Python Cookbook

Composing Functions

Credit: Scott David Daniels

Problem

You need to construct a new function by composing existing functions (i.e., each call of the new function must call one existing function on its arguments, then another on the result of the first one).

Solution

Composition is a fundamental operation between functions that yields a new function as a result—the new function must call one existing function on its arguments, then another on the result of the first one. For example, a function that, given a string, returns a copy that is lowercase and does not have leading and trailing blanks, is the composition of the existing string.lower and string.trim functions (in this case, it does not matter in which order the two existing functions are applied, but generally, it could be important). A class defining the special method _ _call_ _ is often the best Pythonic approach to constructing new functions:

class compose: '''compose functions. compose(f,g,x...)(y...) = f(g(y...),x...))''' def _ _init_ _(self, f, g, *args, **kwargs): self.f = f self.g = g self.pending = args[:] self.kwargs = kwargs.copy( ) def _ _call_ _(self, *args, **kwargs): return self.f(self.g(*args, **kwargs), *self.pending, **self.kwargs) class mcompose(compose): '''compose functions. mcompose(f,g,x...)(y...) = f(*g(y...),x...))''' TupleType = type(( )) def _ _call_ _(self, *args, **kwargs): mid = self.g(*args, **kwargs) if isinstance(mid, self.TupleType): return self.f(*(mid + self.pending), **self.kwargs) return ...