We noticed recently that:

>>> None in 'foo'

raises (at least in Python 2.7)

TypeError: 'in <string>' requires string as left operand, not NoneType

This is surprising.  The description of the 'in' operatator is, 'True if an 
item of s is equal to x, else False '.  From that, I would assume it behaves as 
if it were written:

for item in iterable:
    if item == x:
        return True
    return False

why the extra type check for str.__contains__()?  That seems very unpythonic.  
Duck typing, and all that.

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