What is the difference between groupby.agg, transform, and filter?
PICTURE THIS: GIT FLOW
Simple meaning
agg reduces groups to summaries, transform returns aligned same-shape columns, and filter keeps or drops entire groups based on a group-level condition.
WHY — Pandas instead of guessing?
Why interviewers care about Pandas:
contrast on Pandas, not two memorised paragraphs.
the developer, then one case where picking wrong hurts.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1agg reduces groups to
summaries, transform returns aligned same-shape columns, and filter keeps or drops entire groups based on a group-level condition.
- 2Filter is how you
drop groups with too few rows before modeling.
- 3Mixing them up is
a common source of merge mistakes after aggregation.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
agg reduces groups to summaries, transform returns aligned same-shape columns, and filter keeps or drops entire groups based on a group-level condition. Filter is how you drop groups with too few rows before modeling.