Why Python Closures Print 6,6,6,6 and How to Get 0,2,4,6

Accidentally encountered a Python interview question: what does the following code output?

def testFun():
    temp = [lambda x : i*x for i in range(4)]
    return temp

for everyLambda in testFun():
    print (everyLambda(2))

Many assume the output is 0,2,4,6, but the actual result is 6,6,6,6 due to Python's late binding in closures.

Late binding means the variable i is looked up when the inner function is called, after the loop has finished, so i equals its final value 3 for all lambdas, producing 3*2 = 6.

To obtain the expected 0,2,4,6, you can bind the current value of i at definition time. Four solutions are presented:

Solution 1: Use default argument to capture i

def testFun():
    temp = [lambda x, i=i: i*x for i in range(4)]
    return temp

for everyLambda in testFun():
    print (everyLambda(2))

Solution 2: Use functools.partial

from functools import partial
from operator import mul

def testFun():
    return [partial(mul, i) for i in range(4)]

for everyLambda in testFun():
    print (everyLambda(2))

Solution 3: Use a generator expression

def testFun():
    return (lambda x, i=i: i*x for i in range(4))

for everyLambda in testFun():
    print (everyLambda(2))

Solution 4: Use yield with lazy evaluation

def testFun():
    for i in range(4):
        yield lambda x: i*x

for everyLambda in testFun():
    print (everyLambda(2))

All four approaches produce the desired output 0,2,4,6.