Deep copy index integer using lambda

Question

func = []

for value in range(5):
    func.append(lambda: print_index(value))


def print_index(index_to_print):
    print(index_to_print)


for a in func:
    a()

When I run the code above, the output is

4
4
4
4
4

Why is it not ?

0
1
2
3
4

What can I do to make it like the above.

I have tried importing copy and using copy.deepcopy(index). I didn't work probably since index is an integer.

Is lambda part of the reason it is not working.

Thank you for your help!

Answer 1

Not quite sure what you are trying to achieve but the reason that it is printing all 4s is because Python uses dynamic name resolution of variables, that is the value of value when the functions are executed (not when it is declared) is used.

If you really need a function to print the value then you need to create a closure, which means creating the function inside another function, e.g.:

def make_closure(v):
    return lambda: print_index(v)

func = [] 
for value in range(5):
    func.append(make_closure(value))

Now this outputs:

In []:
for a in func:
    a()

Out[]:
0
1
2
3
4