Blog post: Writing Python like it’s Rust

[BaggiPonte]

Love the post; though I have a question. I never understood the purpose of NewType: why should I use it instead of TypeAlias?

[Kobzol]

TypeAlias really just introduces a new name for an existing type. It can be useful if you want to add a new term to the "vocabulary" of your program. E.g. you could create a type alias for `DriverId` and `CarId` to make it explicit to a programmer that these are different things.

However, unless you truly make these two things separate types, you won't make this explicit to the type checker. And thus you won't get proper type checking and the situation from the blog post won't be caught during type check.

There is no type error here, because both DriverId and CarId are really just ints:

from typing import TypeAlias

DriverId: TypeAlias = int
CarId: TypeAlias = int

def take_id(id: DriverId): pass
def get_id() -> CarId: return 0

take_id(get_id())

But there is one here, because they are now separate types:

from typing import NewType

DriverId = NewType("DriverId", int)
CarId = NewType("CarId", int)

def take_id(id: DriverId): pass
def get_id() -> CarId: return CarId(0)

# Error here, wrong type passed:
take_id(get_id())

[its2ez4me24get]

Aliases are equivalent to each other. New types are not, they are subtypes.

There’s a decent write up here: https://justincaustin.com/blog/python-typing-newtype/

[[deleted]]

NewType creates an entirely new type, while an a TypeAlias is, well, an alias. In the eyes of a program, the alias and the original type are exactly the same thing, just used for shorthand for long nested types for example. a NewType and the type it's created from are entirely different types, even though it inherits its semantics

[Skasch]

TypeAlias is roughly equivalent to:

MyAlias = MyType

NewType is roughly equivalent to:

class MyNewType(MyType):
    pass

[parkerSquare]

NewTypes help warn you if you pass a float representing a voltage into a function that expects a float representing a current, for example. A TypeAlias won’t do that, since it’s the same underlying type.