Why Would you use None as a Sentinel in a Function in Python?

Question

version 1

def add_author(authors_books, current_books=None):
  if current_books is None:
    current_books = []

  current_books.extend(authors_books)
  return current_books

version 2

def add_author(authors_books):
  current_books = []
  current_books.extend(authors_books)
  return current_books

In version 1 why would you use "current_books = None" and the following "if" statement if you could get the same outcome using the function in version 2?

Is there certain use cases where you would put a parameter as = to None?

When `add_author` is called for the second time, we might not want to initialise `current_books` to empty list, maybe? — Austin, Mar 29 '20 at 14:37

actual_panda · Answer 1 · 2020-03-29T15:06:46.427

For example when writing a recursive function that has to pass down something mutable like a list. Toy example:

def foo(collected=None, level=0, max_depth=3):
    if collected is None: # first call!
        collected = []

    collected.append(level)    
    print(collected)

    if level < max_depth:
        foo(collected, level + 1, max_depth)

foo()

# output:
# [0]
# [0, 1]
# [0, 1, 2]
# [0, 1, 2, 3]

A default argument for collected is used such that the first caller does not have to supply collected=[].

None is used instead of [] as the default value such that mutations to collected are not preserved over calls (the Python mutable default argument gotcha).

def foo(collected=[], level=0, max_depth=3):
    collected.append(level)    
    print(collected)

    if level < max_depth:
        foo(collected, level + 1, max_depth)

foo()

# output:
# [0]
# [0, 1]
# [0, 1, 2]
# [0, 1, 2, 3]

foo()

# output:    
# [0, 1, 2, 3, 0]
# [0, 1, 2, 3, 0, 1]
# [0, 1, 2, 3, 0, 1, 2]
# [0, 1, 2, 3, 0, 1, 2, 3]

But if we supply collected = [ ] , it would accomplish the same result? — Gojilla, Mar 30 '20 at 05:55
@Gojilla correct. But you don't want to force the user to supply a parameter when only one value is acceptable for that parameter. — actual_panda, Mar 30 '20 at 08:57

Why Would you use None as a Sentinel in a Function in Python?

1 Answers1