How do Optional, unions, and type narrowing work in Python?

Models alternative value types accurately and narrows them through checks without confusing optionality with default arguments.

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TL;DR

A union such as User | ServiceAccount means a value may be either type. Optional[User] means the same as User | None; it does not mean the function parameter has a default or may be omitted. Type narrowing uses evidence such as is None, isinstance, a discriminating field, or a type guard so the checker and reader know which case remains in a branch.

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Union semantics, `None`, flow-sensitive narrowing, sentinels, exhaustive handling, and honest API contracts.

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Overview
A union describes possible values; control flow must establish which possibility is present before type-specific behavior is safe.
A union such as User | ServiceAccount means a value may be either type. Optional[User] means the same as User | None; it does not mean the function parameter has a default or may be omitted. Type narrowing uses evidence such as is None, isinstance, a discriminating field, or a type guard so the checker and reader know which case remains in a branch.
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Details

Saying Optional[T] means the argument has a default.
Using an assertion only to silence the checker.
Returning None for absence, failure, and invalid input alike.
Accessing a union member's specific attribute before narrowing.

How do Optional, unions, and type narrowing work in Python?

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