Python - passing a function as an argument to another function

Question

I have two files;

The first file, myProgram.py, is a python file with a number of functions
one of the functions contained inside is myFunction this function would be called with
```
import myProgram
myProgram.thisFunction()
myProgram.thatFunction()
```
The second file contains a menu system, with calls to the functions in myProgram.py

I'd like to call a particular function based on an argument passed to a function in file2

def file2_function(function):
    myProgram.function

file2_function(thisFunction(x,y,z))

which would essentially create myProgram.thisfunction(x,y,z) and execute it.

I guess I could do this using a bunch of if/elif statements, say:

def file2_function(function):
    if function == 1:
        myProgram.thisFunction
    elif function == 2: 
        myProgram.thatFunction

Though that could get messy as I'm already using a lot of if/elif/else statements for the menu system.

Is there a better way (or is the if/elif route the way to go?)

It's not clear why you need this `file2_function` in the first place. Why cannot you call `myProgram.whatever` directly? — georg, Nov 07 '13 at 11:37
Use [`getattr`](http://docs.python.org/2/library/functions.html#getattr), see also http://stackoverflow.com/questions/4075190/what-is-getattr-exactly-and-how-do-i-use-it — l4mpi, Nov 07 '13 at 11:45
with so many answers in a short space of time it's hard to decide who to give the answer to - getattr seems to be the easiest to maintain though — Simon, Nov 09 '13 at 04:52

score 4 · Answer 1 · answered Nov 07 '13 at 12:18

4

The *args in the function file2_function means the arguments to be passed to the function in which it calls:

def func1(a):
    print a

def file2_function(function,*args):
    function(*args)

file2_function(func1,4)
#4

answered Nov 07 '13 at 12:18

K DawG

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aga · Answer 2 · 2013-11-07T11:41:23.033

3

You can create a dictionary where key is the name of the function, and value is the function itself. Short example:

functions = { 'add' : lambda x,y : x + y, 'mult': lambda x,y : x * y, 'div' : lambda x,y : x / y, 'sub' : lambda x,y : x - y  }
functions['add'](1,2) # returns 3

edited Nov 07 '13 at 11:41

answered Nov 07 '13 at 11:34

aga

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Two downvotes in a row. Could you guys explain me what's wrong with the dictionary-based approach so I could correct my answer? – aga Nov 07 '13 at 12:20

score 2 · Answer 3 · answered Nov 07 '13 at 11:45

2

@aga gave you a good hint. When I'm writing cli applications I usually do smth like:

def func1():
    pass

def func2():
    pass

def func3():
    pass

def func_caller(name):
    func_dict = {
        'func1': func1,
        'func2': func2,
        'func3': func3
        }
    try:
        func_dict[name]()
    except KeyError:
        print "no such func"

answered Nov 07 '13 at 11:45

oleg.foreigner

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for imported functions it is pretty easy to write smth similar with `__all__`, `import *`, and `func.__name__` – oleg.foreigner Nov 07 '13 at 11:53
+1; when checking only for equality and you are just deciding which function to call, a dictionary is perfect for looking that function up. – poke Nov 07 '13 at 11:59
1

I actually upvoted, but there is a maintenance problem with coded dictionary approach, when you add or modify a function, you have to reflet this in dict, prone to errors. general solutions, such as django commands mechanism is looks more promizing for me. – alko Nov 07 '13 at 12:06
@alko yes, you are right. it really depends on the system you are coding. Django commands work with only `handle` method. all the logic to process input data is on a developer. sometimes it is pretty useful to look at interfaces(I know there are no interfaces in Python, I mean a concept of interfaces) and design patterns. – oleg.foreigner Nov 07 '13 at 12:19

score 0 · Accepted Answer · answered Nov 07 '13 at 11:43

0

Alternatively you have getattr. See following snippet:

>>> import sys
>>> getattr(sys,'exc_info')
<built-in function exc_info>
>>> getattr(sys,'exc_info')()
(None, None, None)

answered Nov 07 '13 at 11:43

alko

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downvoter, any comments, please :) there are reasons under this solution, I am pretty sure OP wants call functions by names – alko Nov 07 '13 at 11:44
1

Didn't downvote but fail to see how `sys.modules` is helpful in this case. `getattr` is, of course... – l4mpi Nov 07 '13 at 11:44
overkill, yep, i primarily thought of `sys.modules['__builtin__']` – alko Nov 07 '13 at 11:46
The question is asking about how to pass a function as a parameter. Your solution could be almost functionally equivalent but misses the gist of the question. – JPCF Oct 17 '14 at 13:19

Python - passing a function as an argument to another function

4 Answers4