Python produce function names from string

Question

I would like to use functions based on strings passed in argument as below. The code gives the following error: AttributeError: 'Obj' object has no attribute 'funA'

The functions funA and funB are defined within fun because they are only used within fun and nowhere else

class Obj(object):
    def __init__(self):
        self.A = 2
        self.B = 3

    def fun(self, ar):
        def funA(self):
            print self.A

        def funB(self):
            x = self.B + 4
            print self.B

        for letter in ar:
            name = 'fun' + letter
            getattr(self, name)()

obj_instance = Obj()
obj_instance.fun(['A', 'B'])

The problem is not with the getattr call, but with the functions themselves. Those are not attributes of self (the Obj instance), but local variables inside fun. — Daniel Roseman
– Daniel Roseman, Commented Dec 6, 2017 at 10:17
I don't see why you would declare a function inside a function if you don't need to call it several times and/or for code readability. — IMCoins
– IMCoins, Commented Dec 6, 2017 at 10:22

Tobias Scheck · Accepted Answer · 2017-12-06 10:35:06Z

1

As an alternative you can try the following code. It gives you the opportunity to check whether a function exists or not.

class Obj(object):
    def __init__(self):
        self.A = 2
        self.B = 3

    def fun(self, ar):
        def funA():
            print self.A

        def funB():
            x = self.B + 4
            print self.B
            print x

        for letter in ar:
            name = 'fun' + letter
            if name in locals():
                locals()[name]()

obj_instance = Obj()
obj_instance.fun(['A', 'B'])

answered Dec 6, 2017 at 10:35

Tobias Scheck

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2 Comments

user58925 Over a year ago

Thanks Tobias. Your code works perfectly well for my purposes. Is there a reason why you use "if name in locals()", why not just leave is out?

Tobias Scheck Over a year ago

As I wrote, it's just a sample how to check whether a function exists in your local scope or not. You can leave it out, but it's not safe. E.g. if you try to call obj_instance.fun(['A', 'B', 'C']) than you get an exception for 'C' because there is no funC in your local scope. This happens not if you use the check condition.

XaMi · Accepted Answer · 2017-12-06 10:29:39Z

0

I wouldn't recommend it, but if you really need it, then you can do it this way:

class Obj(object):
    def __init__(self):
        self.A = 2
        self.B = 3

    def fun(self, ar):
        def funA(self):
            print self.A

        def funB(self):
            x = self.B + 4
            print self.B

        for letter in ar:
            name = 'fun' + letter + '()'
            exec(name)

obj_instance=Obj()
obj_instance.fun(['A', 'B'])

exec() executes any string that you put in parentheses as if it was python code.

answered Dec 6, 2017 at 10:29

XaMi

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1 Comment

user58925 Over a year ago

This code generates the following error message: TypeError: funA() takes exactly 1 argument (0 given)

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