How does pandas Dataframe.loc accept the [...] syntax?

Question

I have read this documentation:

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.loc.html

You can use a syntax like df.loc[df['shield'] > 6, ['max_speed']].

I tried using Github and found out:

Suppose you have a pandas.core.frame.DataFrame object, i.e. a DataFrame called df.

The type of df.loc is pandas.core.indexing._LocIndexer.

Nevertheless, I could not sort out these questions:

1. How do you make a Python function/class accepting a syntax like above?

2. Where in the source code of pandas.core.frame.DataFrame is the property self.loc defined??

Answer 1

1. How you make a class accept that syntax in general is by implementing __getitem__ which is an example of operator overloading. This allows an object of that class to be indexed with []. For example:

   class get_item_example(object):
    def __getitem__(self, key):
            print(key)
   

   Try it out:

   >>> gi = get_item_example()
   >>> gi['a']
   a
   >>> gi[['a','b','c']]
   ['a', 'b', 'c']
   >>> gi['a','b','c']
   ('a', 'b', 'c')
   

   In the case of df.loc[df['shield'] > 6, ['max_speed']] what happens is that the key passed to __getitem__ is a tuple containing the pandas series returned by df['shield'] > 6 and the single item list ['max_speed'].

2. In the pandas source, pandas.core.indexing._LocIndexer inherits an implementation of __getitem__ from pandas.core.indexing. _LocationIndexer. The implementation is here: https://github.com/pandas-dev/pandas/blob/61362be9ea4d69b33ae421f1f98b8db50be611a2/pandas/core/indexing.py#L1374