What's the difference between a constrained TypeVar and a Union?
If I want to have a type that can be multiple possible types, Union
s seem to be how I represent that:
U = Union[int, str]
U
can be an int
or a str
.
I noticed though that TypeVar
s allow for optional var-arg arguments that also seem to do the same thing:
T = TypeVar("T", int, str)
Both T
and U
seem to only be allowed to take on the types str
and int
.
What are the differences between these two ways, and when should each be preferred?
Solution 1:
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.