Python 3: Ellipsis in function parameters?

Question

I recently ran into Ellipsis (...) that is used in function parameters in aiohttp code and then in that function's body:

def make_mocked_request(method, path, headers=None, *,
                        match_info=sentinel,
                        version=HttpVersion(1, 1), closing=False,
                        app=None,
                        writer=sentinel,
                        protocol=sentinel,
                        transport=sentinel,
                        payload=sentinel,
                        sslcontext=None,
                        client_max_size=1024**2,
                        loop=...):
    """Creates mocked web.Request testing purposes.

    Useful in unit tests, when spinning full web server is overkill or
    specific conditions and errors are hard to trigger.

    """

    task = mock.Mock()
    if loop is ...:
        loop = mock.Mock()
        loop.create_future.return_value = ()

Can you explain this new python 3 feature?

Answer 1

Ellipsis is a Built-in constant in Python. In Python 3 it has a literal syntax ... so it can be used like any other literal. This was accepted by Guido for Python 3 because some folks thought it would be cute.

The code you have found (use as function argument default) is apparently one such "cute" usage. Later in that code you'll see:

if loop is ...:
    loop = mock.Mock()
    loop.create_future.return_value = ()

It's just being used as a sentinel, here, and may as well be object() or anything else - there is nothing specific to Ellipsis. Perhaps the usual sentinel, None, has some other specific meaning in this context, although I can not see any evidence of that in the commit (it seems like None will have worked just as well).

Another use-case for an ellipsis literal sometimes seen in the wild is a placeholder for code not written yet, similar to pass statement:

class Todo:
    ...

For the more typical use-cases, which are involving the extended slice syntax, see What does the Python Ellipsis object do?