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_*() 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. 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
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
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.
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!)
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
|int PyErr_ExceptionMatches(||PyObject *exc)|
Equivalent to "PyErr_GivenExceptionMatches(PyErr_Occurred(),
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 occur.
|void PyErr_NormalizeException(||PyObject**exc, PyObject**val, PyObject**tb)|
Under certain circumstances, the values returned by
PyErr_Fetch() below can be ``unnormalized'', meaning
*exc is a class object but
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.
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:
function is normally only used by code that needs to handle
exceptions or by code that needs to save and restore the error
|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
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 PyErr_Fetch() to save the current
|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
|PyObject* PyErr_Format(||PyObject *exception,
const char *format, ...)|
This function sets the error indicator and returns NULL.
exception should be a Python exception (class, not
an instance). format should be a string, containing format
codes, similar to printf(). The
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|
||A hex pointer, as a void * 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
|void PyErr_SetNone(||PyObject *type)|
This is a shorthand for "PyErr_SetObject(type,
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
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 then calls
"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(type);" when the system call
returns an error.
|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
|PyObject* PyErr_SetFromWindowsErr(||int ierr)|
This is a convenience function to raise WindowsError.
If called with ierr of 0, the error code returned by a
call to GetLastError() is used instead. It calls the
Win32 function FormatMessage() to retrieve the Windows
description of error code given by ierr or
GetLastError(), then it constructs a tuple object whose
first item is the ierr value and whose second item is the
corresponding error message (gotten from
FormatMessage()), and then calls
This function always returns NULL.
|PyObject* PyErr_SetExcFromWindowsErr(||PyObject *type,
Similar to PyErr_SetFromWindowsErr(), with an additional
parameter specifying the exception type to be raised.
New in version 2.3.
|PyObject* PyErr_SetFromWindowsErrWithFilename(||int ierr,
Similar to PyErr_SetFromWindowsErr(), with the
additional behavior that if filename is not NULL, it is
passed to the constructor of WindowsError as a third
|PyObject* PyErr_SetExcFromWindowsErrWithFilename(||PyObject *type, int ierr, char *filename)|
Similar to PyErr_SetFromWindowsErrWithFilename(), with
an additional parameter specifying the exception type to be raised.
New in version 2.3.
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_Warn(||PyObject *category, char *message)|
Issue a warning message. The category argument is a warning
category (see below) or NULL; the message argument is a
This function normally prints a warning message to 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 warnings module to do the heavy
lifting). The return value is
0 if no exception is raised,
-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, Py_DECREF() owned references and
return an error value).
Warning categories must be subclasses of Warning; the
default warning category is RuntimeWarning. The standard
Python warning categories are available as global variables whose
names are "PyExc_" followed by the Python exception name.
These have the type PyObject*; they are all class objects.
Their names are PyExc_Warning, PyExc_UserWarning,
PyExc_RuntimeWarning, and PyExc_FutureWarning.
PyExc_Warning is a subclass of PyExc_Exception; the
other warning categories are subclasses of PyExc_Warning.
For information about warning control, see the documentation for the
warnings module and the -W option in the
command line documentation. There is no C API for warning control.
|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 warnings.warn_explicit(), see there for more
information. The module and registry arguments may be
set to NULL to get the default effect described there.
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
otherwise the function returns
0. The error indicator may or
may not be cleared if it was previously set.
This function 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
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 the context in which the unraisable exception occurred.
The repr of obj will be printed in the warning message.
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