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

Time Series and Method Chaining

"1/2/2025" sorts after "1/15/2025" as plain text. A date is only chronological once something has actually told pandas it's a date.

Hiring signal: Verifies a datetime column is actually parsed as a datetime before trusting any date-based sort or resample, and writes multi-step transformations as one readable chain instead of a pile of intermediate variables

What you will learn

Parse a real datetime column and resample it to a daily trend
Write a multi-step transformation as one readable method chain
Explain why an unparsed date column sorts lexicographically instead of chronologically
Diagnose a wrong 'sorted by date' result as evidence of an unparsed datetime column

Introduction

Type: Learn Languages: Python, pandas Prerequisites: Lesson 05 (Missing Data, For Real) Time: ~30 minutes

Objective

Learning objectives

Parse a real datetime column and resample it to a daily trend
Write a multi-step transformation as one readable method chain
Explain why an unparsed date column sorts lexicographically instead of chronologically
Diagnose a wrong "sorted by date" result as evidence of an unparsed datetime column

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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