FAQ: Learn Python: Function Arguments - Using None as a Sentinel

This community-built FAQ covers the “Using None as a Sentinel” exercise from the lesson “Learn Python: Function Arguments”.

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Learn Python 3

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2 posts were split to a new topic: How do I check for equals in a conditional?

What is the point of using None? Below are two functions, and to my understanding they both do the same thing. Is there a use case where we would put = None as a parameter?

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

  current_books.extend(authors_books)
  return current_books

Why does adding None as a default value for current_order fix the code since we still assign current_order different values later in the code

def update_order(new_item, current_order=None):
  if current_order is None:
    current_order = []
  #see how we still assign current_order new values
  current_order.append(new_item)
  return current_order

Assigning a parameter a default argument lets the code in the function run with the default if no argument is provided and with the argument if you input a value.

#function with default parameter
def print_num(num=0):
  print(num)
#prints 0
print_num()
#prints 5
print_num(5)
1 Like

I see I see thanks a lot

I got the same answer by using the clear function to get around the list being populated by previous calls. Is this in bad form compared to using None?

def update_order(new_item, current_order=[]):
  current_order.clear()
  current_order.append(new_item)
  return current_order

vs

def update_order(new_item, current_order=None):
  if current_order is None:
    current_order = []
  current_order.append(new_item)
  return current_order

What happens if you do pass two arguments for your first function? That’s not going to do what you want.

I believe None is the generally accepted solution (but you could use something else on the occasion None is a valid argument). There’s some discussion on this here-

What is the purpose of the nomenclature “if variable is None:” when you could just as easily use “if variable == None:”?

It seems like it would be more consistent to not introduce new nomenclature especially when using “==” would be more consistent with other operations?

It’s a little bit of a style thing (similar to certain if boolean uses) but it could result in unexpected behaviour. This SO answer adds a little more info in that some objects may, rarely, return None in a comparison operation despite not actually being the None object-