[tqa236]

Closes #11470

Make urllib.request.Request generic over the type of data, so Request(url, data=b"...").data is bytes instead of the full _DataType union, as suggested in the linked issue.

There is a trade-off with this implementation though.

A plain Request annotation, as used by urlopen(), now means Request[Any]. That keeps Request[bytes] accepted everywhere, but .data is unchecked on code annotated with a plain Request. Before, it was the union.

from urllib.request import BaseHandler, Request, build_opener


class LogBodyHandler(BaseHandler):
    # Pre-processor called for every HTTP request before it is sent.
    def http_request(self, request: Request) -> Request:
        print("sending:", request.data.decode())  # no type error because assuming the body is bytes, but will crash at runtime: AttributeError: 'list' object has no attribute 'decode'
        return request


opener = build_opener(LogBodyHandler())

# Streaming the body as an iterable of chunks is allowed by urllib.
opener.open(Request("http://localhost:1/upload", data=[b"chunk1", b"chunk2"]))  

The alternative I considered but didn't choose is the covariant option, which will also slip through a runtime crash.

_DataT_co = TypeVar("_DataT_co", bound=_DataType, default=_DataType, covariant=True)

req = Request("http://x", data=b"body")   # Request[bytes]
req.data = [b"a", b"b"]                    # no errors from type checkers
req.data.decode()                          # no errors from type checkers; crashes at runtime

I did a small web scrapping to see which option is better and chose the current implementation because the scenario it slips through seem to be rarer (small samples though).

AI Disclosure: I used Claude Opus 5.5 to help resolve this issue.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉