Author:Robert Kirkpatrick


Beginner to coding standards? Pylint can be your guide to reveal what's really going on behind the scenes and help you to become a more aware programmer.

Sharing code is a rewarding endeavor. Putting your code out there can be either an act of philanthropy, coming of age, or a basic extension of belief in open source. Whatever the motivation, your good intentions may not have the desired outcome if people find your code hard to use or understand. The Python community has formalized some recommended programming styles to help everyone write code in a common, agreed-upon style that makes the most sense for shared code. This style is captured in PEP 8, the "Style Guide for Python Code". Pylint can be a quick and easy way of seeing if your code has captured the essence of PEP 8 and is therefore friendly to other potential users.

Perhaps you're not ready to share your code but you'd like to learn a bit more about writing better code and don't know where to start. Pylint can tell you where you may have run astray and point you in the direction to figure out what you have done and how to do better.

This tutorial is all about approaching coding standards with little or no knowledge of in-depth programming or the code standards themselves. It's the equivalent of skipping the manual and jumping right in.

My command line prompt for these examples is:

robertk01 Desktop$

Getting Started

Running Pylint with no arguments will invoke the help dialogue and give you an idea of the arguments available to you. Do that now, i.e.:

robertk01 Desktop$ pylint
a bunch of stuff

A couple of the options that we'll focus on here are:

Messages control:

If you need more detail, you can also ask for an even longer help message, like so:

robertk01 Desktop$ pylint --long-help
Even more stuff

Pay attention to the last bit of this longer help output. This gives you a hint of what Pylint is going to pick on:

   Using the default text output, the message format is :
  There are 5 kind of message types :
  * (C) convention, for programming standard violation
  * (R) refactor, for bad code smell
  * (W) warning, for python specific problems
  * (E) error, for probable bugs in the code
  * (F) fatal, if an error occurred which prevented pylint from doing
  further processing.

When Pylint is first run on a fresh piece of code, a common complaint is that it is too noisy. The current default configuration is set to enforce all possible warnings. We'll use some of the options I noted above to make it suit your preferences a bit better (and thus make it emit messages only when needed).

Your First Pylint'ing

We'll use a basic Python script as fodder for our tutorial. The starting code we will use is called simplecaesar.py and is here in its entirety:

 1  #!/usr/bin/env python3
 3  import string;
 5  shift = 3
 6  choice = input("would you like to encode or decode?")
 7  word = input("Please enter text")
 8  letters = string.ascii_letters + string.punctuation + string.digits
 9  encoded = ''
10  if choice == "encode":
11      for letter in word:
12          if letter == ' ':
13              encoded = encoded + ' '
14          else:
15              x = letters.index(letter) + shift
16              encoded = encoded + letters[x]
17  if choice == "decode":
18      for letter in word:
19          if letter == ' ':
20              encoded = encoded + ' '
21          else:
22              x = letters.index(letter) - shift
23              encoded = encoded + letters[x]
25  print(encoded)

Let's get started.

If we run this:

robertk01 Desktop$ pylint simplecaesar.py
************* Module simplecaesar
simplecaesar.py:3:0: W0301: Unnecessary semicolon (unnecessary-semicolon)
simplecaesar.py:1:0: C0114: Missing module docstring (missing-module-docstring)
simplecaesar.py:5:0: C0103: Constant name "shift" doesn't conform to UPPER_CASE naming style (invalid-name)
simplecaesar.py:9:0: C0103: Constant name "encoded" doesn't conform to UPPER_CASE naming style (invalid-name)
simplecaesar.py:13:12: C0103: Constant name "encoded" doesn't conform to UPPER_CASE naming style (invalid-name)

Your code has been rated at 7.37/10

Previous experience taught me that the default output for the messages needed a bit more info. We can see the second line is:

"simplecaesar.py:1:0: C0114: Missing module docstring (missing-module-docstring)"

This basically means that line 1 violates a convention C0111. It's telling me I really should have a docstring. I agree, but what if I didn't fully understand what rule I violated. Knowing only that I violated a convention isn't much help if I'm a newbie. Another piece of information there is the message symbol between parens, missing-module-docstring here.

If I want to read up a bit more about that, I can go back to the command line and try this:

robertk01 Desktop$ pylint --help-msg=missing-module-docstring
:missing-module-docstring (C0114): *Missing module docstring*
  Used when a module has no docstring.Empty modules do not require a docstring.
  This message belongs to the basic checker.

Yeah, ok. That one was a bit of a no-brainer, but I have run into error messages that left me with no clue about what went wrong, simply because I was unfamiliar with the underlying mechanism of code theory. One error that puzzled my newbie mind was:

:too-many-instance-attributes (R0902): *Too many instance attributes (%s/%s)*

I get it now thanks to Pylint pointing it out to me. If you don't get that one, pour a fresh cup of coffee and look into it - let your programmer mind grow!

The Next Step

Now that we got some configuration stuff out of the way, let's see what we can do with the remaining warnings.

If we add a docstring to describe what the code is meant to do that will help. There are 5 invalid-name messages that we will get to later. Lastly, I put an unnecessary semicolon at the end of the import line so I'll fix that too. To sum up, I'll add a docstring to line 2, and remove the ; from line 3.

Here is the updated code:

 1  #!/usr/bin/env python3
 2  """This script prompts a user to enter a message to encode or decode
 3  using a classic Caesar shift substitution (3 letter shift)"""
 5  import string
 7  shift = 3
 8  choice = input("would you like to encode or decode?")
 9  word = input("Please enter text")
10  letters = string.ascii_letters + string.punctuation + string.digits
11  encoded = ''
12  if choice == "encode":
13      for letter in word:
14          if letter == ' ':
15              encoded = encoded + ' '
16          else:
17              x = letters.index(letter) + shift
18              encoded = encoded + letters[x]
19  if choice == "decode":
20      for letter in word:
21          if letter == ' ':
22              encoded = encoded + ' '
23          else:
24              x = letters.index(letter) - shift
25              encoded = encoded + letters[x]
27  print(encoded)

Here is what happens when we run it:

robertk01 Desktop$ pylint simplecaesar.py
************* Module simplecaesar
simplecaesar.py:7:0: C0103: Constant name "shift" doesn't conform to UPPER_CASE naming style (invalid-name)
simplecaesar.py:11:0: C0103: Constant name "encoded" doesn't conform to UPPER_CASE naming style (invalid-name)
simplecaesar.py:15:12: C0103: Constant name "encoded" doesn't conform to UPPER_CASE naming style (invalid-name)

Your code has been rated at 8.42/10 (previous run: 7.37/10, +1.05)

Nice! Pylint told us how much our code rating has improved since our last run, and we're down to just the invalid-name messages.

There are fairly well defined conventions around naming things like instance variables, functions, classes, etc. The conventions focus on the use of UPPERCASE and lowercase as well as the characters that separate multiple words in the name. This lends itself well to checking via a regular expression, thus the should match (([A-Z_][A-Z1-9_]*)|(__.*__))$.

In this case Pylint is telling me that those variables appear to be constants and should be all UPPERCASE. This is an in-house convention that has lived with Pylint since its inception. You too can create your own in-house naming conventions but for the purpose of this tutorial, we want to stick to the PEP 8 standard. In this case, the variables I declared should follow the convention of all lowercase. The appropriate rule would be something like: "should match [a-z_][a-z0-9_]{2,30}$". Notice the lowercase letters in the regular expression (a-z versus A-Z).

If we run that rule using a --const-rgx='[a-z\_][a-z0-9\_]{2,30}$' option, it will now be quite quiet:

robertk01 Desktop$ pylint --const-rgx='[a-z_][a-z0-9_]{2,30}$' simplecaesar.py

Your code has been rated at 10.00/10 (previous run: 8.42/10, +1.58)

Regular expressions can be quite a beast so take my word on this particular example but go ahead and read up on them if you want.


It would really be a pain to specify that regex on the command line all the time, particularly if we're using many other options. That's what a configuration file is for. We can configure our Pylint to store our options for us so we don't have to declare them on the command line. Using a configuration file is a nice way of formalizing your rules and quickly sharing them with others. Invoking pylint --generate-rcfile will create a sample rcfile with all the options set and explained in comments.

That's it for the basic intro. More tutorials will follow.