Lesson 42 of 55
15 mins readPython Mapped Types and Mapping Modifiers
In Plain English
Mapped types build on index signature syntax to iterate over keys (using [P in K]) and apply transformations like making fields optional, readonly, or nullable.
Core Rules to Remember
[P in keyof T]: Iterates across every property key in type T.
Modifiers: Add or strip readonly (+readonly / -readonly) and optionality (? / -?).
Live Interactive Example
Hit Run Code to see it liveMapping Over Keys with Modifiers
Python 3.12
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Output Console
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How it works: Mapped types iterate over keyof T with [P in keyof T], transforming each property according to specified modifiers.
Your Turn: Micro Challenge
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Micro Exercise
Create a Nullable Mapped Type
- Define type Nullable<T> = { [P in keyof T]: T[P] | null }.
- Apply to interface User { name: string; age: number }.
- Create obj: Nullable<User> = { name: null, age: 25 } and print `${obj.name} ${obj.age}`.
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Sandbox Output
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