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2016, Jul 23 - Dimitri Merejkowsky
License: CC By 4.0

I've been using pylint[1] for quite some time now, so today I'd like to share a few tips with you.

1: https://www.pylint.org

2: https://codewithoutrules.com/2016/10/19/pylint/







What is pylint?







It's a static analyzer for Python code. "Static" means that it won't execute your code, it will just parse it to find mistakes or things that do not respect a given coding style.







Pylint is capable of emitting very interesting warnings.







Here are some examples:







def foo(bar, baz):
    pass

foo(42)







<cite>
No value for argument 'baz' in function call
(no-value-for-parameter)
</cite>








class MyThread(threading.Thread):
    def __init__(self, name):
        self.name = name

    def run(self):
        ...


my_thread = MyThread()
my_thread.start()







<cite>
\_\_init\_\_ method from base class 'Thread' is not called (super-init-not-called)
</cite>







This is nice because it saves you a fatal assertion at runtime:







AssertionError: Thread.__init__() not called







Fixing pylint output







By default, pylint is *very* verbose:







$ pylint my_module.py
No config file found, using default configuration

C:  1, 0: Missing module docstring (missing-docstring)
...

Report
======
8 statements analysed.

Statistics by type
------------------

+---------+-------+-----------+-----------+------------+---------+
|type     |number |old number |difference |%documented |%badname |
+=========+=======+===========+===========+============+=========+
|module   |1      |1          |=          |0.00        |100.00   |
+---------+-------+-----------+-----------+------------+---------+
...
+---------+-------+-----------+-----------+------------+---------+


Raw metrics
-----------

...

Messages by category
--------------------

+-----------+-------+---------+-----------+
|type       |number |previous |difference |
+===========+=======+=========+===========+
|convention |3      |3        |=          |

...

+-----------+-------+---------+-----------+
|error      |1      |1        |=          |
+-----------+-------+---------+-----------+


Global evaluation
-----------------
Your code has been rated at -3.75/10 (previous run: 1.43/10, -5.18)







Let's fix that!







The goal is to have no output at all when everything is fine, only have the errors if something is wrong, and make sure output of `pylint` can then be used by an other program if required.







Get rid of "No config file found, using default configuration"







Just go to the root of your project and run:







$ pylint --generate-rcfile > pylintrc







More readable output







Edit the `pylintrc` file to have:







[REPORTS]

output-format=parseable







That way you'll get a more standard output, with the file name, a colon, the line number and the error message:







my_module.py:11: [C0103(invalid-name), ] Invalid constant name "my_thread"







This is useful if you want to run `pylint` from your editor and quickly jump to the lines that contains errors.







Get rid of the useless stuff







You don't really care about all the stats, so let's just disable everything:







[REPORTS]

reports = no







There! Now we only get only the warning or error messages, except there's a big line to separate the modules:








my_module.py:4: [W0231(super-init-not-called), MyThread.__init__] __init__ method from base
                class 'Thread' is not called

other_stuff.py:5: [W0311(bad-indentation), ] Bad indentation. Found 3 spaces, expected 4







Correcting false positives







Sometimes `pylint` thinks there's a problem in your code even though it's perfectly fine.







Here's an example using the excellent path.py[3] library:







3: https://pythonhosted.org/path.py/






my_path = path.Path(".").abspath()
my_path.joinpath("foo")







<cite>
my_module.py:5: [E1120(no-value-for-parameter), main] No value for argument
'first' in unbound method call
</cite>







If you take a look, it seems that `pylint` gets confused by upstream code:







class multimethod(object):
    """
    Acts like a classmethod when invoked from the class and like an
    instancemethod when invoked from the instance.
    """
    ...


class Path():
    ...

    @multimethod
    def joinpath(cls, first, *others):
        if not isinstance(first, cls):
            first = cls(first)
        return first._next_class(first.module.join(first, *others))







It's some dark magic (yeah Python) to make sure you can use both:







path_1 = my_path.joinpath("bar")
path_2 = path.Path.joinpath(my_path, "bar")







and `pylint` only "gets" the second usage...







Here the solution is to use a "pragma" to tell `pylint` that the code is fine







my_path = path.Path(".").abspath()
# pylint: disable=no-value-for-parameter
a_path = my_path.joinpath("foo")







But if you do that, you'll get:







<cite>
my_module.py:5: [I0011(locally-disabled), ] Locally disabling no-value-for-parameter (E1120)
</cite>







The solution is to disable warnings about disabled warnings (so meta):







[MESSAGES CONTROL]

disable=reduce-builtin,dict-iter-method,reload-builtin, ... ,locally-disabled







Freezing pylint version







If you're like me, you probably have a `dev-requirements.txt` containing a line about `pylint` in order to use in a `virtualenv`.







It's also possible you're using buildout[4].







4: http://www.buildout.org/en/latest






But anyway, I highly recommend you have a separate installation of `pylint` just for your project, isolated from the rest of your system.







The fact is that `pylint` depends on `astroid`, and both projects are constantly evolving.







So if you're not careful, you may end up upgrading `astroid` or `pylint` and suddenly some false positives will get fixed, and some other will appear.







So to make sure this does not happen, always freeze `pylint` and `astroid` version numbers, like so:







pylint==1.5.5
astroid==1.4.7







(you can use `pip freeze` to see the version of all the packages in your `virtualenv`)







Running pylint







To run `pylint`, you have to give it a list of packages or modules do check on the command line.







For instance, let's assume your sources look like this:







src
  pylintrc
  bar.py
  spam
     __init__.py
     eggs.py







Then you have to call `pylint` like this:







$ cd src
$ pylint bar.py spam







You may try to run `pylint .` or just `pylint` but it won't work :/







So this means that anytime you add a package or a new module, you have to change the way you call `pylint`.







This is rather annoying, that's why I suggest you use invoke[5].







5: http://www.pyinvoke.org






Write a `tasks.py` file looking like:







import path
import invoke

def get_pylint_args():
    top_path = path.Path(".")
    top_dirs = top_path.dirs()
    for top_dir in top_dirs:
        if top_dir.joinpath("__init__.py").exists():
            yield top_dir
    yield from (x for x in top_path.files("*.py"))


@invoke.task
def pylint():
    invoke.run("pylint " + " ".join(get_pylint_args()), echo=True)







And then you just have to use:







$ invoke pylint







Speeding up pylint







`pylint` also knows how to use multiple jobs so that it runs faster.







Since you are already using `tasks.py`, you can specify the number of jobs to use in a cross-platform way easily:







import multiprocessing


def get_pylint_args():
    ...

    num_cpus = multiprocessing.cpu_count()
    yield "-j%i" % num_cpus







Shorter version







You can also *not* do all of the above and just fire up `pylint` like so:







$ pylint -E *.py







It will only show errors, (`-E` is short for `--errors-only`), and will have a good enough output.












That's all for today, see you next time!


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