Not necessarily. Python objects can declare a `__contains__` function (such that `x in y` corresponds to `x.__contains__(y)`): https://docs.python.org/3.5/refe

Not necessarily. Python objects can declare a `__contains__` function (such that `x in y` corresponds to `x.__contains__(y)`): https://docs.python.org/3.5/reference/datamodel.html#object.__contains__ https://docs.python.org/3.5/reference/datamodel.html#object....