[2.7] bpo-38216, bpo-36274: Allow subclasses to separately override validation and encoding behavior (GH-16448)

[jaraco]

• bpo-38216: Allow bypassing input validation

• bpo-36274: Also allow the URL encoding to be overridden.

• bpo-38216, bpo-36274: Add tests demonstrating a hook for overriding validation, test demonstrating override encoding, and a test to capture expectation of the interface for the URL.

• Call with skip_host to avoid tripping on the host checking in the URL.

• Remove obsolete comment.

• Make _prepare_path_encoding its own attr.

This makes overriding just that simpler.

Also, don't use the := operator to make backporting easier.

• Add a news entry.

• _prepare_path_encoding -> _encode_prepared_path()

• Once again separate the path validation and request encoding, drastically simplifying the behavior. Drop the guarantee that all processing happens in _prepare_path..
  (cherry picked from commit 7774d78)

Co-authored-by: Jason R. Coombs [email protected]

https://bugs.python.org/issue38216

[jaraco]

Backporting this change, I observe a couple of things:

1. The _encode_request call is no longer meaningful because the request construction will implicitly encode the request using the default encoding when the format string is used (request = '%s %s %s'...). In order to keep the code as consistent as possible, I decided to include the call as a pass-through. I'd be just as happy to remove it entirely, but I'll leave that up to the reviewer to decide. It's okay that this functionality is disabled on Python 2 because this functionality was mainly around bpo-36274, which was mainly a concern with the transition to Python 3.
2. Because _encode_request is no longer meaningful, neither is the test for it, so I've removed that test. Therefore, the meaningful part of this test is that for bpo-38216, adding a (underscore-protected) hook to customize/disable validation.

[jaraco]

As a reminder, this PR is for your consideration. The reported failure cases for bpo-38216 already have workarounds implemented, so there are no known pressing needs for this corrective bugfix. Accepting this change will, however, provide a consistent workaround that applies to other (not-yet-identified) use-cases across all (affected) Python versions, so it seems worthwhile to me.

[benjaminp]

Since 2.7 wasn't ever released with the fix for bpo-38216, I'm going to say we should take this just in case.

I'm going to edit your PR to remove the usage of locals(). Otherwise, it looks good. Thanks for doing this.