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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.
2: https://codewithoutrules.com/2016/10/19/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
By default, pylint is *very* verbose:
$ pylint my_module.py No config file found, using default configuration
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.
Just go to the root of your project and run:
$ pylint --generate-rcfile > pylintrc
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.
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:
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
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`)
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].
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
`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
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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