pass by reference - Python: about [:] and its behavior in a function -

def call(nums):     nums[:] = [x x in nums if x != 4]  numbers = [4, 5] print(numbers) call(numbers) print(numbers) 

the output above code is:

[4, 5] [5] 

but if remove "[:]", output becomes following:

[4, 5] [4, 5] 

i know [:] makes copy of full list, why function argument modified in 1 case , not in other?

as suspected issue hiding in slicer in line:

nums[:] = [x x in nums if x != 4] 

python "pass value" means you're not passing pointer numbers function, copy of it. re-assigning value copy of reference not change original reference.

but, when use slicer, you're accessing object behind reference , changing directly why see side effect after exit function - when using "slice".