How to create a closure

Since loops in Python don't create scopes, you usually need an extra function to create closures. The straightforward way doesn't work:

multipliers = []
for i in range(10):
    multipliers.append(lambda x: x * i)

[multipliers[i](2) for i in range(5)]
# [18, 18, 18, 18, 18]

Let's add the extra function:

multiplier_creator = lambda i: lambda x: x * i
for i in range(10):
    multipliers.append(multiplier_creator(i))

It works this way, but the code can be clumsy, especially if you need def, not lambda:

def multiplier_creator(i):
    def multiplier(x):
        return x * i
    for i in range(10):
        return multiplier

    multipliers.append(multiplier_creator(i))

To make it slightly more readable, you can write universal function and get partials of it:

multiplier = lambda x, i: x * i
for i in range(10):
    multipliers.append(partial(multiplier, i=i))

You can always emulate partial with custom lambda, but repr of partials are generally more readable:

from functools import partial

In : partial(int, base=2)
Out: functools.partial(<class 'int'>, base=2)

In : lambda x: int(x, base=2)
Out: <function __main__.<lambda>>

Fun fact: thanks to the operator module this particular example can be expressed even more appealing:

from functools import partial
import operator
multipliers = []

for i in range(10):
    multipliers.append(partial(operator.mul, i))