Use and meaning of “in” in an if statement?

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It depends on what next is.

If it’s a string (as in your example), then in checks for substrings.

>>> "in" in "indigo"
True
>>> "in" in "violet"
False
>>> "0" in "10"
True
>>> "1" in "10"
True

If it’s a different kind of iterable (list, tuple, set, dictionary…), then in checks for membership.

>>> "in" in ["in", "out"]
True
>>> "in" in ["indigo", "violet"]
False

In a dictionary, membership is seen as “being one of the keys”:

>>> "in" in {"in": "out"}
True
>>> "in" in {"out": "in"}
False

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