lesson 5 of 6 · groupby, joins, and first featuresgroupby, joins, and first aggregateslesson 5 of 6 · groupby, joins, and first featuresgroupby, joins, and first aggregateslesson 5 of 6 · groupby, joins, and first featuresgroupby, joins, and first aggregateslesson 5 of 6 · groupby, joins, and first featuresgroupby, joins, and first aggregateslesson 5 of 6 · groupby, joins, and first featuresgroupby, joins, and first aggregateslesson 5 of 6 · groupby, joins, and first featuresgroupby, joins, and first aggregateslesson 5 of 6 · groupby, joins, and first featuresgroupby, joins, and first aggregateslesson 5 of 6 · groupby, joins, and first featuresgroupby, joins, and first aggregateslesson 5 of 6 · groupby, joins, and first featuresgroupby, joins, and first aggregateslesson 5 of 6 · groupby, joins, and first featuresgroupby, joins, and first aggregateslesson 5 of 6 · groupby, joins, and first featuresgroupby, joins, and first aggregates
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: