Pythonic? ...Dynamically calling nested functions

Question

Is this a a pythonic implementation?

I'm calling nested functions dynamically from a string argument with wrappers to reduce the chance of calling a non-existent function. Here's an example where I want to perform different comparisons on arg1 and arg2 (in terms of ==, >=, < etc)...

class ComparisonClass(object):
    def__init__(self):
        pass

    def comparison(self,arg1,arg2,comparison):
        def equal_to():               
            pass
        def greater_than():
            pass
        def less_than():
            pass

        return locals()[comparison]()

    def comparison_equal_to(self,arg1,arg2):
        return self.comparison(arg1,arg2,'equal_to')

    def comparison_greater_than(self,arg1,arg2):
        return self.comparison(arg1,arg2,'greater_than')

    def comparison_less_than(self,arg1,arg2):
        return self.comparison(arg1,arg2,'less_than')

Karl Knechtel · Accepted Answer · 2011-12-16 12:48:48Z

6

Implementation is irrelevant here because the very thing you seek to implement isn't Pythonic.

In Python, we use the __eq__ family of methods for comparisons, and there exist tools in the standard library for implementing some of them in terms of others.

answered Dec 16, 2011 at 12:48

Karl Knechtel

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2 Comments

burn5 Over a year ago

Thanks for your answer! I'm fairly new to Python and am not familiar with the eq family. Naturally I will seek out the documentation but could you maybe give me a brief explanation of what it is? It might also help if I add that there is much more to the comparisons (ie greater_than() involves more than asking 'is arg1 > arg2') but I have omitted that here for simplicity. I guess I really shouldn't have specified comparisons and merely asked if it was a pythonic implementation of dynamically calling nested functions? Apologies if I am missing the point anywhere!

Karl Knechtel Over a year ago

I'm mostly just having difficulty figuring out what sense you make of the phrase "dynamically calling nested functions". To the extent that I understand it, Daniel's answer is more than adequate. For __eq__ et. al., see the documentation: docs.python.org/reference/datamodel.html#object.__lt__

Daniel Roseman · Accepted Answer · 2011-12-16 13:25:06Z

1

What you're missing is that in Python, functions are first-class objects. That means they can be assigned to variables and passed around just like any other object.

So, rather than nesting a function and calling it within the parent function, you just want to assign the relevant function as the return value of your function, and call it on return. Or even better, assign a dictionary at class level containing the functions:

def equal_to():
       pass

   (etc)

COMPARISONS = {
    'equal_to': equal_to,
    etc
}

Now you can call COMPARISONS['equal_to'](arg1, arg2).

answered Dec 16, 2011 at 13:25

Daniel Roseman

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