Lesson 28 of 55
15 mins readPython Generic Constraints with extends
In Plain English
Generic constraints (<T extends Shape>) enforce structural prerequisites on type parameters, allowing safe access to required properties.
Core Rules to Remember
extends Constraint: <T extends Shape> ensures T satisfies the given structural contract.
Safe Property Access: Unlocks member property access (like .length or .id) directly on generic arguments.
Live Interactive Example
Hit Run Code to see it liveConstraining Type Parameters
Python 3.12
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Output Console
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How it works: Because T extends HasLength, accessing a.length is completely type-safe.
Your Turn: Micro Challenge
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Micro Exercise
Constrain to Identifiable Entity
- Declare interface HasId { id: string | number }.
- Write generic function printEntityId<T extends HasId>(entity: T): string returning `Entity ID: ${entity.id}`.
- Call with { id: "srv-99", name: "Cluster" } and print.
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Sandbox Output
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