Python Cookbook

Associating Parameters with a Function (Currying)

Credit: Scott David Daniels, Ben Wolfson, Nick Perkins, and Alex Martelli

Problem

You need to tweak a function (or other callable) to get another callable with fewer formal arguments, keeping the original arguments’ given values fixed (i.e., you need to curry a callable to make another).

Solution

Curry is not just a spice used in Asian cuisine; it’s also an important technique in Python and other programming languages:

class curry:
    def _ _init_ _(self, fun, *args, **kwargs):
        self.fun = fun
        self.pending = args[:]
        self.kwargs = kwargs.copy(  )

    def _ _call_ _(self, *args, **kwargs):
        if kwargs and self.kwargs:
            kw = self.kwargs.copy(  )
            kw.update(kwargs)
        else:
            kw = kwargs or self.kwargs

        return self.fun(*(self.pending + args), **kw)

Discussion

Popular in functional programming, currying is a way to bind some of the function’s arguments and wait for the rest of the arguments to show up later. Currying is named in honor of Haskell Curry, a mathematician who laid some of the cornerstones in the theory of formal systems and processes. The curry function defined in this recipe is called with a callable and some or all of the arguments to the callable. The curry function returns a function that takes subsequent parameters as arguments, and curry calls the original with all of those parameters. This recipe uses a class instance to hold the curried parameters until they’re needed. For example:

double = curry(operator.mul, 2) triple = curry(operator.mul, ...