What Can Be a Key?

Dictionary keys must be immutable (unchangeable) types. This is because Python uses a special technique called hashing to make key lookups extremely fast. Immutable types can be hashed; mutable types cannot. Keys Must Be Unique Each key in a dictionary must be unique. If you create a dictionary with duplicate keys, only the last value is kept:

About This Interactive Section

This section is part of the Dictionaries: Beginner lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live PostgreSQL database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

How DataDriven Lessons Work

DataDriven combines four interview rounds (SQL, Python, Data Modeling, Pipeline Architecture) with adaptive difficulty and spaced repetition. Easy problems get harder as you improve. Weak concepts resurface until you master them. Your readiness score tracks progress across every topic interviewers test. Every lesson section ends with problems you solve by writing and running real code, not by picking multiple-choice answers.