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 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 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 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_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(): 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 the 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. 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 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 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.

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, ...)

Return value: Always NULL.

This function sets the error indicator and returns NULL.. exception should be a Python exception (string or class, not an instance). format should be a string, containing format codes, similar to printf(). The width.precision before a format code is parsed, but the width part is ignored.

Character Meaning

c Character, as an int parameter
d Number in decimal, as an int parameter
x Number in hexadecimal, as an int parameter
x 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.

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();" when the system call returns an error.

PyObject* PyErr_SetFromErrnoWithFilename(PyObject *type, char *filename): Return value: Always NULL.
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 instance.

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_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.

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 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_DeprecationWarning, PyExc_SyntaxWarning, and PyExc_RuntimeWarning. 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.

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 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, PyObject *base, PyObject *dict): Return value: New reference.
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 message.

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