Pythonic function composition
Oliver Fromme
olli at haluter.fromme.com
Mon Oct 25 17:26:02 CEST 2004
Alan G Isaac <aisaac at american.edu> 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?
There are probably several ways to do it.
The following is the one which come to my mind first.
The trick is to first define a function that composes
two functions, and then use reduce() to apply it to an
arbitrary number of functions.
>>> def compose2 (f, g):
... def h (x):
... return f(g(x))
... return h
...
>>> def compose (fns):
... return reduce(compose2, fns)
...
Some testing:
>>> def add42 (x): return x + 42
...
>>> def mul2 (x): return x * 2
...
>>> def sub5 (x): return x - 5
...
>>> def div3 (x): return x / 3
...
>>> a = compose((add42, mul2, sub5, div3))
>>> a(1)
27
There's no need to juggle with lambda in this case.
Lambda has its uses, but this isn't one of them.
Best regards
Oliver
--
Oliver Fromme, Konrad-Celtis-Str. 72, 81369 Munich, Germany
``All that we see or seem is just a dream within a dream.''
(E. A. Poe)
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