Tutorial

Author:Robert Kirkpatrick

Intro

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. 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:

Master:
  --generate-rcfile=<file>
Commands:
  --help-msg=<msg-id>
Commands:
  --help-msg=<msg-id>
Message control:
  --disable=<msg-ids>
Reports:
  --files-output=<y_or_n>
  --reports=<y_or_n>
  --output-format=<format>

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

Output:
   Using the default text output, the message format is :
  MESSAGE_TYPE: LINE_NUM:[OBJECT:] MESSAGE
  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 much probably 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. I borrowed extensively from the code here: http://www.daniweb.com/code/snippet748.html The starting code we will use is called simplecaeser.py and is here in its entirety:

 1  #!/usr/bin/env python
 2
 3  import string
 4
 5  shift = 3
 6  choice = raw_input("would you like to encode or decode?")
 7  word = (raw_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]
24
25  print encoded

Let’s get started.

If we run this:

robertk01 Desktop$ pylint simplecaeser.py
No config file found, using default configuration
************* Module simplecaeser
C:  1, 0: Missing module docstring (missing-docstring)
W:  3, 0: Uses of a deprecated module 'string' (deprecated-module)
C:  5, 0: Invalid constant name "shift" (invalid-name)
C:  6, 0: Invalid constant name "choice" (invalid-name)
C:  7, 0: Invalid constant name "word" (invalid-name)
C:  8, 0: Invalid constant name "letters" (invalid-name)
C:  9, 0: Invalid constant name "encoded" (invalid-name)
C: 16,12: Operator not preceded by a space
            encoded=encoded + letters[x]
                   ^ (no-space-before-operator)


Report
======
19 statements analysed.

Duplication
-----------

+-------------------------+------+---------+-----------+
|                         |now   |previous |difference |
+=========================+======+=========+===========+
|nb duplicated lines      |0     |0        |=          |
+-------------------------+------+---------+-----------+
|percent duplicated lines |0.000 |0.000    |=          |
+-------------------------+------+---------+-----------+



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

+----------+-------+------+---------+-----------+
|type      |number |%     |previous |difference |
+==========+=======+======+=========+===========+
|code      |21     |87.50 |21       |=          |
+----------+-------+------+---------+-----------+
|docstring |0      |0.00  |0        |=          |
+----------+-------+------+---------+-----------+
|comment   |1      |4.17  |1        |=          |
+----------+-------+------+---------+-----------+
|empty     |2      |8.33  |2        |=          |
+----------+-------+------+---------+-----------+



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

+---------+-------+-----------+-----------+------------+---------+
|type     |number |old number |difference |%documented |%badname |
+=========+=======+===========+===========+============+=========+
|module   |1      |1          |=          |0.00        |0.00     |
+---------+-------+-----------+-----------+------------+---------+
|class    |0      |0          |=          |0.00        |0.00     |
+---------+-------+-----------+-----------+------------+---------+
|method   |0      |0          |=          |0.00        |0.00     |
+---------+-------+-----------+-----------+------------+---------+
|function |0      |0          |=          |0.00        |0.00     |
+---------+-------+-----------+-----------+------------+---------+



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

+-----------+-------+---------+-----------+
|type       |number |previous |difference |
+===========+=======+=========+===========+
|convention |7      |7        |=          |
+-----------+-------+---------+-----------+
|refactor   |0      |0        |=          |
+-----------+-------+---------+-----------+
|warning    |1      |1        |=          |
+-----------+-------+---------+-----------+
|error      |0      |0        |=          |
+-----------+-------+---------+-----------+



Messages
--------

+-------------------------+------------+
|message id               |occurrences |
+=========================+============+
|invalid-name             |5           |
+-------------------------+------------+
|no-space-before-operator |1           |
+-------------------------+------------+
|missing-docstring        |1           |
+-------------------------+------------+
|deprecated-module        |1           |
+-------------------------+------------+



Global evaluation
-----------------
Your code has been rated at 5.79/10

Wow. That’s a lot of stuff. The first part is the ‘messages’ section while the second part is the ‘report’ section. There are two points I want to tackle here.

First point is that all the tables of statistics (i.e. the report) are a bit overwhelming so I want to silence them. To do that, I will use the –reports=n option.

Tip

Many of Pylint’s commonly used command line options have shortcuts. for example, –reports=n can be abbreviated to -rn. Pylint’s man page lists all these shortcuts.

Second, previous experience taught me that the default output for the messages needed a bit more info. We can see the first line is:

"C:  1: Missing docstring (missing-docstring)"

This basically means that line 1 violates a convention C. 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 information there is the message symbol between parens, missing-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-docstring
No config file found, using default configuration
:missing-docstring (C0111): *Missing docstring*
  Used when a module, function, class or method has no docstring. Some special
  methods like __init__ doesn't necessary 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. I’m also going to be a bit cowboy and ignore the deprecated-module message because I like to take risks in life. A deprecation warning means that future versions of Python may not support that code so my code may break in the future. There are 5 invalid-name messages that we will get to later. Lastly, I violated the convention of using spaces around an operator such as “=” so I’ll fix that too. To sum up, I’ll add a docstring to line 2, put spaces around the = sign on line 16 and use the --disable=deprecated-module to ignore the deprecation warning.

Here is the updated code:

 1  #!/usr/bin/env python
 2  """This script prompts a user to enter a message to encode or decode
 3  using a classic Caeser shift substitution (3 letter shift)"""
 4
 5  import string
 6
 7  shift = 3
 8  choice = raw_input("would you like to encode or decode?")
 9  word = (raw_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]
26
27  print encoded

And here is what happens when we run it with our --disable=deprecated-module option:

robertk01 Desktop$ pylint --reports=n --disable=deprecated-module simplecaeser.py
No config file found, using default configuration
************* Module simplecaeser
C:  7, 0: Invalid constant name "shift" (invalid-name)
C:  8, 0: Invalid constant name "choice" (invalid-name)
C:  9, 0: Invalid constant name "word" (invalid-name)
C: 10, 0: Invalid constant name "letters" (invalid-name)
C: 11, 0: Invalid constant name "encoded" (invalid-name)

Nice! 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 lives 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 --reports=n --disable=deprecated-module --const-rgx='[a-z_][a-z0-9_]{2,30}$'  simplecaeser.py
No config file found, using default configuration

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.

Tip

It would really be a pain to have to use all these options on the command line all the time. That’s what the 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 the 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.