Nested Functions

You can define functions inside other functions. The inner function, called a nested function, is only accessible within the outer function. This keeps helper logic private and encapsulated, preventing pollution of the broader namespace. Basic Nested Function Define helper functions inside the main function when they are only meaningful in that context: Organizing Complex Logic Nested functions help organize complex functions into clear, named steps without creating module-level functions that are only used once: Each step of the calculation has a clear, descriptive name. The main flow at the bottom reads like documentation: get subtotal, apply discount, add tax. The helper functions encapsulate the details. Nested functions excel at keeping helper logic close to the code that uses it. Whe

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This section is part of the Functional Programming: Intermediate 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.

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