Calling unknown Python functions

Question

This was the best name I could come up with for the topic and none of my searches yielded information relevant to the question.

How do I call a function from a string, i.e.

functions_to_call = ["func_1", "func_2", "func_3"]

for f in functions_to_call:
    call f

Answer 1

You can use the python builtin locals() to get local declarations, eg:

def f():
    print "Hello, world"

def g():
    print "Goodbye, world"

for fname in ["f", "g"]:
    fn = locals()[fname]
    print "Calling %s" % (fname)
    fn()

You can use the "imp" module to load functions from user-specified python files which gives you a bit more flexibility.

Using locals() makes sure you can't call generic python, whereas with eval, you could end up with the user setting your string to something untoward like:

f = 'open("/etc/passwd").readlines'
print eval(f+"()")

or similar and end up with your programming doing things you don't expect to be possible. Using similar tricks with locals() and dicts in general will just give attackers KeyErrors.

Answer 2

how do you not know the name of the function to call? Store the functions instead of the name:

functions_to_call = [int, str, float]

value = 33.5

for function in functions_to_call:
    print "calling", function
    print "result:", function(value)