Pythonic function composition
Peter Otten
__peter__ at web.de
Mon Oct 25 17:23:09 CEST 2004
Alan G Isaac wrote:
> Given a list of functions, it seems there must be a
> Pythonic approach to composition. Something like
>
> def compose(fns): return lambda x: reduce(lambda f,g: f(g),fns)(x)
>
> This will not work because the argument 'x' is not "inside".
> What is the proper formulation?
You need to pass a function that makes a function (a "factory") to reduce():
>>> fns = [lambda x: x+2, lambda x: x*2, lambda x: x*x]
>>> def compose(f, g):
... def fog(x):
... return f(g(x))
... return fog
...
>>> g1 = reduce(compose, fns)
>>> g1(2)
10
The same with lambdas:
>>> g2 = reduce(lambda f, g: lambda x: f(g(x)), fns)
>>> g2(2)
10
Peter
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