Converting a method with 2 variables to one variable

Question

I have a function foo that takes 2 arguments a and b. I want to create a list of functions that are similar to foo, but value of a is fixed.

def foo(a,b): 
   return a*b

fooList = []
for i in range(n):
   fooList.append(foo(i))

I want fooList[i] to return a function that is similar to foo(i, b).

Use `lambda` or `functools.partial`. But beware late binding: https://stackoverflow.com/q/19837486/3001761 — jonrsharpe, Jul 01 '17 at 20:29
This is a simple example. I might use a more complex function instead of simple foo. — 2ank3th, Jul 01 '17 at 20:29

cs95 · Answer 1 · 2017-07-01T20:34:26.093

1

You can use functools.partial:

from functools import partial

def foo(a, b):
    return a * b

fooList = []
for i in range(n):
   fooList.append(partial(foo, i))

Then you'd do fooList[i](5) to calculate i * 5.

You can also curry lambda functions, somewhat like this:

for i in range(n):
    fooList.append((lambda x: lambda y: x * y)(i))

You can then call that the same way as above.

edited Jul 01 '17 at 20:34

answered Jul 01 '17 at 20:30

cs95

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@idjaw https://en.wikipedia.org/wiki/Currying :) – cs95 Jul 01 '17 at 20:32
3

...you test this? Binding `lambda`s in a loop has behaviour that may surprise you. – jonrsharpe Jul 01 '17 at 20:32
2

If you are naming your lambda, you shouldn't be using a lambda! – juanpa.arrivillaga Jul 01 '17 at 20:33
@juanpa.arrivillaga Erm, good point. Fixed. – cs95 Jul 01 '17 at 20:37
@jonrsharpe I had no idea about late binding. Thanks, I learned something new. – cs95 Jul 01 '17 at 20:39
Tested this code and it works as expected for the curried lambda functions. – cs95 Jul 01 '17 at 20:41

Converting a method with 2 variables to one variable

1 Answers1