Fredrik Tolf wrote: > On Mon, 2006-03-06 at 20:25 -0800, James Stroud wrote: > >>Fredrik Tolf wrote: >> >>>If I have a variable which points to a function, can I check if certain >>>argument list matches what the function wants before or when calling it? >>> >>>Currently, I'm trying to catch a TypeError when calling the function >>>(since that is what is raised when trying to call it with an illegal >>>list), but that has the rather undesirable side effect of also catching >>>any TypeErrors raised inside the function. Is there a way to avoid that? >>> >>>Fredrik Tolf >>> >>> >> >>Since python is "weakly typed", you will not be able to check what >>"type" of arguments a function expects. [...] > > > Sorry, it seems I made myself misunderstood. I don't intend to check the > type of the values that I pass to a function (I'm well aware that Python > is dynamically typed). The reason I refer to TypeError is because that > seems to be the exception raised by Python when I attempt to call a > function with an argument list that wouldn't be valid for it (for > example due to the number of parameters being wrong). I just want to > check in advance whether a certain arglist would be valid to call a > certain function. > > >>So, if you want to know the number of >>arguments expected, I've found this works: >> >>py> def func(a,b,c): >>... print a,b,c >>... >>py> func.func_code.co_argcount >>3 > > > Not very well, though: > >>>>def a(b, c, *d): pass > > ... > >>>>print a.func_code.co_argcount > > 2 > > Here, that would indicate that I could only call `a' with two arguments, > while in fact I could call it with two or more arguments. >
What is your goal? To avoid TypeError? In that case the minimum for no error is 2, as *d could be empty. It would then be safe to check against co_argcount to avoid errors: py> def a(a,b,*c): ... print a,b ... py> a(*range(a.func_code.co_argcount)) 0 1 py> a(*range(a.func_code.co_argcount + 5)) 0 1 py> a(*range(a.func_code.co_argcount - 1)) Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: a() takes at least 2 arguments (1 given) If the function requires 3 arguments to work, you may want to change the definition to reflect that fact as the *args are implicitly optional. This will help with using co_argcount as a test. James -- James Stroud UCLA-DOE Institute for Genomics and Proteomics Box 951570 Los Angeles, CA 90095 http://www.jamesstroud.com/ -- http://mail.python.org/mailman/listinfo/python-list
