No TypeError comparing int with None in Python3

Question

I understand that comparing int and None types is not valid in Python3 (3.6.1) as I see here:

>>> largest = None
>>> number = 5
>>> number > largest
TypeError: '>' not supported between instances of int and NoneType

But inside this script it doesn't give a TypeError.

largest = None
for number in [5, 20, 11]:
    if largest is None or number > largest:
        largest = number

When I run this script with python3 it runs without a TypeError. Why?

Answer 1

You're witnessing short circuiting.

if largest is None or number > largest:
        (1)        or      (2)

When condition (1) is evaluated to be true, condition (2) is not executed. In the first iteration, largest is None is True, so the whole expression is true.

---

As an illustrative example, consider this little snippet.

test = 1
if test or not print('Nope!'):
     pass

# Nothing printed 

Now, repeat with test=None:

test = None
if test or not print('Nope!'):
     pass

Nope!

Answer 2

If you examine your code carefully, you'll notice that you initialize largest to None, then conditionally ask if it is None, thus the if statement evaluates as True.