How to convert a string to a function name in python?

Question

For example, if I have a function called add like

def add(x,y):
    return x+y

and I want the ability to convert a string or a

Answer 1

[I got here via a duplicate question. My first thought was to use argparse and shlex and I didn't see that here, so I'm adding it as another option.]

You could use argparse to set up a registry of functions/commands and safely parse their args. This will provide some level of user-friendliness too by, e.g., letting you know when you've entered a command that doesn't exist.

import argparse
import shlex

def hello(name):
    print('hello,', name)

def main():
    parser = argparse.ArgumentParser()
    subparsers = parser.add_subparsers()

    hello_parser = subparsers.add_parser('hello')
    hello_parser.add_argument('name')
    hello_parser.set_defaults(func=hello)

    print('Enter q to quit')

    while True:
        command = input('command> ')
        command = command.strip()

        if not command:
            continue

        if command.lower() == 'q':
            break

        words = shlex.split(command)

        try:
            args = parser.parse_args(words)
        except SystemExit:
            # argparse will sys.exit() on -h and errors; prevent that
            continue

        func_args = {name: value for name, value in vars(args).items()}
        del func_args['func']
        args.func(**func_args)

if __name__ == '__main__':
    try:
        main()
    except KeyboardInterrupt:
        print()

Answer 2

The built-in function eval will do what you want. All the usual warnings about executing arbitrary user-supplied code apply.

If there are a finite number of predefined functions, you should avoid eval and use a lookup table instead (i.e. Dict). Never trust your users.

Answer 3

Since you are taking user input, the safest way is to define exactly what is valid input:

dispatcher={'add':add}
w='add'
try:
    function=dispatcher[w]
except KeyError:
    raise ValueError('invalid input')

If you want to evaluate strings like 'add(3,4)', you could use safe eval:

eval('add(3,4)',{'__builtins__':None},dispatcher)

eval in general could be dangerous when applied to user input. The above is safer since __builtins__ is disabled and locals is restricted to dispatcher. Someone cleverer than I might be able to still cause trouble, but I couldn't tell you how to do it.

WARNING: Even eval(..., {'__builtins__':None}, dispatcher) is unsafe to be applied to user input. A malicious user could run arbitrary functions on your machine if given the opportunity to have his string evaluated by eval.