Vermillion - 1 year ago 81

Python Question

I am trying to write a function that composes any number of lambda functions.

I have two simple lambda functions.

`f = lambda x: x + 1`

g = lambda x: x**2

My attempt at a composing function is this:

`def compose(*functions):`

composed = lambda x: x # Function that returns input

for function in reversed(functions):

composed = lambda x: function(composed(x))

return composed

My thinking was to loop through the variable number of functions, each time making the

`composed`

Then I could make a function that is a composition of

`f`

`g`

`c = compose(f, g)`

So calling

`c(5)`

`f(g(5))`

`RecursionError: maximum recursion depth exceeded`

I thought introducing an intermediate variable might fix the problem.

`def compose(*functions):`

composed = lambda x: x # Function that returns input

for function in reversed(functions):

intermediate = lambda x: function(composed(x))

composed = intermediate

return composed

But the same error is raised.

Is there a way to fix this?

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Answer Source

First, I think your approach will suffer from *late closure binding*, as `function`

in lambda will only take the last value of function at the end of the iteration. Secondly, `composed`

will only call itself recursively in the end also due to the first reason; `composed`

- the lambda - calls the last value of `composed`

- itself!

One possible fix is to bind `composed`

and `function`

to the `lambda`

at each iteration:

```
def compose(*functions):
composed = lambda x: x
for function in reversed(functions):
composed = lambda x, function=function, composed=composed: function(composed(x))
return composed
print(compose(f, g)(5))
# 26
```

But your overall problem looks like a good use case for `functools.reduce`

:

```
from functools import reduce
def compose(*functions):
def inner(v):
return reduce(lambda x, y: y(x), reversed(functions), v)
return inner
print(compose(f, g)(5))
# 26
```

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