Improve TypeVar name regex

[cdce8p]

Description

Small improvements for the TypeVar name regex.

• Simplify [^\W\da-z_] to [A-Z]
• Add negative lookahead to prevent T or Type prefix. E.g. TAnyStr should be marked as invalid-name.

[coveralls]

Pull Request Test Coverage Report for Build 2888845479

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.252%

---

Totals
Change from base Build 2888370961:	0.0%
Covered Lines:	16872
Relevant Lines:	17713

---

💛 - Coveralls

[cdce8p]

@DanielNoord Do you know what I need to do for the checks to pass? Should I change the fragment suffix to .other or do I always need a Ref?

[DanielNoord]

@Pierre-Sassoulas We still can't seem to merge without updating branches 😢

[Pierre-Sassoulas]

We still can't seem to merge without updating branches

I'm going to look into it, I'm not on mobile anymore. I hope you'll get admin-ship soon, be it the easy way or the hard way 😄

[Pierre-Sassoulas]

Seems like requiring a new review if the code change also auto-checked the "up to date code" option.

[github-actions]

🤖 Effect of this PR on checked open source code: 🤖

Effect on astroid:
The following messages are now emitted:

1. invalid-name:
   Type variable name "_NodesT2" doesn't conform to predefined naming style
   https://github.com/PyCQA/astroid/blob/main/astroid/nodes/node_ng.py#L54
2. invalid-name:
   Type variable name "_NodesT3" doesn't conform to predefined naming style
   https://github.com/PyCQA/astroid/blob/main/astroid/nodes/node_ng.py#L55

Effect on pandas:
The following messages are now emitted:

1. invalid-name:
   Type variable name "_TDT" doesn't conform to predefined naming style
   https://github.com/pandas-dev/pandas/blob/main/pandas/core/indexes/datetimelike.py#L75

Effect on pytest:
The following messages are now emitted:

1. invalid-name:
   Type variable name "TResult" doesn't conform to predefined naming style
   https://github.com/pytest-dev/pytest/blob/main/src/_pytest/runner.py#L264

Effect on sentry:
The following messages are now emitted:

1. invalid-name:
   Type variable name "_K1" doesn't conform to predefined naming style
   https://github.com/getsentry/sentry/blob/master/src/sentry/release_health/metrics.py#L90
2. invalid-name:
   Type variable name "_K2" doesn't conform to predefined naming style
   https://github.com/getsentry/sentry/blob/master/src/sentry/release_health/metrics.py#L91
3. invalid-name:
   Type variable name "TDecoded" doesn't conform to predefined naming style
   https://github.com/getsentry/sentry/blob/master/src/sentry/utils/codecs.py#L12
4. invalid-name:
   Type variable name "TEncoded" doesn't conform to predefined naming style
   https://github.com/getsentry/sentry/blob/master/src/sentry/utils/codecs.py#L13

This comment was generated for commit a9ab795

[mattclay]

@cdce8p Where does the recommendation to avoid a T prefix come from?

[DanielNoord]

@mattclay I think this follows from what we have "seen in the wild". Note that you can allow them again by setting the typevar-rgx pattern config in your configuration file or on the command line.

[yilei]

The new regex also disallows things like _T1 _T2 which is a quite common practice

[mattclay]

@DanielNoord I don't know how common the T prefix is, but it's definitely used "in the wild". For example, it's used in the ansible project, where I counted at least 24 uses. Here's one example:

https://github.com/ansible/ansible/blob/f8e83264372d583a8f819f7bfe03a0f53f1abc0a/test/lib/ansible_test/_internal/config.py#L39

I realize the pattern can be customized, but it seems like an unnecessarily arbitrary stylistic restriction for a default.

[DanielNoord]

The new regex also disallows things like _T1 _T2 which is a quite common practice

This is problematic indeed.

I realize the pattern can be customized, but it seems like an unnecessarily arbitrary stylistic restriction for a default.

@cdce8p Do you have on opinion on this? If not I think we might want to revert this change. _T1 seems like it should be allowed and perhaps allowing other examples with a prefix of T is also better?

[cdce8p]

@cdce8p Where does the recommendation to avoid a T prefix come from?

I mainly looked at a number of typing PEPs as well as the TypeVar names used within typeshed as reference. Those do strongly prefer a T suffix over prefix which is why I advocated for that pattern.

The new regex also disallows things like _T1 _T2 which is a quite common practice

It's not as common as you may think. I would even argue that it's an antipattern which should be avoided except for very specific cases. It's much better to use different / more verbose names to avoid confusion and improve readability. Some common once include S, T, U, V, KT, VT. Just as an example take a look at the TypeVars defined in typeshed/stdlib/typing.pyi.