Python Pandas — why does the `in` operator work with indices and not with the data?

Question

I discovered the hard way that Pandas in operator, applied to Series operates on indices and not on the actual data:

In [1]: import         


        

Answer 1

It is may be helpful to think of the pandas.Series as being a bit like a dictionary, where the index values are equivalent to the keys. Compare:

>>> d = {'a': 1}
>>> 1 in d
False
>>> 'a' in d
True

with:

>>> s = pandas.Series([1], index=['a'])
>>> 1 in s
False
>>> 'a' in s
True

However, note that iterating over the series iterates over the data, not the index, so list(s) would give [1], not ['a'].

Indeed, per the documentation, the index values "must be unique and hashable", so I'd guess there's a hashtable under there somewhere.