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

Answer 1

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.

Answer 2

One safe way is to map from names to functions. It's safer than using eval.

function_mappings = {         'add': add, }  def select_function():     while True:         try:             return function_mappings[raw_input('Please input the function you want to use')]         except KeyError:             print 'Invalid function, try again.'