceback to \code{sys.stderr} and clear the error
- indicator. Call this function only when the error indicator is
- set. (Otherwise it will cause a fatal error!)
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{PyObject*}{PyErr_Occurred}{}
- Test whether the error indicator is set. If set, return the
- exception \emph{type} (the first argument to the last call to one of
- the \cfunction{PyErr_Set*()} functions or to
- \cfunction{PyErr_Restore()}). If not set, return \NULL. You do
- not own a reference to the return value, so you do not need to
- \cfunction{Py_DECREF()} it. \note{Do not compare the return value
- to a specific exception; use \cfunction{PyErr_ExceptionMatches()}
- instead, shown below. (The comparison could easily fail since the
- exception may be an instance instead of a class, in the case of a
- class exception, or it may the a subclass of the expected
- exception.)}
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{int}{PyErr_ExceptionMatches}{PyObject *exc}
- Equivalent to \samp{PyErr_GivenExceptionMatches(PyErr_Occurred(),
- \var{exc})}. This should only be called when an exception is
- actually set; a memory access violation will occur if no exception
- has been raised.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{int}{PyErr_GivenExceptionMatches}{PyObject *given, PyObject *exc}
- Return true if the \var{given} exception matches the exception in
- \var{exc}. If \var{exc} is a class object, this also returns true
- when \var{given} is an instance of a subclass. If \var{exc} is a
- tuple, all exceptions in the tuple (and recursively in subtuples)
- are searched for a match. If \var{given} is \NULL, a memory access
- violation will occur.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{void}{PyErr_NormalizeException}{PyObject**exc, PyObject**val, PyObject**tb}
- Under certain circumstances, the values returned by
- \cfunction{PyErr_Fetch()} below can be ``unnormalized'', meaning
- that \code{*\var{exc}} is a class object but \code{*\var{val}} is
- not an instance of the same class. This function can be used to
- instantiate the class in that case. If the values are already
- normalized, nothing happens. The delayed normalization is
- implemented to improve performance.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{void}{PyErr_Clear}{}
- Clear the error indicator. If the error indicator is not set, there
- is no effect.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{void}{PyErr_Fetch}{PyObject **ptype, PyObject **pvalue,
- PyObject **ptraceback}
- Retrieve the error indicator into three variables whose addresses
- are passed. If the error indicator is not set, set all three
- variables to \NULL. If it is set, it will be cleared and you own a
- reference to each object retrieved. The value and traceback object
- may be \NULL{} even when the type object is not. \note{This
- function is normally only used by code that needs to handle
- exceptions or by code that needs to save and restore the error
- indicator temporarily.}
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{void}{PyErr_Restore}{PyObject *type, PyObject *value,
- PyObject *traceback}
- Set the error indicator from the three objects. If the error
- indicator is already set, it is cleared first. If the objects are
- \NULL, the error indicator is cleared. Do not pass a \NULL{} type
- and non-\NULL{} value or traceback. The exception type should be a
- class. Do not pass an invalid exception type or value.
- (Violating these rules will cause subtle problems later.) This call
- takes away a reference to each object: you must own a reference to
- each object before the call and after the call you no longer own
- these references. (If you don't understand this, don't use this
- function. I warned you.) \note{This function is normally only used
- by code that needs to save and restore the error indicator
- temporarily; use \cfunction{PyErr_Fetch()} to save the current
- exception state.}
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{void}{PyErr_SetString}{PyObject *type, const char *message}
- This is the most common way to set the error indicator. The first
- argument specifies the exception type; it is normally one of the
- standard exceptions, e.g. \cdata{PyExc_RuntimeError}. You need not
- increment its reference count. The second argument is an error
- message; it is converted to a string object.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{void}{PyErr_SetObject}{PyObject *type, PyObject *value}
- This function is similar to \cfunction{PyErr_SetString()} but lets
- you specify an arbitrary Python object for the ``value'' of the
- exception.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{PyObject*}{PyErr_Format}{PyObject *exception,
- const char *format, \moreargs}
- This function sets the error indicator and returns \NULL.
- \var{exception} should be a Python exception (class, not
- an instance). \var{format} should be a string, containing format
- codes, similar to \cfunction{printf()}. The \code{width.precision}
- before a format code is parsed, but the width part is ignored.
-
- % This should be exactly the same as the table in PyString_FromFormat.
- % One should just refer to the other.
-
- % The descriptions for %zd and %zu are wrong, but the truth is complicated
- % because not all compilers support the %z width modifier -- we fake it
- % when necessary via interpolating PY_FORMAT_SIZE_T.
-
- % %u, %lu, %zu should have "new in Python 2.5" blurbs.
-
- \begin{tableiii}{l|l|l}{member}{Format Characters}{Type}{Comment}
- \lineiii{\%\%}{\emph{n/a}}{The literal \% character.}
- \lineiii{\%c}{int}{A single character, represented as an C int.}
- \lineiii{\%d}{int}{Exactly equivalent to \code{printf("\%d")}.}
- \lineiii{\%u}{unsigned int}{Exactly equivalent to \code{printf("\%u")}.}
- \lineiii{\%ld}{long}{Exactly equivalent to \code{printf("\%ld")}.}
- \lineiii{\%lu}{unsigned long}{Exactly equivalent to \code{printf("\%lu")}.}
- \lineiii{\%zd}{Py_ssize_t}{Exactly equivalent to \code{printf("\%zd")}.}
- \lineiii{\%zu}{size_t}{Exactly equivalent to \code{printf("\%zu")}.}
- \lineiii{\%i}{int}{Exactly equivalent to \code{printf("\%i")}.}
- \lineiii{\%x}{int}{Exactly equivalent to \code{printf("\%x")}.}
- \lineiii{\%s}{char*}{A null-terminated C character array.}
- \lineiii{\%p}{void*}{The hex representation of a C pointer.
- Mostly equivalent to \code{printf("\%p")} except that it is
- guaranteed to start with the literal \code{0x} regardless of
- what the platform's \code{printf} yields.}
- \end{tableiii}
-
- An unrecognized format character causes all the rest of the format
- string to be copied as-is to the result string, and any extra
- arguments discarded.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{void}{PyErr_SetNone}{PyObject *type}
- This is a shorthand for \samp{PyErr_SetObject(\var{type},
- Py_None)}.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{int}{PyErr_BadArgument}{}
- This is a shorthand for \samp{PyErr_SetString(PyExc_TypeError,
- \var{message})}, where \var{message} indicates that a built-in
- operation was invoked with an illegal argument. It is mostly for
- internal use.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{PyObject*}{PyErr_NoMemory}{}
- This is a shorthand for \samp{PyErr_SetNone(PyExc_MemoryError)}; it
- returns \NULL{} so an object allocation function can write
- \samp{return PyErr_NoMemory();} when it runs out of memory.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{PyObject*}{PyErr_SetFromErrno}{PyObject *type}
- This is a convenience function to raise an exception when a C
- library function has returned an error and set the C variable
- \cdata{errno}. It constructs a tuple object whose first item is the
- integer \cdata{errno} value and whose second item is the
- corresponding error message (gotten from
- \cfunction{strerror()}\ttindex{strerror()}), and then calls
- \samp{PyErr_SetObject(\var{type}, \var{object})}. On \UNIX, when
- the \cdata{errno} value is \constant{EINTR}, indicating an
- interrupted system call, this calls
- \cfunction{PyErr_CheckSignals()}, and if that set the error
- indicator, leaves it set to that. The function always returns
- \NULL, so a wrapper function around a system call can write
- \samp{return PyErr_SetFromErrno(\var{type});} when the system call
- returns an error.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{PyObject*}{PyErr_SetFromErrnoWithFilename}{PyObject *type,
- const char *filename}
- Similar to \cfunction{PyErr_SetFromErrno()}, with the additional
- behavior that if \var{filename} is not \NULL, it is passed to the
- constructor of \var{type} as a third parameter. In the case of
- exceptions such as \exception{IOError} and \exception{OSError}, this
- is used to define the \member{filename} attribute of the exception
- instance.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{PyObject*}{PyErr_SetFromWindowsErr}{int ierr}
- This is a convenience function to raise \exception{WindowsError}.
- If called with \var{ierr} of \cdata{0}, the error code returned by a
- call to \cfunction{GetLastError()} is used instead. It calls the
- Win32 function \cfunction{FormatMessage()} to retrieve the Windows
- description of error code given by \var{ierr} or
- \cfunction{GetLastError()}, then it constructs a tuple object whose
- first item is the \var{ierr} value and whose second item is the
- corresponding error message (gotten from
- \cfunction{FormatMessage()}), and then calls
- \samp{PyErr_SetObject(\var{PyExc_WindowsError}, \var{object})}.
- This function always returns \NULL.
- Availability: Windows.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{PyObject*}{PyErr_SetExcFromWindowsErr}{PyObject *type,
- int ierr}
- Similar to \cfunction{PyErr_SetFromWindowsErr()}, with an additional
- parameter specifying the exception type to be raised.
- Availability: Windows.
- \versionadded{2.3}
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{PyObject*}{PyErr_SetFromWindowsErrWithFilename}{int ierr,
- const char *filename}
- Similar to \cfunction{PyErr_SetFromWindowsErr()}, with the
- additional behavior that if \var{filename} is not \NULL, it is
- passed to the constructor of \exception{WindowsError} as a third
- parameter.
- Availability: Windows.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{PyObject*}{PyErr_SetExcFromWindowsErrWithFilename}
- {PyObject *type, int ierr, char *filename}
- Similar to \cfunction{PyErr_SetFromWindowsErrWithFilename()}, with
- an additional parameter specifying the exception type to be raised.
- Availability: Windows.
- \versionadded{2.3}
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{void}{PyErr_BadInternalCall}{}
- This is a shorthand for \samp{PyErr_SetString(PyExc_TypeError,
- \var{message})}, where \var{message} indicates that an internal
- operation (e.g. a Python/C API function) was invoked with an illegal
- argument. It is mostly for internal use.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{int}{PyErr_WarnEx}{PyObject *category, char *message, int stacklevel}
- Issue a warning message. The \var{category} argument is a warning
- category (see below) or \NULL; the \var{message} argument is a
- message string. \var{stacklevel} is a positive number giving a
- number of stack frames; the warning will be issued from the
- currently executing line of code in that stack frame. A \var{stacklevel}
- of 1 is the function calling \cfunction{PyErr_WarnEx()}, 2 is
- the function above that, and so forth.
-
- This function normally prints a warning message to \var{sys.stderr};
- however, it is also possible that the user has specified that
- warnings are to be turned into errors, and in that case this will
- raise an exception. It is also possible that the function raises an
- exception because of a problem with the warning machinery (the
- implementation imports the \module{warnings} module to do the heavy
- lifting). The return value is \code{0} if no exception is raised,
- or \code{-1} if an exception is raised. (It is not possible to
- determine whether a warning message is actually printed, nor what
- the reason is for the exception; this is intentional.) If an
- exception is raised, the caller should do its normal exception
- handling (for example, \cfunction{Py_DECREF()} owned references and
- return an error value).
-
- Warning categories must be subclasses of \cdata{Warning}; the
- default warning category is \cdata{RuntimeWarning}. The standard
- Python warning categories are available as global variables whose
- names are \samp{PyExc_} followed by the Python exception name.
- These have the type \ctype{PyObject*}; they are all class objects.
- Their names are \cdata{PyExc_Warning}, \cdata{PyExc_UserWarning},
- \cdata{PyExc_UnicodeWarning}, \cdata{PyExc_DeprecationWarning},
- \cdata{PyExc_SyntaxWarning}, \cdata{PyExc_RuntimeWarning}, and
- \cdata{PyExc_FutureWarning}. \cdata{PyExc_Warning} is a subclass of
- \cdata{PyExc_Exception}; the other warning categories are subclasses
- of \cdata{PyExc_Warning}.
-
- For information about warning control, see the documentation for the
- \module{warnings} module and the \programopt{-W} option in the
- command line documentation. There is no C API for warning control.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{int}{PyErr_Warn}{PyObject *category, char *message}
- Issue a warning message. The \var{category} argument is a warning
- category (see below) or \NULL; the \var{message} argument is a
- message string. The warning will appear to be issued from the function
- calling \cfunction{PyErr_Warn()}, equivalent to calling
- \cfunction{PyErr_WarnEx()} with a \var{stacklevel} of 1.
-
- Deprecated; use \cfunction{PyErr_WarnEx()} instead.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{int}{PyErr_WarnExplicit}{PyObject *category,
- const char *message, const char *filename, int lineno,
- const char *module, PyObject *registry}
- Issue a warning message with explicit control over all warning
- attributes. This is a straightforward wrapper around the Python
- function \function{warnings.warn_explicit()}, see there for more
- information. The \var{module} and \var{registry} arguments may be
- set to \NULL{} to get the default effect described there.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{int}{PyErr_CheckSignals}{}
- This function interacts with Python's signal handling. It checks
- whether a signal has been sent to the processes and if so, invokes
- the corresponding signal handler. If the
- \module{signal}\refbimodindex{signal} module is supported, this can
- invoke a signal handler written in Python. In all cases, the
- default effect for \constant{SIGINT}\ttindex{SIGINT} is to raise the
- \withsubitem{(built-in exception)}{\ttindex{KeyboardInterrupt}}
- \exception{KeyboardInterrupt} exception. If an exception is raised
- the error indicator is set and the function returns \code{-1};
- otherwise the function returns \code{0}. The error indicator may or
- may not be cleared if it was previously set.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{void}{PyErr_SetInterrupt}{}
- This function simulates the effect of a
- \constant{SIGINT}\ttindex{SIGINT} signal arriving --- the next time
- \cfunction{PyErr_CheckSignals()} is called,
- \withsubitem{(built-in exception)}{\ttindex{KeyboardInterrupt}}
- \exception{KeyboardInterrupt} will be raised. It may be called
- without holding the interpreter lock.
- % XXX This was described as obsolete, but is used in
- % thread.interrupt_main() (used from IDLE), so it's still needed.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{PyObject*}{PyErr_NewException}{char *name,
- PyObject *base,
- PyObject *dict}
- This utility function creates and returns a new exception object.
- The \var{name} argument must be the name of the new exception, a C
- string of the form \code{module.class}. The \var{base} and
- \var{dict} arguments are normally \NULL. This creates a class
- object derived from \exception{Exception} (accessible in C as
- \cdata{PyExc_Exception}).
-
- The \member{__module__} attribute of the new class is set to the
- first part (up to the last dot) of the \var{name} argument, and the
- class name is set to the last part (after the last dot). The
- \var{base} argument can be used to specify alternate base classes;
- it can either be only one class or a tuple of classes.
- The \var{dict} argument can be used to specify a dictionary of class
- variables and methods.
-\end{cfuncdesc}
-
-\begin{cfuncdesc}{void}{PyErr_WriteUnraisable}{PyObject *obj}
- This utility function prints a warning message to \code{sys.stderr}
- when an exception has been set but it is impossible for the
- interpreter to actually raise the exception. It is used, for
- example, when an exception occurs in an \method{__del__()} method.
-
- The function is called with a single argument \var{obj} that
- identifies the context in which the unraisable exception occurred.
- The repr of \var{obj} will be printed in the warning message.
-\end{cfuncdesc}
-
-\section{Standard Exceptions \label{standardExceptions}}
-
-All standard Python exceptions are available as global variables whose
-names are \samp{PyExc_} followed by the Python exception name. These
-have the type \ctype{PyObject*}; they are all class objects. For
-completeness, here are all the variables:
-
-\begin{tableiii}{l|l|c}{cdata}{C Name}{Python Name}{Notes}
- \lineiii{PyExc_BaseException\ttindex{PyExc_BaseException}}{\exception{BaseException}}{(1), (4)}
- \lineiii{PyExc_Exception\ttindex{PyExc_Exception}}{\exception{Exception}}{(1)}
- \lineiii{PyExc_StandardError\ttindex{PyExc_StandardError}}{\exception{StandardError}}{(1)}
- \lineiii{PyExc_ArithmeticError\ttindex{PyExc_ArithmeticError}}{\exception{ArithmeticError}}{(1)}
- \lineiii{PyExc_LookupError\ttindex{PyExc_LookupError}}{\exception{LookupError}}{(1)}
- \lineiii{PyExc_AssertionError\ttindex{PyExc_AssertionError}}{\exception{AssertionError}}{}
- \lineiii{PyExc_AttributeError\ttindex{PyExc_AttributeError}}{\exception{AttributeError}}{}
- \lineiii{PyExc_EOFError\ttindex{PyExc_EOFError}}{\exception{EOFError}}{}
- \lineiii{PyExc_EnvironmentError\ttindex{PyExc_EnvironmentError}}{\exception{EnvironmentError}}{(1)}
- \lineiii{PyExc_FloatingPointError\ttindex{PyExc_FloatingPointError}}{\exception{FloatingPointError}}{}
- \lineiii{PyExc_IOError\ttindex{PyExc_IOError}}{\exception{IOError}}{}
- \lineiii{PyExc_ImportError\ttindex{PyExc_ImportError}}{\exception{ImportError}}{}
- \lineiii{PyExc_IndexError\ttindex{PyExc_IndexError}}{\exception{IndexError}}{}
- \lineiii{PyExc_KeyError\ttindex{PyExc_KeyError}}{\exception{KeyError}}{}
- \lineiii{PyExc_KeyboardInterrupt\ttindex{PyExc_KeyboardInterrupt}}{\exception{KeyboardInterrupt}}{}
- \lineiii{PyExc_MemoryError\ttindex{PyExc_MemoryError}}{\exception{MemoryError}}{}
- \lineiii{PyExc_NameError\ttindex{PyExc_NameError}}{\exception{NameError}}{}
- \lineiii{PyExc_NotImplementedError\ttindex{PyExc_NotImplementedError}}{\exception{NotImplementedError}}{}
- \lineiii{PyExc_OSError\ttindex{PyExc_OSError}}{\exception{OSError}}{}
- \lineiii{PyExc_OverflowError\ttindex{PyExc_OverflowError}}{\exception{OverflowError}}{}
- \lineiii{PyExc_ReferenceError\ttindex{PyExc_ReferenceError}}{\exception{ReferenceError}}{(2)}
- \lineiii{PyExc_RuntimeError\ttindex{PyExc_RuntimeError}}{\exception{RuntimeError}}{}
- \lineiii{PyExc_SyntaxError\ttindex{PyExc_SyntaxError}}{\exception{SyntaxError}}{}
- \lineiii{PyExc_SystemError\ttindex{PyExc_SystemError}}{\exception{SystemError}}{}
- \lineiii{PyExc_SystemExit\ttindex{PyExc_SystemExit}}{\exception{SystemExit}}{}
- \lineiii{PyExc_TypeError\ttindex{PyExc_TypeError}}{\exception{TypeError}}{}
- \lineiii{PyExc_ValueError\ttindex{PyExc_ValueError}}{\exception{ValueError}}{}
- \lineiii{PyExc_WindowsError\ttindex{PyExc_WindowsError}}{\exception{WindowsError}}{(3)}
- \lineiii{PyExc_ZeroDivisionError\ttindex{PyExc_ZeroDivisionError}}{\exception{ZeroDivisionError}}{}
-\end{tableiii}
-
-\noindent
-Notes:
-\begin{description}
-\item[(1)]
- This is a base class for other standard exceptions.
-
-\item[(2)]
- This is the same as \exception{weakref.ReferenceError}.
-
-\item[(3)]
- Only defined on Windows; protect code that uses this by testing that
- the preprocessor macro \code{MS_WINDOWS} is defined.
-
-\item[(4)]
- \versionadded{2.5}
-\end{description}
-
-
-\section{Deprecation of String Exceptions}
-
-All exceptions built into Python or provided in the standard library
-are derived from \exception{BaseException}.
-\withsubitem{(built-in exception)}{\ttindex{BaseException}}
-
-String exceptions are still supported in the interpreter to allow
-existing code to run unmodified, but this will also change in a future
-release.