From Python's sources object.c:
/* Test whether an object can be called */
int
PyCallable_Check(PyObject *x)
{
if (x == NULL)
return 0;
if (PyInstance_Check(x)) {
PyObject *call = PyObject_GetAttrString(x, "__call__");
if (call == NULL) {
PyErr_Clear();
return 0;
}
/* Could test recursively but don't, for fear of endless
recursion if some joker sets self.__call__ = self */
Py_DECREF(call);
return 1;
}
else {
return x->ob_type->tp_call != NULL;
}
}
It says:
- If an object is an instance of some class then it is callable iff it has
__call__
attribute.
- Else the object
x
is callable iff x->ob_type->tp_call != NULL
Desciption of tp_call
field:
ternaryfunc tp_call
An optional
pointer to a function that implements
calling the object. This should be
NULL if the object is not callable.
The signature is the same as for
PyObject_Call(). This field is
inherited by subtypes.
You can always use built-in callable
function to determine whether given object is callable or not; or better yet just call it and catch TypeError
later. callable
is removed in Python 3.0, use callable = lambda o: hasattr(o, '__call__')
Example, a simplistic cache implementation:
class Cached:
def __init__(self, function):
self.function = function
self.cache = {}
def __call__(self, *args):
try: return self.cache[args]
except KeyError:
ret = self.cache[args] = self.function(*args)
return ret
Usage:
@Cached
def ack(x, y):
return ack(x-1, ack(x, y-1)) if x*y else (x + y + 1)
Example from standard library, file site.py
, definition of built-in exit()
and quit()
functions:
class Quitter(object):
def __init__(self, name):
self.name = name
def __repr__(self):
return 'Use %s() or %s to exit' % (self.name, eof)
def __call__(self, code=None):
# Shells like IDLE catch the SystemExit, but listen when their
# stdin wrapper is closed.
try:
sys.stdin.close()
except:
pass
raise SystemExit(code)
__builtin__.quit = Quitter('quit')
__builtin__.exit = Quitter('exit')