Variable function call in Python

Question

I'm designing a tool which recursively identifies upstream pathways in a vector river network. I'd like to be able to truncate the search if the cumulative path cost passes a test which is defined by the user, and I'm not sure whether this is possible. Below is an example of the sort of thing I'd like to do:

def test1(value):
    if value > 1: return True
    else: return False

def test2(value):
    if value % 4: return True
    else: return False

def main(test):
    for i in range(20):
        if SPECIFIEDTEST(i):
            print i
            break

main(test1)

I know exec() can be used for this purpose but I also understand this is frowned upon? Is there another, better method for passing a function to another function?

Functions are first-class entities in python: you can simply pass a function to another function, or bind a local variable to the function and call it that way. So I'm confused by what you're confused about, e.g., did you mean you're looking for something like f = {'test1':test1, 'test2':test2}[key]? (after which f(i) calls the desired function) — torek
– torek, Commented Feb 25, 2014 at 17:44
def test1(value): return value > 1, etc. You rarely need to use the actual literals True and False. — chepner
– chepner, Commented Feb 25, 2014 at 18:03

sgarza62 · Accepted Answer · 2014-02-25 18:01:17Z

4

Building a dictionary for this is unnecessary, as functions can be passed in as parameters to other functions. Simply pass the function into main()

def test1(value):
    if value > 1: return True
    else: return False

def test2(value):
    if value % 4: return True
    else: return False

# using test1 as the default input value...
def main(test = test1):
    for i in range(20):
        if test(i):
            print i
            break

main(test1)
# prints 2

main(test2)
# prints 1

answered Feb 25, 2014 at 18:01

sgarza62

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3 Comments

tzaman Over a year ago

Given the poster mentions user-definition and asks about exec I'm assuming string input is involved; that's why a dict is needed.

triphook Over a year ago

Sorry, this was a really basic question. I simply wasn't sure whether it was possible to pass a function to another function directly. Thanks for the help.

sgarza62 Over a year ago

@tzaman You're right, that's a good assumption. In the case of user inputs as strings, a dictionary would be a good fit. Either way, a variable holding a function is still being passed in.

tzaman · Accepted Answer · 2014-02-25 17:45:38Z

4

You can create a dictionary that maps function names to the actual functions, and use that:

tests = {'test1': test1, 'test2': test2}
if tests[specifiedtest](i):
    ...

answered Feb 25, 2014 at 17:45

tzaman

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