process(python): Explore the possibility of using a single linter

[parthea]

In the templated nox_file.py for python, there is a lint nox session which runs both black and flake8 linters.

synthtool/synthtool/gcp/templates/python_library/noxfile.py.j2

Lines 52 to 64 in 02193e4

	def lint(session):
	"""Run linters.
	Returns a failure if the linters find linting errors or sufficiently
	serious code quality issues.
	"""
	session.install("flake8", BLACK_VERSION)
	session.run(
	"black",
	"--check",
	*BLACK_PATHS,
	)
	session.run("flake8", "google", "tests")

googleapis/python-aiplatform#1087, there is a conflict between the 2 lint tools that we use where a formatting change made by black causes the flake8 check to fail.

In the current configuration, black and flake8 check for different things. black tests/applies formatting while flake checks for syntax.

To check this, create an empty file test_lint.py with a single import statement and no new line at the end of the file. As an example, I used import google.api in an empty file without a blank line at the end.

If you run black --check test_lint.py which tests the file without changing it.

black --check test_lint.py
would reformat test_lint.py
All done! 💥 💔 💥
1 file would be reformatted.

Then run black test_lint.py, which adds an empty line at the end of the file.

black test_lint.py
reformatted test_lint.py
All done! ✨ 🍰✨
1 file reformatted.

On the same file run flake8 test_lint.py which tests for syntax.

flake8 test_lint.py
test_lint.py:1:1: F401 'google.api' imported but unused

I've opened this issue to understand the impact of removing black, and confirm that flake8 can fill the gap.

[parthea]

I was able to re-create the issue reported in googleapis/python-aiplatform#1087 in this PR. See the build log here. I'm going to open a PR to add E231 to the .flake8 ignore list. That should resolve the issue where the linters are battling with each other. Let's continue to add flake8 formatting errors to the ignore list as needed as we find compatibility issues with black.