"Not in" operator doesn't take "or" operator

Question

I have this little python code, it doesn't work as below.

while True:
    activity = input("What would you like to do today? ")
    a="cinema"
    b="tennis"
    if a or b not in activity.lower():
        print(f"Instead of {activity} I want to go to the cinema")
    else:
        print("Sure Let's do that!!")

However when I extract the or b , it works fine


while True:
    activity = input("What would you like to do today? ")
    a="cinema"
    b="tennis"
    if a  not in activity.lower():
        print(f"Instead of {activity} I want to go to the cinema")
    else:
        print("Sure Let's do that!!")

The question is why doesn't a or b doesn't when used with not in work?

EDIT: I have done the below modification, But still doesn't work.

while True:
    activity = input("What would you like to do today? ")
    a="cinema"
    b="tennis"
    if (a not in activity.lower()) or (b not in activity.lower()):
        print(f"Instead of {activity} I want to go to the cinema")
    else:
        print("Sure Let's do that!!")

Below link is also suggested for this post. It is also very good. So I share it for the ones who will benefit from

How to test multiple variables against a value?

You must write it fully as "a not in activity.lower() or b not in activity.lower()". — Michael Butscher, Nov 14 '20 at 21:38
Admittedly, creating `not in` as a separate operator compared to `not (x in y)` migh make it seem so... — juanpa.arrivillaga, Nov 14 '20 at 21:41
@juanpa.arrivillaga sure I get that now. What we need to think is as True or False in this case. Hope I am right? — Çağlar Yıldız, Nov 14 '20 at 21:49
Very related: https://stackoverflow.com/questions/47007680/how-do-and-and-or-act-with-non-boolean-values — juanpa.arrivillaga, Nov 14 '20 at 21:53
@juanpa.arrivillaga thanks the answers are amazing at the link. — Çağlar Yıldız, Nov 14 '20 at 21:54
Also, the relevant section of the docs: https://docs.python.org/3/library/stdtypes.html#truth-value-testing — juanpa.arrivillaga, Nov 14 '20 at 21:57
"EDIT: I have done the below modification, But still doesn't work." The reason this one doesn't work is a simple logical error. Please see https://en.wikipedia.org/wiki/De_Morgan%27s_laws — Karl Knechtel, Nov 14 '20 at 22:10

score 1 · Answer 1 · answered Nov 14 '20 at 21:36

1

Look at the value of a or b:

>>> a = "cinema"
>>> b = "tennis"
>>> a or b
'cinema'

The "or" operator doesn't have a special different meaning when it's part of an in.

answered Nov 14 '20 at 21:36

Samwise

68,105
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While it's true that `or` works this way, the operator precedence is not what you expect. For example, if you add `c = {'cinema'}`, then `a or b not in c` results in `'cinema'` (the same as `a or (b not in c)`, whereas `(a or b) not in c` results in `False`. – Karl Knechtel Nov 14 '20 at 22:08

Óscar López · Accepted Answer · 2020-11-14T21:51:04.003

1

This won't work, it's not doing what you think:

if a or b not in activity.lower():

The above is equivalent to:

if (a) or (b not in activity.lower()):

Which means something different, and will be True as long as a is not null or empty. Besides you have the comparison backwards, the correct syntax you're looking for is:

if activity.strip().lower() not in (a, b):

edited Nov 14 '20 at 21:51

answered Nov 14 '20 at 21:37

Óscar López

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The syntax you gave doesn't work either. I guess this many operators doesn't work in python. – Çağlar Yıldız Nov 14 '20 at 21:42
Your answer explains a lot though – Çağlar Yıldız Nov 14 '20 at 21:44
`if activity.lower() not in (a,b)`: works too, and is less verbose. – Mark Tolonen Nov 14 '20 at 21:45
1

@MarkTolonen yep, you're right. I didn't realize that OP has the comparison backwards. Thank you, I've updated my answer! – Óscar López Nov 14 '20 at 21:49
1

@ÇağlarYıldız it's not a problem with the number of operators :) is just that you had the comparison backwards. I've updated my answer. – Óscar López Nov 14 '20 at 21:50
@MarkTolonen I also updated the question, can you tell what is wrong? – Çağlar Yıldız Nov 14 '20 at 21:52
1

Mmm in the case of `in` strings don't act like a container of characters, because `in` checks if one string is a substring of another – juanpa.arrivillaga Nov 14 '20 at 22:10
The version you had originally would work for things like `activity = "the cinema"` since `a in activity.lower()` will give true since it is a substring, the version you took from @MarkTolonen only works when the input is the same. – Tadhg McDonald-Jensen Nov 15 '20 at 00:02
@TadhgMcDonald-Jensen that wasn’t from me. See the 3rd comment above. – Mark Tolonen Nov 15 '20 at 01:50

Mark Tolonen · Answer 3 · 2020-11-14T23:53:23.340

a or b not in c is treated as (a) or (b not in c). a is considered True if it is non-zero or non-empty. So if a is a non-empty string, a or b not in c will always be True.

c not in (a,b) is what you want. It tests that the value of c is not a member of the container, equivalent to c != a and c != b.

In your edit, activity is a string, and in in strings works more like contains. So this code:

activity = "run"
a="cinema"
b="tennis"
if (a not in activity.lower()) or (b not in activity.lower()):

is equivalent to:

'cinema' not in 'run' or 'tennis' not in 'run'

which is:

True or True  # result True

Note 'tennis' in 'thetennisball' is True because 'thetennisball' contains the word 'tennis'. If you are looking for exact matches this isn't what you want.

Another issue is using or. When testing that a thing is not one thing or another, one of them will always be false, since a thing can't be two things at once. You'll never get a True for x != 2 or x != 1 for example. If x is 1 it is not 2 and if x is 2 it is not 1. Use and instead.

To test for words in an exact list of words, use a list (or other container like tuple):

'tennis' not in ('cinema','tennis') # False

The only part I don't understand is about my edit. You say tennis="run" but in the "tennis" !="r" part you claim it is going to search for each letter. That surely returns false. Doesn't it search as a whole container? — Çağlar Yıldız, Nov 14 '20 at 22:21
@ÇağlarYıldız Actually, I misspoke. *string* in *string* works differently. `'def' in 'abcdefghi'` returns True. `in` for strings is more like *contains*. I'll edit the answer to correct that, but still not what you want. — Mark Tolonen, Nov 14 '20 at 23:33

Shark Coding · Answer 4 · 2020-11-14T22:30:05.487

-1

First, it is checking if (a) or (b not in activity.lower()) This is not right. What you want is something to check for a string in a string like if activty.count(a) == 0 and activty.count(b) == 0 becuase this checks how many times a and b appear in activity.

edited Nov 14 '20 at 22:30

answered Nov 14 '20 at 21:38

Shark Coding

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1
10

Thanks I learned a lot from your answer. – Çağlar Yıldız Nov 14 '20 at 21:44
Sadly, if (a not in activity.lower()) or (b not in activity.lower()):-> didn't work – Çağlar Yıldız Nov 14 '20 at 21:58
`activity` is a string and `in` won't return True unless `a` is a single character matching a character of `activity`. – Mark Tolonen Nov 14 '20 at 22:08
you should try `if activity.find(a) == -1 and activity.find(b) == -1` becuase this will find a string inside a string – Shark Coding Nov 14 '20 at 22:12
or you could use this `if activity.count(a) == 0 and activity.count(b) == 0` it checks how many times `a` and `b` appear inside of activity – Shark Coding Nov 14 '20 at 22:15

"Not in" operator doesn't take "or" operator

4 Answers4