pythonlistfunctionsyntaxcall-by-value

Python change list item

I am wondering why when changing values to a list by calling a function, there is such a difference in the following two scenarios (assign new value to list v.s. to list[:]). I guess it has something to do with call-by-value/call-by-reference.

Code

def rotate1(nums, k) -> None:   
  nums = nums[-k:]+nums[:-k]
  print(f"In rotate1: {nums}")
  
def rotate2(nums, k) -> None:   
  nums[:] = nums[-k:]+nums[:-k]
  print(f"In rotate2: {nums}")  

ls1 = [1,2,3,4]
rotate1(ls1, 2)
print(f"Outside rotate1: {ls1}\n")

ls2 = [1,2,3,4]
rotate2(ls2, 2)
print(f"Outside rotate2: {ls2}\n")

Output

In rotate1: [3, 4, 1, 2]
Outside rotate1: [1, 2, 3, 4]

In rotate2: [3, 4, 1, 2]
Outside rotate2: [3, 4, 1, 2]

ls1 value does not change after calling rotate1; whereas ls2 value changes after calling rotate2.

Solution

  • The same problem is explained exactly in this page Is Python pass-by-reference or pass-by-value?

    As we know, in Python, “Object references are passed by value”.

    In both rotate methods, the nums creates a new variable to point to object [1,2,3,4]. The difference is:

    So, outside the methods, only the rotate2 really changes the object of ls2.