Groupby, joins, and first features (step 4/7) · dataframes with numpy and pandas

Fill in the dict method that means 'get this key's list, creating an empty one if it doesn't exist yet' — the split step of every groupby.

Method name:

Fill in the dict method that means 'get this key's list, creating an empty one if it doesn't exist yet' — the split step of every groupby.

Method name:

Groupby, joins, and first features — step 4 of 7