List lambda

Question

Hi!

def cm():
return [lambda x: i*x for i in range(3)]

for m in cm():
print m(1)

Output:222

And my question is about lambda. The expression in this
[lambda x: i*x for i in range(3)]
Work as
lambda x: 2*x for i in range(3)]
And return 222

I do not understand why that

After all range(3)=0,1,2 but no 2,2,2

Why it returns 2,2,2?

Answer 1

If we unpack the list comprehension and lambda function, cm() rougly looks like

def cm():
def f(x):
return i*x
funcs = []
for i in range(3):
funcs.append(f)
return funcs

Now the issue seems clearer: All three functions in the list refer to the same variable i, which is not local to them. So when these fuctions are called, 2, the last value of i coming from iterating over range(3), is used. For more experiments, we can try

def cm():
def f(x):
return i*x
funcs = []
for i in range(3):
funcs.append(f)
i = 10
return funcs

This time output is three 10s, as 10 is the last value of i. A quick way to avoid this is to save i as a variable local to the lambdas:

def cm():
return [lambda x, j=i: j*x for i in range(3)]

A variant of this is also in the Python FAQ:
https://docs.python.org/3/faq/programming.html#why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result

Answer 2

Maybe this is what you want to do🤔
https://code.sololearn.com/cJ070QKCnhBy