1.2 Intermezzo: Errors and Exceptions
An important convention throughout the Python interpreter is the
following: when a function fails, it should set an exception condition
and return an error value (usually a NULL pointer). Exceptions
are stored in a static global variable inside the interpreter; if this
variable is NULL no exception has occurred. A second global
variable stores the ``associated value'' of the exception (the second
argument to raise). A third variable contains the stack
traceback in case the error originated in Python code. These three
variables are the C equivalents of the Python variables
sys.exc_type, sys.exc_value and sys.exc_traceback (see
the section on module sys in the
Python Library Reference). It is
important to know about them to understand how errors are passed
around.
The Python API defines a number of functions to set various types of exceptions.
The most common one is PyErr_SetString(). Its arguments are an exception object and a C string. The exception object is usually a predefined object like PyExc_ZeroDivisionError. The C string indicates the cause of the error and is converted to a Python string object and stored as the ``associated value'' of the exception.
Another useful function is PyErr_SetFromErrno(), which only takes an exception argument and constructs the associated value by inspection of the global variable errno. The most general function is PyErr_SetObject(), which takes two object arguments, the exception and its associated value. You don't need to Py_INCREF() the objects passed to any of these functions.
You can test non-destructively whether an exception has been set with PyErr_Occurred(). This returns the current exception object, or NULL if no exception has occurred. You normally don't need to call PyErr_Occurred() to see whether an error occurred in a function call, since you should be able to tell from the return value.
When a function f that calls another function g detects
that the latter fails, f should itself return an error value
(usually NULL or -1). It should not call one of the
PyErr_*() functions -- one has already been called by g.
f's caller is then supposed to also return an error indication
to its caller, again without calling PyErr_*(),
and so on -- the most detailed cause of the error was already
reported by the function that first detected it. Once the error
reaches the Python interpreter's main loop, this aborts the currently
executing Python code and tries to find an exception handler specified
by the Python programmer.
(There are situations where a module can actually give a more detailed error message by calling another PyErr_*() function, and in such cases it is fine to do so. As a general rule, however, this is not necessary, and can cause information about the cause of the error to be lost: most operations can fail for a variety of reasons.)
To ignore an exception set by a function call that failed, the exception condition must be cleared explicitly by calling PyErr_Clear(). The only time C code should call PyErr_Clear() is if it doesn't want to pass the error on to the interpreter but wants to handle it completely by itself (possibly by trying something else, or pretending nothing went wrong).
Every failing malloc() call must be turned into an exception -- the direct caller of malloc() (or realloc()) must call PyErr_NoMemory() and return a failure indicator itself. All the object-creating functions (for example, PyInt_FromLong()) already do this, so this note is only relevant to those who call malloc() directly.
Also note that, with the important exception of
PyArg_ParseTuple() and friends, functions that return an
integer status usually return a positive value or zero for success and
-1 for failure, like Unix system calls.
Finally, be careful to clean up garbage (by making Py_XDECREF() or Py_DECREF() calls for objects you have already created) when you return an error indicator!
The choice of which exception to raise is entirely yours. There are predeclared C objects corresponding to all built-in Python exceptions, such as PyExc_ZeroDivisionError, which you can use directly. Of course, you should choose exceptions wisely -- don't use PyExc_TypeError to mean that a file couldn't be opened (that should probably be PyExc_IOError). If something's wrong with the argument list, the PyArg_ParseTuple() function usually raises PyExc_TypeError. If you have an argument whose value must be in a particular range or must satisfy other conditions, PyExc_ValueError is appropriate.
You can also define a new exception that is unique to your module. For this, you usually declare a static object variable at the beginning of your file:
static PyObject *SpamError;
and initialize it in your module's initialization function (initspam()) with an exception object (leaving out the error checking for now):
PyMODINIT_FUNC
initspam(void)
{
PyObject *m;
m = Py_InitModule("spam", SpamMethods);
if (m == NULL)
return;
SpamError = PyErr_NewException("spam.error", NULL, NULL);
Py_INCREF(SpamError);
PyModule_AddObject(m, "error", SpamError);
}
Note that the Python name for the exception object is spam.error. The PyErr_NewException() function may create a class with the base class being Exception (unless another class is passed in instead of NULL), described in the Python Library Reference under ``Built-in Exceptions.''
Note also that the SpamError variable retains a reference to the newly created exception class; this is intentional! Since the exception could be removed from the module by external code, an owned reference to the class is needed to ensure that it will not be discarded, causing SpamError to become a dangling pointer. Should it become a dangling pointer, C code which raises the exception could cause a core dump or other unintended side effects.
We discuss the use of PyMODINIT_FUNC as a function return type later in this sample.
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Extending and Embedding the Python Interpreter |  |
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Release 2.4.4c1, documentation updated on 11 October 2006.