A Beginner's Guide to Code Standards in Python - Pylint Tutorial
Author: Logilab
Keywords: logilab

| A Beginner's Guide to Code Standards in Python - Pylint Tutorial

| A Beginner's Guide to Code Standards in Python - Pylint Tutorial For a detailed description of Pylint, see http://www.logilab.org/project/pylint. 1. 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$ 2. Getting Started Running Pylint with no arguments will invoke the help dialogue and give you a 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: --rcfile=<file> Commands: --help-msg=<msg-id> Commands: --help-msg=<msg-id> Message control: --disable-msg=<msg-ids> Reports: --files-output=<y_or_n> --reports=<y_or_n> --include-ids=<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 occured 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 'scream only when needed'). 3. 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 it's 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: Missing docstring W: 3: Uses of a deprecated module 'string' C: 5: Invalid name "shift" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C: 6: Invalid name "choice" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C: 7: Invalid name "word" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C: 8: Invalid name "letters" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C: 9: Invalid name "encoded" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C: 16: Operator not preceded by a space encoded=encoded + letters[x] ^ 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 |occurences | +===========+===========+ |C0103 |5 | +-----------+-----------+ |W0402 |1 | +-----------+-----------+ |C0322 |1 | +-----------+-----------+ |C0111 |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. 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" 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. So let's turn on a bit more info by using the option "--include-ids=y". Let's do it again! robertk01 Desktop$ pylint --reports=n --include-ids=y simplecaeser.py No config file found, using default configuration ************* Module simplecaeser C0111: 1: Missing docstring W0402: 3: Uses of a deprecated module 'string' C0103: 5: Invalid name "shift" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C0103: 6: Invalid name "choice" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C0103: 7: Invalid name "word" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C0103: 8: Invalid name "letters" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C0103: 9: Invalid name "encoded" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C0322: 16: Operator not preceded by a space encoded=encoded + letters[x] Oooh. I like that better. Now I know that I violated the convention number C0111 and now I can read up a bit more about that. Let's go back to the command line and try this: robertk01 Desktop$ pylint --help-msg=C0111 No config file found, using default configuration :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: :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! 4. 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 W0402 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 C0103 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-msg=W0402" to ignore the deprecation warning. Here's the updated code: 1 #!/usr/bin/env python 2 """This script prompts a user to enter a messsage 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's what happens when we run it with our --disable-msg=W0402 option: robertk01 Desktop$ pylint --reports=n --include-ids=y --disable-msg=W0402 simplecaeser.py No config file found, using default configuration ************* Module simplecaeser C0103: 7: Invalid name "shift" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C0103: 8: Invalid name "choice" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C0103: 9: Invalid name "word" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C0103: 10: Invalid name "letters" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) C0103: 11: Invalid name "encoded" (should match (([A-Z_][A-Z1-9_]*)|(__.*__))$) Nice! We're down to just the C0103 messages. There are fairly well defined conventions around naming things like instance variables, functions, classes, etc. The conventions focus on the use of UPPERCASE and lowercase as well as the characters that separate multiple words in the name. This lends itself well to checking via a regular expression, thus the "should match (([A-Z_][A-Z1-9_]*)|(__.*__))$". In this case Pylint is telling me that those variables appear to be constants and should be all UPPERCASE. This rule is in fact a naming convention that is specific to the folks at Logilab who created Pylint. That is the way they have chosen to name those variables. 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 --include-ids=y --disable-msg=W0402 --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. It would really be a pain in the butt to have to use all these options on the command line all the time. That's what the rc 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 rc file is a nice way of formalizing your rules and quickly sharing them with others and/or forcing your style on them. Go ahead, conquer the standards world by spreading your rc file...I dare you. That's it for the basic intro. More tutorials will follow.