Why does "in" behave differently for lists and tuples?

Question

The code

for item in ('sin', 'cos', 'sinc', 'sin^2'):
    print(item not in ('sin^2'))

produces the result

False
True
True
False

but

for item in ('sin', 'cos', 'sinc', 'sin^2'):
    print(item not in ['sin^2'])

produces the result

True
True
True
False

Why?

You don't have a tuple in the first one. You just have a string. — Daniel Roseman, Apr 17 '19 at 21:07
`('sin^2')` is not a tuple, that is a string. The *comma* makes the tuple, not the parentheses. Note, `my_tuple = 1,2` is a valid tuple, so is `my_tuple = 3,` and `my_tuple = (3,)`, whereas `my_tuple = (3)` is an `int` — juanpa.arrivillaga, Apr 17 '19 at 21:09

score 1 · Accepted Answer · answered Apr 17 '19 at 21:08

The expression is being treated as a parenthesized string rather than a tuple. So it ends up being a substring match. To define it as a single-element tuple, there must be a trailing comma:

print(item not in ('sin^2',))

This is described here: https://wiki.python.org/moin/TupleSyntax

Why does "in" behave differently for lists and tuples?

1 Answers1