Add style checker and formatter

[ofek]

What does this PR do?

This adds the ability to easily test for style in our CI and format code locally at will.

Motivation

Consistent code formatting across https://github.com/DataDog/integrations-core and https://github.com/DataDog/integrations-extras.

Implementation

We use a tox plugin that, when dd_check_style is set to true in tox.ini, will dynamically expose 2 new environments:

1. style - This will check the formatting using a variety of tools. Using the -s/--style flag of ddev test will execute this environment rather than standalone flake8.
2. format_style - This will format the code for you, often without the need for any additional manual edits. You can run the formatter by using the new -fs/--format-style flag of ddev test.

New checks will have this enabled upon creation.

Style

We now use all of the main style checkers the Python community has coalesced on.

• black - An opinionated formatter, like JavaScript's prettier and Golang's gofmt.
• isort - A tool to sort imports lexicographically, by section, and by type. We use the 5 standard sections: __future__, stdlib, third party, first party, and local. datadog_checks is configured as a first party namespace.
• flake8 - An easy-to-use wrapper around pycodestyle and pyflakes. We select everything it provides and only ignore a few things to give precedence to other tools.
• bugbear - A flake8 plugin for finding likely bugs and design problems in programs. We use:
  • B001: "Do not use bare except:, it also catches unexpected events like memory errors, interrupts, system exit, and so on. Prefer except Exception:."
  • B003: "Assigning to os.environ doesn't clear the environment. Subprocesses are going to see outdated variables, in disagreement with the current process. Use os.environ.clear() or the env= argument to Popen."
  • B006: "Do not use mutable data structures for argument defaults. All calls reuse one instance of that data structure, persisting changes between them."
  • B007: "Loop control variable not used within the loop body. If this is intended, start the name with an underscore."
  • B301: "Python 3 does not include .iter* methods on dictionaries. The default behavior is to return iterables. Simply remove the iter prefix from the method. For Python 2 compatibility, also prefer the Python 3 equivalent if you expect that the size of the dict to be small and bounded. The performance regression on Python 2 will be negligible and the code is going to be the clearest. Alternatively, use six.iter*."
  • B306: BaseException.message has been deprecated as of Python 2.6 and is removed in Python 3. Use str(e) to access the user-readable message. Use e.args to access arguments passed to the exception."
  • B902: Invalid first argument used for method. Use self for instance methods, and cls for class methods."

Config files

black and isort configuration resides in the now-standardized pyproject.toml at the root. flake8 will follow suit when support lands, but for now resides in its usual .flake8.

Once this is merged we'll start syncing these with https://github.com/DataDog/integrations-extras.

Future

Add the black readme badge for each repo when finished

Also, we may want to eventually:

• Get rid of the standalone flake8 environment
• Set a threshold for cyclomatic complexity
• Use bandit to find common security issues

Notes

There are only 2 things to look out for when formatting:

• Merge implicitly concatenated string literals that fit on one line psf/black#26
• isort removes newlines before standalone comments if the following line is an import of the same group psf/black#251

[codecov]

Codecov Report

Merging #3299 into master will decrease coverage by 7.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3299      +/-   ##
==========================================
- Coverage   84.78%   77.75%   -7.04%     
==========================================
  Files         680       31     -649     
  Lines       35594     1043   -34551     
  Branches     4249       92    -4157     
==========================================
- Hits        30180      811   -29369     
+ Misses       4199      200    -3999     
+ Partials     1215       32    -1183

[zippolyte]

LGTM

[jeffwidman]

❤️