CODEWITHSUNDEEP
pythonpython-3.x
user5711693
user5711693

Reputation: 113

Using __contains__ method in Python

I am using ____contains__ method in python as per below:

Case 1: True

if ('A','B','D').__contains__('A'or'E'):
    print(True)
else:
    print(False)

Case 2: False

if ('A','B','D').__contains__('E'or'B'):
    print(True)
else:
    print(False)

If I replace or with and, that works and also using expressions such as (('E' and 'A') and ('B' and 'C')) works.

Please could you confirm why or does not work in standard way like if I were to do something like if (B or C) condition? Also is there other ways of doing the above using regular expressions or lamda expressions?

Upvotes: 0

Views: 767

Answers (2)

Jacinator
Jacinator

Reputation: 1413

First of all, don't use __contains__ like that. Use in like this.

if ('A' or 'E') in ('A', 'B', 'C'):
    pass

Hopefully this will start to show where your problem is. The a or b is evaluating before the __contains__ is being called and only one of the values is being tested for inclusion.

The statement ('A' or 'E') will simplify to 'A'. That is why your first case is printing True.

if ('E' or 'B') in ('A', 'B', 'C'):
    pass

For your second case ('E' or 'B') will evaluate to 'E', which isn't in ('A', 'B', 'C'), so your second case prints False.

Addendum

To correct this code you should be thinking a little bit more verbosely. 

values = ('A', 'B', 'C')

if 'A' in values or 'E' in values:
    pass

if 'E' in values or 'B' in values:
    pass

Upvotes: 0

Gnudiff
Gnudiff

Reputation: 4305

You are using and and or incorrectly.

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned. // https://docs.python.org/3/reference/expressions.html#boolean-operations

Also, double underscore functions are marked with the double underscore especially as a marker that people shouldn't usually use them.

(One of the) correct ways to write the above is:

my_tuple=('A','B','D')
if 'A' in my_tuple or 'E' in my_tuple:
    print(True)
else:
    print(False)

Upvotes: 1

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