Correct way of using "not in" operator


We know that,

a = 1
b = 2

print(not a > b)

is the correct way of using the "not" keyword and the below throws an error

a = 1
b = 2

print(a not > b)

since "not" inverts the output Boolean.

Thus, by this logic the correct way for checking the presence of a member in a list should be

a = 1
b = [2,3,4,5]

print(not a in b)

But I find the most common way is

a = 1
b = [2,3,4,5]

print(a not in b)

which from the logic given in previous example should throw an error.

So what is the correct way of using the "not in" operator in Python3.x?

Asked By: Mohit Lamba



what is the correct way of using the "not in" operator

There is only one way to use the not in operator. Your not a in b instead uses the not operator and the in operator.

PEP 8 doesn’t seem to have an opinion about which to use, but about the similar is not operator (thanks Silvio) it says:

Use is not operator rather than not ... is. While both expressions are functionally identical, the former is more readable and preferred:

# Correct:
if foo is not None:
# Wrong:
if not foo is None:

So I’d say not in should also be preferred, for the same reason.

Answered By: Kelly Bundy

not in is a special case that simplifies to exactly what you tried first. Namely,

a not in b

literally simplifies to not (a in b). It also works (slightly differently, but same idea) for is.

a is not b`

is equivalent to not (a is b). Python added these because they flow naturally like English prose. On the other hand, a not < b doesn’t look or feel natural, so it’s not allowed. The not in and is not are special cases in the grammar, not small parts of a general rule about where not can go. The only general rule in play is that not can always be used as a prefix operator (like in not (a < b))

Answered By: Silvio Mayolo

not, not in and in are all valid operators. Transitively, not (in_expression) is also valid

Correct way? Refer Zen of Python.

Answered By: OneCricketeer

First of all not in, is not a two separate operator, is constituently a single operator ,and also known as membership operator. There is another membership operator that is in. Membership operator has high precedence than logical NOT, AND and OR.

print(not a in b) -> This is actually first evaluating a in b then result is inverted by the logical ‘not’ and then result is printed.

So as per your example it should print True as a in b gives False then it is inverted to True via logical NOT operator.

print(a not in b) -> Here python checks if a is not a part of the b, if it is return ‘False’ else ‘True` .

So as per your example it should return True as a is not a part of b.

I think a not in b is more clear than not a in b.I would suggest to use membership operator for testing the membership.

However the result will remain same for both kind of expression but the process of evaluating is completely different.

Answered By: novice
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