Phase 1: pandas Mastery · ~30 minutes · Python · pandas

Split-Apply-Combine (groupby)

groupby isn't one operation. It's three — split the data apart, apply a calculation to each piece, combine the results — and most groupby confusion comes from not knowing which of the three just happened.

Hiring signal: Understands groupby as three distinct real steps, not one opaque function call, and knows exactly why a grouped result's shape depends on as_index

What you will learn

Explain groupby as three distinct steps: split, apply, combine
Compute a real grouped aggregate on genuine NYC taxi trip data
Explain the difference between a groupby result with and without as_index
Diagnose a downstream error caused by an unexpected as_index default

Introduction

Type: Learn Languages: Python, pandas Prerequisites: Lesson 02 (Sorting and Ranking) Time: ~30 minutes

Objective

Learning objectives

Explain groupby as three distinct steps: split, apply, combine
Compute a real grouped aggregate on genuine NYC taxi trip data
Explain the difference between a groupby result with and without as_index
Diagnose a downstream error caused by an unexpected as_index default

Unlock the full lesson

You've read the first 2 sections. The rest of this lesson covers The Problem, The Concept, Build, Key Terms & Next — plus a hands-on lab, quiz, and project artifact.

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