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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 propogated, 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
string or class; if it is a class, the value should be an instance
of that 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.}
\end{cfuncdesc}
\begin{cfuncdesc}{void}{PyErr_SetString}{PyObject *type, 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. You need not increment its reference count.
\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 (string or 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.
\begin{tableii}{c|l}{character}{Character}{Meaning}
\lineii{c}{Character, as an \ctype{int} parameter}
\lineii{d}{Number in decimal, as an \ctype{int} parameter}
\lineii{x}{Number in hexadecimal, as an \ctype{int} parameter}
\lineii{x}{A string, as a \ctype{char *} parameter}
\end{tableii}
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();} when the system call returns an
error.
\end{cfuncdesc}
\begin{cfuncdesc}{PyObject*}{PyErr_SetFromErrnoWithFilename}{PyObject *type,
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}{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_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.
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_DeprecationWarning}, \cdata{PyExc_SyntaxWarning}, and
\cdata{PyExc_RuntimeWarning}. \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_WarnExplicit}{PyObject *category, char *message,
char *filename, int lineno, 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 is obsolete. It 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.
\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 the root for all exceptions, the built-in name
\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 an alternate base class.
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 where 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_Exception}{\exception{Exception}}{(1)}
\lineiii{PyExc_StandardError}{\exception{StandardError}}{(1)}
\lineiii{PyExc_ArithmeticError}{\exception{ArithmeticError}}{(1)}
\lineiii{PyExc_LookupError}{\exception{LookupError}}{(1)}
\lineiii{PyExc_AssertionError}{\exception{AssertionError}}{}
\lineiii{PyExc_AttributeError}{\exception{AttributeError}}{}
\lineiii{PyExc_EOFError}{\exception{EOFError}}{}
\lineiii{PyExc_EnvironmentError}{\exception{EnvironmentError}}{(1)}
\lineiii{PyExc_FloatingPointError}{\exception{FloatingPointError}}{}
\lineiii{PyExc_IOError}{\exception{IOError}}{}
\lineiii{PyExc_ImportError}{\exception{ImportError}}{}
\lineiii{PyExc_IndexError}{\exception{IndexError}}{}
\lineiii{PyExc_KeyError}{\exception{KeyError}}{}
\lineiii{PyExc_KeyboardInterrupt}{\exception{KeyboardInterrupt}}{}
\lineiii{PyExc_MemoryError}{\exception{MemoryError}}{}
\lineiii{PyExc_NameError}{\exception{NameError}}{}
\lineiii{PyExc_NotImplementedError}{\exception{NotImplementedError}}{}
\lineiii{PyExc_OSError}{\exception{OSError}}{}
\lineiii{PyExc_OverflowError}{\exception{OverflowError}}{}
\lineiii{PyExc_ReferenceError}{\exception{ReferenceError}}{(2)}
\lineiii{PyExc_RuntimeError}{\exception{RuntimeError}}{}
\lineiii{PyExc_SyntaxError}{\exception{SyntaxError}}{}
\lineiii{PyExc_SystemError}{\exception{SystemError}}{}
\lineiii{PyExc_SystemExit}{\exception{SystemExit}}{}
\lineiii{PyExc_TypeError}{\exception{TypeError}}{}
\lineiii{PyExc_ValueError}{\exception{ValueError}}{}
\lineiii{PyExc_WindowsError}{\exception{WindowsError}}{(3)}
\lineiii{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.
\end{description}
\section{Deprecation of String Exceptions}
All exceptions built into Python or provided in the standard library
are derived from \exception{Exception}.
\withsubitem{(built-in exception)}{\ttindex{Exception}}
String exceptions are still supported in the interpreter to allow
existing code to run unmodified, but this will also change in a future
release.
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