Objects A, B ... have attribute namespace and I have a function that filters a list of such objects by a certain set of values of namespace attribute:
T = TypeVar('T')
def filter(seq: list[T], namespace_values: set[str]) -> list[T]:
# Returns a smaller list containing only the items from
# `seq` whose `namespace` are in `namespace_values`
...
This works well, but it allows passing an object of type X that does not have the attribute namespace without any check error.
Then I created a protocol and changed the function in order to use the protocol:
class Namespaced(Protocol):
namespace: str
def filter(seq: list[Namespaced], namespace_values: set[str]) -> list[Namespaced]:
# Returns a smaller list containing only the items from
# `seq` whose `namespace` are in `namespace_values`
...
Now I get a check error if I pass a list of X (which is what I wanted), but I lost the generics:
list_of_a: list[A] = [a1, a2, a3]
output = filter(list_of_a, ['ns1', 'ns2'])
# output is list[Namespaced] instead of list[A]
How can I combine the generics and protocol so my function returns a list of type T and also checks that the seq's items implement Namespaced protocol?
I tried the below approach but the T is lost.
def filter(seq: list[Namespaced[T]], namespace_values: set[str]) -> list[T]:
# Returns a smaller list containing only the items from
# `seq` whose `namespace` are in `namespace_values`
...
Cheers!
Use a bound type variable with the protocol as the bound. Consider the following module:
(py39) Juans-MacBook-Pro:~ juan$ cat test.py
Which has:
from typing import TypeVar, Protocol
from dataclasses import dataclass
class Namespaced(Protocol):
namespace: str
T = TypeVar("T", bound="Namespaced")
@dataclass
class Foo:
namespace: str
@dataclass
class Bar:
namespace: str
id: int
def frobnicate(namespaced: list[T]) -> list[T]:
for x in namespaced:
print(x.namespace)
return namespaced
result1 = frobnicate([Foo('foo')])
result2 = frobnicate([Bar('bar', 1)])
reveal_type(result1)
reveal_type(result2)
Then mypy gives:
(py39) Juans-MacBook-Pro:~ juan$ mypy --strict test.py
test.py:27: note: Revealed type is "builtins.list[test.Foo*]"
test.py:28: note: Revealed type is "builtins.list[test.Bar*]"