4. Exception Handling
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 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
-1 if they
return an integer (exception: the PyArg_Parse*() functions
1 for success and
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.
The error indicator consists of three Python objects corresponding to
the Python variables
sys.exc_traceback. API functions exist to interact with the
error indicator in various ways. There is a separate error indicator
for each thread.
- void PyErr_Print ()
Print a standard traceback to
sys.stderr and clear the error
indicator. Call this function only when the error indicator is set.
(Otherwise it will cause a fatal error!)
- PyObject* PyErr_Occurred ()
Test whether the error indicator is set. If set, return the exception
type (the first argument to the last call to one of the
PyErr_Set*() functions or to PyErr_Restore()). If
not set, return NULL. You do not own a reference to the return
value, so you do not need to Py_DECREF() it.
Note: Do not compare the return value to a specific
exception; use 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.)
- int PyErr_ExceptionMatches (PyObject *exc)
This should only be called when an exception is actually set; a memory
access violation will occur if no exception has been raised.
- int PyErr_GivenExceptionMatches (PyObject *given, PyObject *exc)
Return true if the given exception matches the exception in
exc. If exc is a class object, this also returns true
when given is an instance of a subclass. If exc is a tuple, all
exceptions in the tuple (and recursively in subtuples) are searched
for a match. If given is NULL, a memory access violation will
- void PyErr_NormalizeException (PyObject**exc, PyObject**val, PyObject**tb)
Under certain circumstances, the values returned by
PyErr_Fetch() below can be ``unnormalized'', meaning that
*exc is a class object but
*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
- void PyErr_Clear ()
Clear the error indicator. If the error indicator is not set, there
is no effect.
- void PyErr_Fetch (PyObject **ptype, PyObject **pvalue,
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
- void PyErr_Restore (PyObject *type, PyObject *value,
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, i.e. 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
- 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. PyExc_RuntimeError. You need not
increment its reference count. The second argument is an error
message; it is converted to a string object.
- void PyErr_SetObject (PyObject *type, PyObject *value)
This function is similar to PyErr_SetString() but lets you
specify an arbitrary Python object for the ``value'' of the exception.
You need not increment its reference count.
- PyObject* PyErr_Format (PyObject *exception,
const char *format, ...)
This function sets the error indicator. exception should be a
Python exception (string or class, not an instance).
fmt should be a string, containing format codes, similar to
width.precision before a format code
is parsed, but the width part is ignored.
||Character, as an int parameter
||Number in decimal, as an int parameter
||Number in hexadecimal, as an int parameter
||A string, as a char * parameter|
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.
A new reference is returned, which is owned by the caller.
- void PyErr_SetNone (PyObject *type)
This is a shorthand for "PyErr_SetObject(type, Py_None)".
- int PyErr_BadArgument ()
This is a shorthand for "PyErr_SetString(PyExc_TypeError,
message)", where message indicates that a built-in operation
was invoked with an illegal argument. It is mostly for internal use.
- PyObject* PyErr_NoMemory ()
This is a shorthand for "PyErr_SetNone(PyExc_MemoryError)"; it
returns NULL so an object allocation function can write
"return PyErr_NoMemory();" when it runs out of memory.
- 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 errno.
It constructs a tuple object whose first item is the integer
errno value and whose second item is the corresponding error
message (gotten from strerror()), and
"PyErr_SetObject(type, object)". On Unix, when
the errno value is EINTR, indicating an interrupted
system call, this calls 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
"return PyErr_SetFromErrno();" when the system call returns an
- PyObject* PyErr_SetFromErrnoWithFilename (PyObject *type,
Similar to PyErr_SetFromErrno(), with the additional
behavior that if filename is not NULL, it is passed to the
constructor of type as a third parameter. In the case of
exceptions such as IOError and OSError, this
is used to define the filename attribute of the exception
- void PyErr_BadInternalCall ()
This is a shorthand for "PyErr_SetString(PyExc_TypeError,
message)", where 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.
- 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
signal module is supported, this can
invoke a signal handler written in Python. In all cases, the default
effect for SIGINT is to raise the
KeyboardInterrupt exception. If an exception is raised the
error indicator is set and the function returns
the function returns
0. The error indicator may or may not be
cleared if it was previously set.
- void PyErr_SetInterrupt ()
This function is obsolete. It simulates the effect of a
SIGINT signal arriving -- the next time
PyErr_CheckSignals() is called,
KeyboardInterrupt will be raised.
It may be called without holding the interpreter lock.
- PyObject* PyErr_NewException (char *name,
This utility function creates and returns a new exception object. The
name argument must be the name of the new exception, a C string
of the form
module.class. The base and
dict arguments are normally NULL. This creates a
class object derived from the root for all exceptions, the built-in
name Exception (accessible in C as
PyExc_Exception). The __module__ attribute of the
new class is set to the first part (up to the last dot) of the
name argument, and the class name is set to the last part (after
the last dot). The base argument can be used to specify an
alternate base class. The dict argument can be used to specify
a dictionary of class variables and methods.
- void PyErr_WriteUnraisable (PyObject *obj)
This utility function prints a warning message to 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 __del__ method.
The function is called with a single argument obj that
identifies where the context in which the unraisable exception
occurred. The repr of obj will be printed in the warning
See About this document... for information on suggesting changes.