The comparisons section of the python 2 docs says:

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https://docs.python.org/2/reference/expressions.html#comparisons

For the list and tuple types, x in y is true if and only if there exists an index i such that x == y[i] is true.

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But it's not strictly speaking correct.  Simplest counter-example:

x = float('nan')
y = [x]

The python 3 docs instead mention correct equivalent which is 

any(x is e or x == e for e in y)