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pythonstringfunctionstring-evaluation

I have a string whose content is a function name, how to refer to the corresponding function in Python?

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 an input to direct to that function like

w=raw_input('Please input the function you want to use')

or

w='add'

Is there any way to use w to refer to the function add?

Solution

  • 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.