Invoking a list of methods with correct arguments in Python

Question

I'm trying to execute a list of functions with the output of some functions being used as inputs to other ones.

A simplified example of what I'm trying to do:

def a():
    return 25

def b():
    return 50

def c(x, y):
    return x+y

Normally, I'd do: print c(a(), b())

But if I have a list of functions: l = [a,b,c]

How can I iterate over this list and get the right arguments passed into method c?

Thanks.

are the functions in the list guaranteed to be in that order? — C.B.
– C.B., Commented Jul 16, 2014 at 0:29
Your probably gonna need to include additional examples as well as a bit more information about how you intend to use it. — Nuclearman
– Nuclearman, Commented Jul 16, 2014 at 0:34
Do the lists of functions change size? Is it always the last function that gets called with the results of the functions leading up to it? — Marius
– Marius, Commented Jul 16, 2014 at 0:43

mhawke · Accepted Answer · 2014-07-16 15:13:49Z

2

My previous answer was not generic enough, so here's another bash at it. This uses inspect.getargspec() to determine the arity of each function prior to calling it. With that knowledge the required number of arguments can be sliced out of a "results list" which contains the results of all previous function calls.

There are some limitations:

inspect.getargspec() doesn't work for built-in functions, only user defined ones.
varargs and keyword args are ignored because this results in possibly infinite arguments which just complicates things.

Code:

import inspect

def a():
    return 25

def b():
    return 50

def c(x, y):
    return x + y

def d(x, y, z):
    return x + y + z

def e(x=100):
    return x * x

def f():
    pass


def execute_function_list(funcs):
    results = []
    for i, fn in enumerate(funcs):
        args_required = len(inspect.getargspec(fn).args)
        if args_required > 0:
            args = results[i-args_required:i]
        else:
            args = ()
        print("calling function %r with args %r" % (fn, args))
        results.append(fn(*args))
        print("after function returned results = %r" % results)
    return results[-1] if len(results) else None

if __name__ == '__main__':
    funcs = [e, a, b, c, d, e]
    print("final result is %r" % execute_function_list(funcs))

answered Jul 16, 2014 at 15:13

mhawke

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1 Comment

mottosan Over a year ago

This is the closest to what I'm trying to do and put me on the right track. Thank you.

mhawke · Accepted Answer · 2014-07-16 00:55:20Z

1

Assuming that the last function in the list is to be called with the results of all of the preceding functions in the list:

def c(*args):
    "Return the sum of all function arguments"
    return sum(args)

>>> l = [a, b, a, b, b, b, c]
>>> args=(f() for f in l[:-1])
>>> args
<generator object <genexpr> at 0x7fd441aabe60>
>>> l[-1](*args)
250

or in one line,

>>> l[-1](*(f() for f in l[:-1]))
250

edited Jul 16, 2014 at 0:55

answered Jul 16, 2014 at 0:49

mhawke

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2 Comments

mottosan Over a year ago

Thank you. While this works for the example in my question, what I have is a more generic list of functions that I need to execute in order. Some of these functions accept arguments whose values are returned by one or more previous functions. So I need to keep track of the value returned by the first method, for instance, and pass it into the 2nd and 3rd one, the output of method 3 goes into method 4 etc.

Nuclearman Over a year ago

In that case, what would be done with the output of the 2nd method? Would it not have an output (returns None) in that example? If the latter is the case, then it seems like a recursive helper method would do the job.

zhujs · Accepted Answer · 2014-07-16 01:07:20Z

0

You can try the built-in function apply:

apply( l[-1], [ func() for func in l[:-1] ] )

edited Jul 16, 2014 at 1:07

answered Jul 16, 2014 at 0:59

zhujs

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1 Comment

mhawke Over a year ago

This doesn't work, besides, apply has been deprecated since 2.3. This will do it though: apply( l[-1], [f() for f in l[:-1]] )

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