CODEWITHSUNDEEP
pythonfunctional-programming
yasar
yasar

Reputation: 13738

Flipping a function's argument order in Python

Nowadays, I am starting to learn haskell, and while I do it, I try to implement some of the ideas I have learned from it in Python. But, I found this one challenging. You can write a function in Haskell, that takes another function as argument, and returns the same function with it's arguments' order flipped. Can one do similiar thing in Python? For example,

def divide(a,b):
    return a / b

new_divide = flip(divide)

# new_divide is now a function that returns second argument divided by first argument

Can you possibly do this in Python?

Upvotes: 15

Views: 17271

Answers (2)

georg
georg

Reputation: 214949

In a pure functional style:

flip = lambda f: lambda *a: f(*reversed(a))

def divide(a, b):
    return a / b

print flip(divide)(3.0, 1.0)

A bit more interesting example:

unreplace = lambda s: flip(s.replace)

replacements = ['abc', 'XYZ']
a = 'abc123'
b = a.replace(*replacements)
print b
print unreplace(b)(*replacements) # or just flip(b.replace)(*replacements)

Upvotes: 18

Raymond Hettinger
Raymond Hettinger

Reputation: 226256

You can create a closure in Python using nested function definitions. This lets you create a new function that reverses the argument order and then calls the original function:

>>> from functools import wraps
>>> def flip(func):
        'Create a new function from the original with the arguments reversed'
        @wraps(func)
        def newfunc(*args):
            return func(*args[::-1])
        return newfunc

>>> def divide(a, b):
        return a / b

>>> new_divide = flip(divide)
>>> new_divide(30.0, 10.0)
0.3333333333333333

Upvotes: 27

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