mutable - Correct Style for Python functions that mutate the argument

Question

I would like to write a Python function that mutates one of the arguments (which is a list, ie, mutable). Something like this:

def change(array):
   array.append(4)

change(array)

I'm more familiar with passing by value than Python's setup (whatever you decide to call it). So I would usually write such a function like this:

def change(array):
  array.append(4)
  return array

array = change(array)

Here's my confusion. Since I can just mutate the argument, the second method would seem redundant. But the first one feels wrong. Also, my particular function will have several parameters, only one of which will change. The second method makes it clear what argument is changing (because it is assigned to the variable). The first method gives no indication. Is there a convention? Which is 'better'? Thank you.

Answer 1

The first way:

def change(array):
   array.append(4)

change(array)

is the most idiomatic way to do it. Generally, in python, we expect a function to either mutate the arguments, or return something¹. The reason for this is because if a function doesn't return anything, then it makes it abundantly clear that the function must have had some side-effect in order to justify it's existence (e.g. mutating the inputs).

On the flip side, if you do things the second way:

def change(array):
  array.append(4)
  return array

array = change(array)

you're vulnerable to have hard to track down bugs where a mutable object changes all of a sudden when you didn't expect it to -- "But I thought change made a copy"...

^{1Technically every function returns something, that _something_ just happens to be None ...}