Python closure

Posted by chunyang on May 5, 2020
TAGS: #python

Python closure usually makes code simpified and are used in functional programming style. It starts with a keyword lambda and the last statement is the result returned. You can also create functions inside another function. A lambda is a anonymous function.

Basic grammar

You can create a closure with the keyword lambda.

import operator
from functools import reduce

a = [1, 2, 3]

# Increase each element in a and get a new list
b = list(map(lambda _: _ + 1, a))

# Get all even numbers
c = list(filter(lambda _: _%2==0, b))

d = reduce(operator.add, c, 0)

Auto capture variables

A lambda function can automatically capture variables in its environment.

def a():
    b = 3
    return lambda: b

c = a()

But you need to be aware that there are boundaries where you can see a new variable.

  • Module: module level variables
  • Class: variables defined in a class
  • Function: variables defined in a function

If you are not familiar with those boundaries, you will make mistakes that you need to refer an existing variable instead of creating a new variable.

def func():
    a = 3
    def b():
        a = 4

The variable a is a new variable in function b, not the same variable in a.

Pitfall for capturing

a = [1, 2, 3]

fns = []

for v in a:
    fns.append(lambda: v)
for fn in fns:

We are expecting three different number printed on the screen. But actually 3 is repeated three times. We are capturing the same variable. To fix the problem, we can add an argument to the anonymous function and pass the argument. Python will evaluate the argument first.

You cannot use print in lambda

You cannot write print statement in a lambda function. But you can use pprint instead.

from pprint import pprint

a = [1,2,3]

list(map(lambda _:pprint(_), a))