Lesson 29 of 55
15 mins readPython Type Parameters in Generic Constraints (keyof)
In Plain English
You can declare a type parameter that is constrained by another type parameter to safely extract property values with getProperty(obj, key).
Core Rules to Remember
K extends keyof T: Constrains K to only valid property names of object type T.
Indexed Return T[K]: Automatically infers the exact property return type matching key K.
Live Interactive Example
Hit Run Code to see it liveSafe Property Access with keyof
Python 3.12
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Output Console
Click "Run Code" to view the rendered output.
How it works: T is serverConfig shape, K is constrained to valid keys, and return type is indexed type T[K].
Your Turn: Micro Challenge
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Micro Exercise
Implement Safe Property Plucker
- Write function pluck<T, K extends keyof T>(items: T[], key: K): T[K][] that maps each item to item[key].
- Pluck "name" from [{ name: "Alice", age: 30 }, { name: "Bob", age: 25 }] and print JSON stringified result.
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Sandbox Output
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