But I guess having a None value representing “no hostname” seems to be better than an empty string that might have a different semantic meaning.
@Dreamsorcerer WDYT?

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@webknjaz - I agree with everything you said and I ran into this problem when I was trying to build a URL for a local file.

The thing is that the same argument could be made for path, query_string, fragment, etc but they are all handled this way (see block starting here).

In other words, with the changes here, the method simply becomes more consistent in the treatment of the various components.

My attempt here is to help people who run into the same issue to get a meaningful error message, rather than having to dig into the code.

@paulpapacz could you research if we would be able to have tests for those cases and maybe submit them first (even if we'll have to mark them as xfail for now)?

Oh, and one other case is relative URIs which only need path+qs.

But I guess having a None value representing “no hostname” seems to be better than an empty string that might have a different semantic meaning. @Dreamsorcerer WDYT?

Looks like the current situation is that it's not designed for it. At a glance, it looks much the same as the other parameters in that block, and fails if you try to use None:

>>> yarl.URL.build(host=None)  
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/ubuntu/.local/lib/python3.10/site-packages/yarl/_url.py", line 246, in build
    cls._validate_authority_uri_abs_path(host=host, path=path)
  File "/home/ubuntu/.local/lib/python3.10/site-packages/yarl/_url.py", line 699, in _validate_authority_uri_abs_path
    if len(host) > 0 and len(path) > 0 and not path.startswith("/"):
TypeError: object of type 'NoneType' has no len()

I don't think there's any meaningful difference to "" or None representing "no hostname", the result would surely look the same, but would require extra code to convert the None.

So, this change looks fine to me. We should get type annotations added though, which would make it more explicit.

Nevermind, it uses stub files. This is already typed as str: https://github.com/aio-libs/yarl/blob/master/yarl/__init__.pyi#L59

@Dreamsorcerer looks like the CI needs to be fixed before merging this: #803