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 return 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_type, sys.exc_value and 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_PrintEx(int set_sys_last_vars)
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!)
If set_sys_last_vars is nonzero, the variables sys.last_type,
sys.last_value and sys.last_traceback will be set to the
type, value and traceback of the printed exception, respectively.
-
void
PyErr_Print()
Alias for PyErr_PrintEx(1).
-
PyObject*
PyErr_Occurred()
- Return value: Borrowed reference.
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 be a subclass of the expected exception.)
-
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.
-
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 performance.
-
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, 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.
-
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 PyErr_Fetch() to save the current
exception state.
-
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. 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.
-
PyObject*
PyErr_Format(PyObject *exception, const char *format, ...)
- Return value: Always NULL.
This function sets the error indicator and returns NULL. exception
should be a Python exception class. The format and subsequent
parameters help format the error message; they have the same meaning and
values as in PyString_FromFormat().
-
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()
- Return value: Always NULL.
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)
- Return value: Always NULL.
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_SetFromErrnoWithFilenameObject(PyObject *type, PyObject *filenameObject)
Similar to PyErr_SetFromErrno(), with the additional behavior that if
filenameObject 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 instance.
-
PyObject*
PyErr_SetFromErrnoWithFilename(PyObject *type, const char *filename)
- Return value: Always NULL.
Similar to PyErr_SetFromErrnoWithFilenameObject(), but the filename
is given as a C string.
-
PyObject*
PyErr_SetFromWindowsErr(int ierr)
- Return value: Always NULL.
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 PyErr_SetObject(PyExc_WindowsError,
object). This function always returns NULL. Availability: Windows.
-
PyObject*
PyErr_SetExcFromWindowsErr(PyObject *type, int ierr)
- Return value: Always NULL.
Similar to PyErr_SetFromWindowsErr(), with an additional parameter
specifying the exception type to be raised. Availability: Windows.
-
PyObject*
PyErr_SetFromWindowsErrWithFilenameObject(int ierr, PyObject *filenameObject)
Similar to PyErr_SetFromWindowsErr(), with the additional behavior that
if filenameObject is not NULL, it is passed to the constructor of
WindowsError as a third parameter. Availability: Windows.
-
PyObject*
PyErr_SetFromWindowsErrWithFilename(int ierr, const char *filename)
- Return value: Always NULL.
Similar to PyErr_SetFromWindowsErrWithFilenameObject(), but the
filename is given as a C string. Availability: Windows.
-
PyObject*
PyErr_SetExcFromWindowsErrWithFilenameObject(PyObject *type, int ierr, PyObject *filename)
Similar to PyErr_SetFromWindowsErrWithFilenameObject(), with an
additional parameter specifying the exception type to be raised.
Availability: Windows.
-
PyObject*
PyErr_SetExcFromWindowsErrWithFilename(PyObject *type, int ierr, const char *filename)
- Return value: Always NULL.
Similar to PyErr_SetFromWindowsErrWithFilename(), with an additional
parameter specifying the exception type to be raised. Availability: Windows.
-
void
PyErr_BadInternalCall()
This is a shorthand for PyErr_SetString(PyExc_SystemError, 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_WarnEx(PyObject *category, char *message, int stacklevel)
Issue a warning message. The category argument is a warning category (see
below) or NULL; the message argument is a message string. 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 stacklevel of 1
is the function calling PyErr_WarnEx(), 2 is the function above that,
and so forth.
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, or -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 PyExc_Warning;
PyExc_Warning is a subclass of PyExc_Exception;
the default warning category is PyExc_RuntimeWarning. The standard
Python warning categories are available as global variables whose names are
enumerated at Standard Warning Categories.
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_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 message string. The warning will
appear to be issued from the function calling PyErr_Warn(), equivalent to
calling PyErr_WarnEx() with a stacklevel of 1.
Deprecated; use PyErr_WarnEx() instead.
-
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
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.
-
int
PyErr_WarnPy3k(char *message, int stacklevel)
Issue a DeprecationWarning with the given message and stacklevel
if the Py_Py3kWarningFlag flag is enabled.
-
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 -1;
otherwise the function returns 0. The error indicator may or may not be
cleared if it was previously set.
-
void
PyErr_SetInterrupt()
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.
-
int
PySignal_SetWakeupFd(int fd)
This utility function specifies a file descriptor to which a '\0' byte will
be written whenever a signal is received. It returns the previous such file
descriptor. The value -1 disables the feature; this is the initial state.
This is equivalent to signal.set_wakeup_fd() in Python, but without any
error checking. fd should be a valid file descriptor. The function should
only be called from the main thread.
-
PyObject*
PyErr_NewException(char *name, PyObject *base, PyObject *dict)
- Return value: New reference.
This utility function creates and returns a new exception class. The name
argument must be the name of the new exception, a C string of the form
module.classname. The base and dict arguments are normally NULL.
This creates a class object derived from 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 alternate
base classes; it can either be only one class or a tuple of classes. The dict
argument can be used to specify a dictionary of class variables and methods.
-
PyObject*
PyErr_NewExceptionWithDoc(char *name, char *doc, PyObject *base, PyObject *dict)
- Return value: New reference.
Same as PyErr_NewException(), except that the new exception class can
easily be given a docstring: If doc is non-NULL, it will be used as the
docstring for the exception class.
-
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 the context
in which the unraisable exception occurred. If possible,
the repr of obj will be printed in the warning message.
Unicode Exception Objects
The following functions are used to create and modify Unicode exceptions from C.
-
PyObject*
PyUnicodeDecodeError_Create(const char *encoding, const char *object, Py_ssize_t length, Py_ssize_t start, Py_ssize_t end, const char *reason)
Create a UnicodeDecodeError object with the attributes encoding,
object, length, start, end and reason.
-
PyObject*
PyUnicodeEncodeError_Create(const char *encoding, const Py_UNICODE *object, Py_ssize_t length, Py_ssize_t start, Py_ssize_t end, const char *reason)
Create a UnicodeEncodeError object with the attributes encoding,
object, length, start, end and reason.
-
PyObject*
PyUnicodeTranslateError_Create(const Py_UNICODE *object, Py_ssize_t length, Py_ssize_t start, Py_ssize_t end, const char *reason)
Create a UnicodeTranslateError object with the attributes object,
length, start, end and reason.
-
PyObject*
PyUnicodeDecodeError_GetEncoding(PyObject *exc)
-
PyObject*
PyUnicodeEncodeError_GetEncoding(PyObject *exc)
Return the encoding attribute of the given exception object.
-
PyObject*
PyUnicodeDecodeError_GetObject(PyObject *exc)
-
PyObject*
PyUnicodeEncodeError_GetObject(PyObject *exc)
-
PyObject*
PyUnicodeTranslateError_GetObject(PyObject *exc)
Return the object attribute of the given exception object.
-
int
PyUnicodeDecodeError_GetStart(PyObject *exc, Py_ssize_t *start)
-
int
PyUnicodeEncodeError_GetStart(PyObject *exc, Py_ssize_t *start)
-
int
PyUnicodeTranslateError_GetStart(PyObject *exc, Py_ssize_t *start)
Get the start attribute of the given exception object and place it into
*start. start must not be NULL. Return 0 on success, -1 on
failure.
-
int
PyUnicodeDecodeError_SetStart(PyObject *exc, Py_ssize_t start)
-
int
PyUnicodeEncodeError_SetStart(PyObject *exc, Py_ssize_t start)
-
int
PyUnicodeTranslateError_SetStart(PyObject *exc, Py_ssize_t start)
Set the start attribute of the given exception object to start. Return
0 on success, -1 on failure.
-
int
PyUnicodeDecodeError_GetEnd(PyObject *exc, Py_ssize_t *end)
-
int
PyUnicodeEncodeError_GetEnd(PyObject *exc, Py_ssize_t *end)
-
int
PyUnicodeTranslateError_GetEnd(PyObject *exc, Py_ssize_t *end)
Get the end attribute of the given exception object and place it into
*end. end must not be NULL. Return 0 on success, -1 on
failure.
-
int
PyUnicodeDecodeError_SetEnd(PyObject *exc, Py_ssize_t end)
-
int
PyUnicodeEncodeError_SetEnd(PyObject *exc, Py_ssize_t end)
-
int
PyUnicodeTranslateError_SetEnd(PyObject *exc, Py_ssize_t end)
Set the end attribute of the given exception object to end. Return 0
on success, -1 on failure.
-
PyObject*
PyUnicodeDecodeError_GetReason(PyObject *exc)
-
PyObject*
PyUnicodeEncodeError_GetReason(PyObject *exc)
-
PyObject*
PyUnicodeTranslateError_GetReason(PyObject *exc)
Return the reason attribute of the given exception object.
-
int
PyUnicodeDecodeError_SetReason(PyObject *exc, const char *reason)
-
int
PyUnicodeEncodeError_SetReason(PyObject *exc, const char *reason)
-
int
PyUnicodeTranslateError_SetReason(PyObject *exc, const char *reason)
Set the reason attribute of the given exception object to reason. Return
0 on success, -1 on failure.
Recursion Control
These two functions provide a way to perform safe recursive calls at the C
level, both in the core and in extension modules. They are needed if the
recursive code does not necessarily invoke Python code (which tracks its
recursion depth automatically).
-
int
Py_EnterRecursiveCall(const char *where)
Marks a point where a recursive C-level call is about to be performed.
If USE_STACKCHECK is defined, this function checks if the OS
stack overflowed using PyOS_CheckStack(). In this is the case, it
sets a MemoryError and returns a nonzero value.
The function then checks if the recursion limit is reached. If this is the
case, a RuntimeError is set and a nonzero value is returned.
Otherwise, zero is returned.
where should be a string such as " in instance check" to be
concatenated to the RuntimeError message caused by the recursion depth
limit.
-
void
Py_LeaveRecursiveCall()
Ends a Py_EnterRecursiveCall(). Must be called once for each
successful invocation of Py_EnterRecursiveCall().
Standard Exceptions
All standard Python exceptions 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. For completeness, here are all
the variables:
Notes:
This is a base class for other standard exceptions.
This is the same as weakref.ReferenceError.
Only defined on Windows; protect code that uses this by testing that the
preprocessor macro MS_WINDOWS is defined.
-
Only defined on VMS; protect code that uses this by testing that the
preprocessor macro __VMS is defined.
Standard Warning Categories
All 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. For completeness, here are all
the variables:
Notes:
- This is a base class for other standard warning categories.
String Exceptions
Changed in version 2.6: All exceptions to be raised or caught must be derived from BaseException.
Trying to raise a string exception now raises TypeError.