Python: exec "function()"

Question

I'm coding a simple test program in python as part of my greater program, but i would l like to pass a sub function name into the main function, so the main function can run the subfunction.

eg:

import datetime;

def cronjob(repeat, interval, task):
    if (str(repeat) == 'inf'):
        repeat = 99999999999999999999;
    position = 0;
    interval = datetime.timedelta(seconds=interval);
    x = datetime.datetime.now()
    while not (position >= repeat):
        while (datetime.datetime.now() - x < interval):
            pass;
        x = datetime.datetime.now();
        position += 1;
        exec task;

def test():
    print "hello";

cronjob(10, 0.1, 'test');

EDIT: Already fixed this, but since nothing is listed here, let me show you how to do it in case someone else needs it.

I fiddled with eval() and exec, and tried just eval(task). that didn't throw an error, so I tried print eval(task) and sure enough, it listed the function's memory address [that is, test()]. finally, I have used eval(task); to then call that function. below is the code fixing this:

import datetime;

def cronjob(repeat, interval, task):
    if (str(repeat) == 'inf'):
        repeat = 99999999999999999999;
    position = 0;
    interval = datetime.timedelta(seconds=interval);
    x = datetime.datetime.now()
    while not (position >= repeat):
        while (datetime.datetime.now() - x < interval):
            pass;
        x = datetime.datetime.now();
        position += 1;
        eval(task);

def test():
    print "hello";

cronjob(10, 0.1, 'test()');

sleeplessnerd · Accepted Answer · 2012-03-04 06:48:54Z

2

Why not pass the function object itself to the scheduler ?

test is an object and can be used as an argument too!

def test():
  print "test"

def exe(func):
  func()

exe(test)

answered Mar 4, 2012 at 6:48

sleeplessnerd

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4 Comments

D. Brumby Over a year ago

Because my main program will only be able to send strings, so i have to use a string.

sleeplessnerd Over a year ago

Are you sure? - This seems like an arbitrary constraint. I dont want to be that guy :) - But You could try to avoid this stringy stuff. Furthermore, if you give a string "test" the cron executor still needs to find the corresponding function. So if your main program is so seperated from the cron executor that you can only pass strings, I doubt, that the specified function is accessible in the namespace of the executor.

D. Brumby Over a year ago

unfortunately, the program i am writing for is closed source. I am making a tcp proxy server, and the messages will need to be interpreted on the go. Though, this task will only be used by internal functions with one other interpreting, so you may be right about the possibility of a work around. I'll look into it. Cheers.

zenpoy Over a year ago

@DwayneBrumby - If your program gets the input through stdin you can use the Python function input to get the string parsed as python script exe(input("enter function to execute:\n"))

Cameron · Accepted Answer · 2012-03-04 06:51:25Z

0

I believe since functions are objects, you can just pass one in to the "controlling" function by name, so you don't need the exec call (which is usually used for dynamic code execution).

e.g.

def foo(a_number, a_function):
    print a_number
    a_number += 1
    a_function(a_number)

def bar(another_number):
    print another_number

foo(5, bar)

should produce the output:
5
6

answered Mar 4, 2012 at 6:51

Cameron

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Comments

bereal · Accepted Answer · 2012-03-04 07:04:34Z

0

In case if you're absolutely sure you want to derive a function from a string, you may want to use a dict as a mapping from strings to functions like follows:

dispatcher = {'dothis': module1.do_this,
              'dothat': module2.to_that}

def foo(fun):
    fun(arg1, arg2)

def bar(action):
    fun = dispatcher.get(action)
    if fun:
       foo(fun)

This will be much more secure (as action is likely to come from the outside) and provide better decoupling of internal code structure from the external API.

answered Mar 4, 2012 at 7:04

bereal

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