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wsgiref.validator requires wsgi.input's read to always give EXACTLY one argument. This is incorrect.
It's own documentation says:
* That wsgi.input is used properly:
 - .read() is called with zero or one argument
PEP3333 says:
A server should allow read() to be called without an argument, and return the remainder of the client's input stream.

The change was suggested before PEP 3333 in issue 4718 (see msg78292), but PEP 3333 says:
 A server should allow read() to be called without an argument, and return the remainder of the client's input stream.
https://www.python.org/dev/peps/pep-3333/#input-and-error-streams
I couldn't find any objection at https://mail.python.org/pipermail/web-sig/2010-September/004655.html 

This is a misreading of the spec. The read() method is defined as taking exactly one parameter. The server *should* support it being optional, but the app invoking it without an argument is not compliant, and wsgiref.validate is validating that the *app* performs according to spec, not the server.
Therefore, the documentation is what's in error, not the code.

I'm not sure I follow, as it has little to say on whether the application's expected behavior here, and only a recommendation that the server allow it. But, it also defers to the "Python Standard Library", which does have an opinion. I feel that (in hindsight) the exact behavior of each method should have been specified in full.
*However*, even if I differ on the reading of the spec, on general principle (and this has changed since I first wrote the issue) I agree that the application should not be omitting it anyway, because reading the entire response at once without checking the content length is dangerous.

This area of the spec is different between 333 and 3333 precisely to clarify this point, without technically changing conformance requirements. PEP 333 was supposed to require the app to supply an argument, while PEP 3333 was changed to encourage the server to allow the app to omit it.
It's an annoying grey area in the spec itself, but not at all a grey area for for the validation tool, which is supposed to validate the strictest interpretation of the spec. A conformant server is allowed to not work with a missing argument, so it's a bad idea for your app to omit it if you want to run on all conformant servers. (And this is entirely independent of whether it's a good idea to read the entire stream in the first place.)

New changeset f8716c88f13f035c126fc1db499ae0ea309c7ece by Cheryl Sabella in branch 'master':
bpo-18610: Update wsgiref.validate docstring for wsgi.input read() (GH-11663)
https://github.com/python/cpython/commit/f8716c88f13f035c126fc1db499ae0ea309c7ece