Introduce Partial for TypedDict

Ideas
63

I see the value of Partial, Required, Pick and Omit, but implementing each of these as custom special forms in the type system doesn’t strike me as the best approach.

In TypeScript, these are not special-cased operators. Rather, they are built on a small number of more general type operators that allow other useful transforms (Readonly, Record, Exclude, Extract, etc.) to be built. These operators give TypeScript a “super power” that Python’s type system currently lacks.

For context, here are the definitions for Partial, Required, Pick, and Omit in TypeScript:

// Make all properties in T optional
type Partial<T> = {
    [P in keyof T]?: T[P];
};

// Make all properties in T required
type Required<T> = {
    [P in keyof T]-?: T[P];

// From T, pick a set of properties whose keys are in the union K
type Pick<T, K extends keyof T> = {
    [P in K]: T[P];
};

// Construct a type with the properties of T except for those in type K.
type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;

Before converging on a concrete proposal for how to implement these in the Python type system, I encourage you to consider more generalized solution like the one found in TypeScript. I started to explore this idea here but didn’t fully flesh it out.

We may decide in the end that hard-coding the behavior of a handful of these transforms is the only viable solution, but that feels to me like a missed opportunity.

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