pip upgrade warning is bypassed when pip is a dependency

[ncoghlan]

Helping to debug a contributor setup issue at the PyCon AU sprints, we eventually tracked the problem down to:

• briefcase needed a more recent pip
• "pip install --upgrade briefcase" implicitly tried to upgrade pip, but had the usual Windows issues with trying to replace a running executable
• the warning about this case didn't get displayed

[pfmoore]

Yeah, the heuristics for working out when we're trying to use pip to upgrade itself aren't 100% reliable. I don't think anyone would object to improved detection of this case, but we don't know how to do it. Suggestions (and working code 🙂) welcome, of course!

[pradyunsg]

Hmm... We're checking if "pip" is in the requirement set, which should be robust to this.

I guess there's something about this assumption when I wrote that, that's not correct. 🙃

[pfmoore]

I think it's that pip is not in the explicit requirement set, but is pulled in as a dependency when installing one of the requested requirements. I don't think we can do much about that, as we only find out once the install process has started (although I may well be wrong, I'm going from a pretty vague memory of how it works...)

[pradyunsg]

Right. We're both speculating from memory. XD

I've fallen for this before so... let's wait till someone finds the time to investigate what's actually happening.

[atugushev]

I've tried to reproduce the issue on AppVeyor and found that adding "pip" to the list below fixes easily the issue:

pip/src/pip/_internal/utils/misc.py

Lines 1189 to 1193 in f377148

	pip_names = [
	"pip.exe",
	"pip{}.exe".format(sys.version_info[0]),
	"pip{}.{}.exe".format(*sys.version_info[:2])
	]

Checkout my test case and AppVeyor log:
https://github.com/atugushev/test-package/blob/master/.appveyor.yml
https://ci.appveyor.com/project/atugushev/test-package/builds/26548167

Tried on atugushev@8462580

[pradyunsg]

Reopening given #7557.