When do Python generics and TypeVar improve an API?

Uses generics to preserve relationships among types and introduces bounds, constraints, and variance only where they clarify a real API.

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TL;DR

Generics improve an API when the input and output types are related. A repository generic over T can return T, and a function that returns the first item of Iterable[T] preserves the caller's item type. A type variable represents one consistent type chosen for that use. A union instead means the value may be any listed alternative and does not automatically preserve which alternative went in.

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The difference between a type variable and a union, relationship-preserving signatures, and safe producer or consumer variance.

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A useful generic contract says that types are related across positions; it is not merely a way to replace every concrete annotation with a letter.
Generics improve an API when the input and output types are related. A repository generic over T can return T, and a function that returns the first item of Iterable[T] preserves the caller's item type. A type variable represents one consistent type chosen for that use. A union instead means the value may be any listed alternative and does not automatically preserve which alternative went in.
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A bound says a type variable must support at least a particular base type or protocol, while constraints restrict it to specified alternatives. I use these only when the implementation genuinely needs that capability. Variance describes safe subtype substitution for generic containers: read-only producers can often be covariant, consumers can be contravariant, and mutable containers are commonly invariant because they both accept and return values.

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Details

Using a union where one consistent type relationship is required.
Adding a type variable that appears only once and preserves nothing.
Declaring a mutable read-write container covariant.
Using bounds or constraints without needing their behavior.

When do Python generics and TypeVar improve an API?

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