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author | Georg Brandl <georg@python.org> | 2007-08-15 14:26:55 (GMT) |
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committer | Georg Brandl <georg@python.org> | 2007-08-15 14:26:55 (GMT) |
commit | f56181ff53ba00b7bed3997a4dccd9a1b6217b57 (patch) | |
tree | 1200947a7ffc78c2719831e4c7fd900a8ab01368 /Doc/api/exceptions.tex | |
parent | af62d9abfb78067a54c769302005f952ed999f6a (diff) | |
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diff --git a/Doc/api/exceptions.tex b/Doc/api/exceptions.tex deleted file mode 100644 index 2dabeee..0000000 --- a/Doc/api/exceptions.tex +++ /dev/null @@ -1,442 +0,0 @@ -\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. |