A project I’m working on at is starting to grow rather large. I spent a good deal of effort yesterday getting a five line change added to our (which is run as part of our CI and CD pipeline on every pull request and merge). After the PR was merged, I gloated to others how awesome my team's project was. Here are the five lines: Python Enigma Makefile lint: pylint --rcfile=.pylintrc api -f parseable -r n && \ mypy --silent-imports api && \ pycodestyle api --max-line-length=120 && \ pydocstyle api For completeness sake, here is the target that actually gets run as part of CI tests: test test: install lint python3 setup.py test --addopts="--cov=api" coverage xml -i -o coverage/cobertura-coverage.xml --omit=$(VIRTUAL_ENV)/*,.eggs/* (So, for those not versed in “Makefile-ese”, the target won't run successfully if the target doesn't do so first). test lint Why was I happy enough to be a jerk to other engineers and brag about those five lines? It all comes down to . complexity As Python projects grow, maintenance becomes a nightmare (I’m more referring to enterprise-sized projects rather than “large” personal projects, but the same idea holds). becomes disorganized, messy, reflects the style of the author (even for teams doing a decent job enforcing and , and docstrings fall by the wayside. It takes new developers longer and longer to ramp up on the project. Simple fixes and feature changes become not so simple when it comes time to actually make them. All of these are due to the necessary increase in complexity as a project grows. So how to we reduce that complexity as much as possible? Code PEP-8 PEP-257 No Panacea Obviously, there’s no one answer here, but there’s a set of actions you can take which requires low overhead and pays huge dividends: right now Make code “linters” part of your build process. If of them report issues, fail your build. any any So what is a code “linter”? static analysis Static analysis but not executed_, so there are no external dependencies and running a linter will never harm your environment. You can think of them as a poor-man’s AI code reviewer._ A code “linter” is a tool that performs on your code and reports coding style violations, possible design flaws, outright bugs, and code patterns known to be dangerous. means that your code is analyzed Linters have various output formats and modes they can be run in (and vary in complexity and usefulness). Here’s example output from , one of the most popular Python linters: pylint ************* Module sandman2.appC:117, 0: Line too long (81/80) (line-too-long)C:163, 0: Line too long (88/80) (line-too-long)C: 24, 0: Invalid constant name "auth" (invalid-name)R: 27, 0: Too many arguments (6/5) (too-many-arguments)************* Module sandman2.decoratorsW: 42,24: Duplicate key 'status' in dictionary (duplicate-key)************* Module sandman2.modelC: 49, 0: Line too long (89/80) (line-too-long)C: 50, 0: Line too long (113/80) (line-too-long)C:141, 0: Invalid constant name "DeclarativeModel" (invalid-name)C: 10, 0: Imports from package sqlalchemy are not grouped (ungrouped-imports)************* Module sandman2.serviceC: 98, 0: Trailing whitespace (trailing-whitespace)C:208, 0: Line too long (87/80) (line-too-long)C:215, 0: Line too long (93/80) (line-too-long)C:224, 0: Line too long (82/80) (line-too-long)R:231, 4: Method could be a function (no-self-use)************* Module sandman2.scripts.sandman2ctlC: 40, 0: Line too long (98/80) (line-too-long) Report======364 statements analysed. Statistics by type------------------ +---------+-------+-----------+-----------+------------+---------+|type     |number |old number |difference |%documented |%badname |+= = = == =+|module   |9      |9          |=          |100.00      |0.00     |+---------+-------+-----------+-----------+------------+---------+|class    |12     |12         |=          |100.00      |0.00     |+---------+-------+-----------+-----------+------------+---------+|method   |21     |21         |=          |100.00      |0.00     |+---------+-------+-----------+-----------+------------+---------+|function |18     |18         |=          |100.00      |0.00     |+---------+-------+-----------+-----------+------------+---------+ =+ + + + + External dependencies---------------------:: flask (sandman2.app,sandman2.decorators,sandman2.service)\-views (sandman2.service)flask_sqlalchemy (sandman2.model)sandman2 (sandman2.scripts.sandman2ctl)\-admin (sandman2.app)\-app (sandman2)\-decorators (sandman2.service)\-exception (sandman2.app,sandman2.decorators,sandman2.service)\-model (sandman2.app,sandman2.service)\-service (sandman2.app)sqlalchemy\-ext| \-automap (sandman2.model)| \-declarative (sandman2.model)\-inspection (sandman2.model)\-sql\-sqltypes (sandman2.app) Raw metrics----------- +----------+-------+------+---------+-----------+|type      |number |%     |previous |difference |+ = =====+|code      |459    |54.38 |459      |=          |+----------+-------+------+---------+-----------+|docstring |243    |28.79 |243      |=          |+----------+-------+------+---------+-----------+|comment   |20     |2.37  |22       |-2.00      |+----------+-------+------+---------+-----------+|empty     |122    |14.45 |118      |+4.00      |+----------+-------+------+---------+-----------+ =+ + + + Duplication----------- +-------------------------+------+---------+-----------+|                         |now   |previous |difference |+= =+|nb duplicated lines      |0     |0        |=          |+-------------------------+------+---------+-----------+|percent duplicated lines |0.000 |0.000    |=          |+-------------------------+------+---------+-----------+ =+ + + Messages by category-------------------- +-----------+-------+---------+-----------+|type       |number |previous |difference |+= = =======+|convention |12     |13       |-1.00      |+-----------+-------+---------+-----------+|refactor   |2      |2        |=          |+-----------+-------+---------+-----------+|warning    |1      |3        |-2.00      |+-----------+-------+---------+-----------+|error      |0      |0        |=          |+-----------+-------+---------+-----------+ =+ + + % errors / warnings by module----------------------------- +-----------------------------+------+--------+---------+-----------+|module                       |error |warning |refactor |convention |+= =====+|sandman2.decorators          |0.00  |100.00  |0.00     |0.00       |+-----------------------------+------+--------+---------+-----------+|sandman2.service             |0.00  |0.00    |50.00    |33.33      |+-----------------------------+------+--------+---------+-----------+|sandman2.app                 |0.00  |0.00    |50.00    |25.00      |+-----------------------------+------+--------+---------+-----------+|sandman2.model               |0.00  |0.00    |0.00     |33.33      |+-----------------------------+------+--------+---------+-----------+|sandman2.scripts.sandman2ctl |0.00  |0.00    |0.00     |8.33       |+-----------------------------+------+--------+---------+-----------+ =+ + + + Messages-------- +--------------------+------------+|message id          |occurrences |+======== +|line-too-long       |8           |+--------------------+------------+|invalid-name        |2           |+--------------------+------------+|ungrouped-imports   |1           |+--------------------+------------+|trailing-whitespace |1           |+--------------------+------------+|too-many-arguments  |1           |+--------------------+------------+|no-self-use         |1           |+--------------------+------------+|duplicate-key       |1           |+--------------------+------------+ + Global evaluation-----------------Your code has been rated at 9.59/10 (previous run: 9.50/10, +0.08) Of course, this is verbose, and is definitely the most comprehensive output of any of the linters I’ve used, but it gives a good sense of what they look for and what they’re capable of. provides a helpful (if arbitrary) "score" at the end of the output which is saved between runs, so you know if your code is getting better or worse by standards. super [pylint](https://www.pylint.org/) somewhat pylint So How Does This Relate To Continuous Integration? In their most basic mode, most linters will print out all violations to . That means, if everything went perfectly, the command just runs to success with no output. If you take a look at my change at the top of the article, that's why we can chain together calls to successive linters and the first one to report a violation will abort the entire set of commands. STDOUT and return a non-zero exit code if any violations were encountered Makefile By hooking this up to your CI platform, Someone doesn’t follow PEP-8? Build fails. Sorry, your PR can’t be merged. Didn’t write a docstring for that package/module/class/function? Sorry, build failed. Wrote a function that takes 14 arguments and named it ? You guessed it... you can enforce good style, catch bugs, and actually require proper documentation for everything (and I do mean ) at build time. everything foo Build failed. This is huge for a number of reasons: Everyone responsible for doing code reviews doesn’t even have to at the PR until the build is passing. You actually have to follow all the rules before anyone will read your code. look Linters are capable of catching without ever executing your code. It’s like another (very small) layer of testing actual bugs New developers and code reviewers can open any file in your project is all identical to any other file they might open. The benefit of this can not be overstated. and the layout, naming conventions, documentation, and everything else I’m Convinced! Let’s Lint This Puppy! Python (like most languages) has a number of linters available, all of which do similar (but not necessarily exactly the same) things. To confuse matters, everyone thinks it’s a great idea every once in a while to write a tool that just packages all the linters that are currently en vogue and fires all of them at your code like a fire hose. But this happens every few years, so you get meta-linters made of meta-linters, until who knows what tool is actually looking at your code (and if it’s still maintained). So I’m going to be “that guy” and just tell you what linters you should run. The following list was chosen very carefully, but I’ll get into linter holy wars in the comments if anyone is game. The linters you should use are: [pylint](http://www.pylint.org/) [pycodestyle](https://github.com/PyCQA/pycodestyle) [pydocstyle](https://github.com/PyCQA/pydocstyle) And that’s it. I’ll briefly cover each one below. pylint has been around forever and is almost certainly the most comprehensive Python linter available. Some complain that it's too picky or too verbose, but that's what config files are for! One of the lesser-known (awesome) features of is the file, which will even if you run pylint pylint .pylintrc pylint create a skeleton with sane defaults for you $ pylint --generate-rcfile I still marvel that more CLI tools do this. It’s simply awesome. don’t You can greatly reduce 's verbosity by running with the flag to suppress that giant report. pylint -r n pycodestyle and pydocstyle Both of these linters used to have different names: the former was called and the latter (after the PEPs they check conformance against). In a , GvR himself asked them to kindly change the names of the tools to avoid people hating on just because the tool of the same name was bugging them. Regardless, these tools are comprehensive and easy to get started with. Just them and then run and find out everything you've been doing wrong all this time! pep8 pep257 somewhat hilarious GitHub issue PEP-8 pip install $ pycodestyle <some-module-or-package-or-file> As one might expect, handles (the official Style Guide for Python Code) conformance while handles (the official Docstring Conventions for Python). The latter is awesomely strict. When and if you get your project to pass it without error (took me a day on the project in question) you'll never have to worry about someone forgetting to write a docstring (or forgetting to write a docstring) again. It even checks for proper capitalization, punctuation, and prescriptive vs. descriptive comments. I am in love with its pedantry. pycodestyle PEP-8 pydocstyle PEP-257 how Automate All The Things In all seriousness, is a startup. We have limited resources. If we want to keep a large project sane, the only way to do that is through automation (because we’re too busy building with public data). I would go so far as to argue that, between our testing suite (unit tests and E2E browser tests), continuous integration pipeline (Jenkins + GitHub + Codecov + a lot of homegrown glue), our continuous deployment pipeline (10% Jenkins/GitHub, 90% Ansible), and tools like the linters mentioned above, Enigma awesome stuff we are saving the cost of two full-time engineers to manage our development and deployment processes while making our code base more readable, more testable, and more maintainable. All of that from a couple of automated tools you can install in the next five minutes… What are you waiting for!? Posted on Dec 09, 2016 by Jeff Knupp Originally published at jeffknupp.com on December 9, 2016.

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