Python Pandas -- Why Does The `in` Operator Work With Indices And Not With The Data?

I discovered the hard way that Pandas in operator, applied to Series operates on indices and not on the actual data: In [1]: import pandas as pd In [2]: x = pd.Series([1, 2, 3])

Solution 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}>>>1in d
False
>>>'a'in d
True

with:

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