Passing arguments to functions in Python using inspect

Question

I have a Python script which creates a dictionary of its own functions and I'd like it to execute them by reading in a function name and arguments using YARP (knowledge of YARP is irrelevant to this question though).

I create a list of strings called "inc" which is populated by values coming into the program. The first item is a function name, and any other strings in the list are arguments. I create a dictionary called "methods" where the key is the function name and the value is a reference to the function object (using the inspect module). I store the return value of the function in a variable "result".

The snippet below shows a simplified version of what I'm using so far, which works fine, but can't handle functions with more than one argument. To circumvent this I use a list if a function needs more parameters:

if len(inc) == 1:
    result = methods[inc[0]]()   # call method with 0 arguments

elif len(inc) == 2:
    result = methods[inc[0]](inc[1])    # call method passing a string

else:
    args = []
    result = methods(inc[0])(inc[1:])    # call method passing a list

Ideally, I'd like to change this so that my functions can have any number of arguments, but I can't figure out how I can do this. I'm new to Python and I have looked at the documentation and various websites - I just can't find a solution. I've tried things like creating a tuple of the arguments, but that doesn't work either as it ends up passing the whole tuple in as one parameter.

Is there a better solution to this problem, like creating some kind of object which represents a set of parameters and passing that into the function? Any suggestions would be greatly appreciated!

Community · Accepted Answer · 2017-05-23 10:34:29Z

2

You should check out https://stackoverflow.com/a/3394898/1395668.

The magic you are looking for is the *. Apply this to your list and it unpacks the items into the argument fields of your function:

a = [ 1, 2, 3]
def myfunc(a, b, c):
    return a + b + c

print myfunc(*a)

Check out ** for the same approach for dict

edited May 23, 2017 at 10:34

CommunityBot

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answered Jul 17, 2012 at 10:32

Andrew Alcock

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Comments

Sven Marnach · Accepted Answer · 2012-07-17 10:32:00Z

0

It's a bit strange to have this kind of mixed structure:

inc = [func_name, arg1, arg2, ...]

Wouldn't it be much more natural to have two separate bits of information?

func_name = ...
args = [arg1, arg2, ...]

The you could do

methods[func_name](*args)

(Usually, I wouldn't bind the functions name to a variable, but preferably the function itself.)

answered Jul 17, 2012 at 10:32

Sven Marnach

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4 Comments

Andrew Alcock Over a year ago

The former approach looks like lisp to me - and nothing wrong with that.

MxB Over a year ago

It is strange and that isn't exactly the structure in my program - I put a list like that just to simplify the example. The actual program uses "Bottle" objects used by YARP.

MxB Over a year ago

Also, thank you for responding as the * was what I was missing! :-)

Sven Marnach Over a year ago

@AndrewAlcock: Different programming languages have different idioms, and it usually helps readability to stick to these idioms. In Python, lists are preferably homogeneous.

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