completion: pip: Only invoke pip when trying to complete it

[NoahGorny]

Description

Invokes pip only when actively trying to complete it.

Motivation and Context

This should greatly reduce shell load time, as the pip invocation takes
a lot of time.

How Has This Been Tested?

Locally

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[NoahGorny]

@cornfeedhobo @davidpfarrell @nwinkler what is your opinion on this?
it greatly improves shell load time for me

[cornfeedhobo]

@NoahGorny I'd love to test this more thoroughly soon, but I'm a little bogged down. If you're okay waiting through this week, I'll probably get to it.

[cornfeedhobo]

Tested. It's beautiful. Let's definitely get this out.

[cornfeedhobo]

Looks great and tests great, but small change requested. Thanks!

[cornfeedhobo]

I'm also thinking we might want to generalize the lazy loading of any function, but I think I could handle that in a follow up PR.

[NoahGorny]

@cornfeedhobo finally got around to actaully doing it, thanks for the CR 😄

[hongyi-zhao]

@cornfeedhobo finally got around to actaully doing it, thanks for the CR smile

What's the meaning of CR used by you here?

[davidpfarrell]

First I was worried about the surprised delay the user might get on first use, but TIL that bash-completion loads completions lazily by default.

We might consider a framework to also lazy-load completions by default, but until then, this seems legit.

[davidpfarrell]

@hongyi-zhao wrote:

What's the meaning of CR used by you here?

I believe @NoahGorny uses CR ubiquitously to mean "Code Review"

[NoahGorny]

@cornfeedhobo ping 😄

[cornfeedhobo]

This looks great. There is a small lag on the first tab, but that's to be expected with lazy loading.