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(python2.1-api.info)Exception Handling
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 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.
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()'
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, ...)'
This function sets the error indicator. 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.
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 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)'
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 (e.g.
`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)'
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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