@overload - which body actually runs

Question

Accepted Answer

The @overload decorator does not create actual dispatch logic. Each @overload variant simply overwrites the previous definition in the module namespace, so none of them survive to runtime. The last definition - the one without @overload - is what actually exists at runtime and handles all calls. It typically uses isinstance checks or a Union type annotation internally. Type checkers like mypy read the @overload signatures to understand the input-output relationship and validate call sites, then ignore the implementation signature for type inference purposes. This split - type checker sees the overloads, runtime sees only the implementation - is why you must always provide the plain implementation after all @overload variants, or you get a NameError at runtime.