Better way to call a chain of functions in python?

Question

I have a chain of operations which needs to occur one after the other and each depends on the previous function's output.

Like this:

out1 = function1(initial_input)
out2 = function2(out1)
out3 = function3(out2)
out4 = function4(out3)

and so on about 10 times. It looks a little ugly in the code.

What is the best way to write it? Is there someway to handle it using some functional programming magic? Is there a better way to call and execute this function chain?

I think that you could use "reduce": take a look [here](http://stackoverflow.com/questions/17122644/compute-a-chain-of-functions-in-python) — smeso, Dec 08 '13 at 13:51
@Faust Looks super interesting. Thank you for sharing that link! — ComputerFellow, Dec 08 '13 at 13:52

Nicolas Barbey · Answer 1 · 2013-12-08T14:06:53.107

7

You can use functools.reduce:

out = functools.reduce(lambda x, y : y(x), [f1, f2, f3, f4], initial_value)

Quoting functools.reduce documentation:

Apply a function of two arguments cumulatively to the items of a sequence, from left to right, so as to reduce the sequence to a single value. For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5). If initial is present, it is placed before the items of the sequence in the calculation, and serves as a default when the sequence is empty.

Here, we use the fact that functions can be treated as any variable in Python, and an anonymous functions which simply does "apply x to function y".

This "reduce" operation is part of a very general pattern which have been applied successfully to parallelize tasks (see http://en.wikipedia.org/wiki/MapReduce).

edited Dec 08 '13 at 14:06

answered Dec 08 '13 at 13:52

Nicolas Barbey

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Haha, this looks magical! Thank you! :) Could you just give a basic explanation for this? – ComputerFellow Dec 08 '13 at 13:54
@phg and Nicolas - just wondering what the performance might be of this versus the iterative (for loop) answer falsetru gave. – ComputerFellow Dec 08 '13 at 14:04
I timed it to 4 us vs 3us with the for loop on a toy example. But I don't know if this is very relevant. – Nicolas Barbey Dec 08 '13 at 14:14

score 5 · Accepted Answer · answered Dec 08 '13 at 13:46

5

Use a loop:

out = initial_input
for func in [function1, function2, function3, function4]:
    out = func(out)

answered Dec 08 '13 at 13:46

falsetru

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Wow. Why didn't I think of this. This is genius! – ComputerFellow Dec 08 '13 at 13:47
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You don't need the `[]` either, this works too: `for fn in f1, f2, f3:` – thebjorn Dec 08 '13 at 14:40
@thebjorn, I am reading [The for statement](http://docs.python.org/2/reference/compound_stmts.html#the-for-statement) :) – falsetru Dec 08 '13 at 14:45
@falsetru :-) Click through to expression_list: http://docs.python.org/2/reference/expressions.html#expression-lists – thebjorn Dec 08 '13 at 14:49

score 2 · Answer 3 · answered Dec 08 '13 at 13:53

To propagate functional programming a bit:

In [1]: compose = lambda f, g: lambda arg: f(g(arg))

In [2]: from functools import reduce

In [3]: funcs = [lambda x:x+1, lambda x:x*2]

In [4]: f = reduce(compose, funcs)

In [5]: f(1)
Out[5]: 3

In [6]: f(3)
Out[6]: 7

score 1 · Answer 4 · answered Dec 08 '13 at 13:47

1

You can pass the return values directly to the next function:

out4 = function4(function3(function2(function1(initial_input))))

But this isn't necessarily better, and is perhaps less readable.

answered Dec 08 '13 at 13:47

jonrsharpe

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Better way to call a chain of functions in python?

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