What’s the difference between a constrained TypeVar and a Union?

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T‘s type must be consistent across multiple uses within a given scope, while U‘s does not.

With a Union type used as function parameters, the arguments as well as the return type can all be different:

U = Union[int, str]

def union_f(arg1: U, arg2: U) -> U:
    return arg1

x = union_f(1, "b")  # No error due to different types
x = union_f(1, 2)  # Also no error
x = union_f("a", 2)  # Also no error
x # And it can't tell in any of the cases if 'x' is an int or string

Compare that to a similar case with a TypeVar where the argument types must match:

T = TypeVar("T", int, str)

def typevar_f(arg1: T, arg2: T) -> T:
    return arg1

y = typevar_f(1, "b")  # "Expected type 'int' (matched generic type 'T'), got 'str' instead
y = typevar_f("a", 2)  # "Expected type 'str' (matched generic type 'T'), got 'int' instead

y = typevar_f("a", "b")  # No error
y  # It knows that 'y' is a string

y = typevar_f(1, 2)  # No error
y  # It knows that 'y' is an int

So, use a TypeVar if multiple types are allowed, but different usages of T within a single scope must match each other. Use a Union if multiple types are allowed, but different usages of U within a given scope don’t need to match each other.

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