There is already a check for request.data being str in do_request_(). Maybe it is okay to remove this check; I think it would be equivalent to the new check.

Regarding the dict check, here is a strange class that currently works, but would be broken by the patch. Maybe it is okay to break it though, since it certainly goes against the urlopen() documentation.

class ReaderMapping(dict):
    def __init__(self):
        super().__init__({"abc": "xyz"})
        self.mode = "r"
        self.data = "123"
    def read(self, size):
        data = self.data
        self.data = ""
        return data

urlopen(Request("http://localhost/", headers={"Content-Length": 3}, data=ReaderMapping()))

If we wanted to continue to support this kind of usage, perhaps the safest option would be to change the test to “if type(data) is dict” rather than use isinstance().

I also left a comment about eliminating urlencode() in the test.