Pylint User Manual
Author: Logilab
Keywords: logilab

| Pylint User Manual

| Pylint User Manual 1. Contents Link or reference ("introduction") to an inexistant target. Introduction Link or reference ("what-is-pylint") to an inexistant target. What is pylint? Link or reference ("installation") to an inexistant target. Installation Link or reference ("dependancies") to an inexistant target. Dependancies Link or reference ("distributions") to an inexistant target. Distributions Link or reference ("source-distribution-installation") to an inexistant target. Source distribution installation Link or reference ("note-for-windows-users") to an inexistant target. Note for Windows users Link or reference ("invoking-pylint") to an inexistant target. Invoking pylint Link or reference ("pylint-output") to an inexistant target. Pylint output Link or reference ("source-code-analysis-section") to an inexistant target. Source code analysis section Link or reference ("reports-section") to an inexistant target. Reports section Link or reference ("command-line-options") to an inexistant target. Command line options Link or reference ("daily-pylint-usage") to an inexistant target. Daily pylint usage Link or reference ("bug-reports-feedback") to an inexistant target. Bug reports, feedback Link or reference ("mailing-lists") to an inexistant target. Mailing lists Link or reference ("advanced-usage") to an inexistant target. Advanced usage Link or reference ("base-configuration") to an inexistant target. Base configuration Link or reference ("environment") to an inexistant target. Environment Link or reference ("messages-control") to an inexistant target. Messages control Link or reference ("about-analysis") to an inexistant target. About analysis Link or reference ("pylint-heuristics") to an inexistant target. Pylint heuristics Link or reference ("about-astng-inference") to an inexistant target. About astng inference Link or reference ("enhancing-pylint") to an inexistant target. Enhancing Pylint Link or reference ("writing-your-own-checker") to an inexistant target. Writing your own checker Link or reference ("contribute") to an inexistant target. Contribute ! Link or reference ("other-information") to an inexistant target. Other information Link or reference ("ide-integration") to an inexistant target. IDE integration Link or reference ("some-projects-using-pylint") to an inexistant target. Some projects using Pylint Link or reference ("frequently-asked-questions-usage-tips-for-pylint") to an inexistant target. Frequently Asked Questions / Usage tips for PyLint This document is meant to be the reference user manual for Pylint. This is a work in progress so some sections or parts may be missing (sometimes marked by a XXX). If you think it's lacking some important information, please talk about it on the python-projects mailing list (see the Link or reference ("mailing-lists") to an inexistant target. Mailing lists section for more information about the list). 2. Introduction 2.1. What is pylint? Pylint is a tool that checks for errors in python code, tries to enforce a coding standard and looks for smelling code. This is similar but nevertheless different from what pychecker provides, especially since pychecker explicitely does not bother with coding style. The default coding style used by pylint is close to PEP 008 (aka Guido's style guide). For more information about code smells, refer to Martin Fowler's refactoring book One important thing to note is that Pylint isn't smarter than you are: it may warn you about things that you have conscientiously done. That's for example because it tries to detect things that may be dangerous in a context, but maybe not in others, or because it checks for some things that you don't care about. Generally, you shouldn't expect pylint to be totally quiet about your code, so don't necessarily be alarmed if it gives you a hell lot of messages for your proudly(XXX) project ;) Pylint will display a number of messages as it analyzes the code, as well as some statistics about the number of warnings and errors found in different files. The messages are classified under various categories such as errors and warnings (more below). If you run pylint twice, it will display the statistics from the previous run together with the ones from the current run, so that you can see if the code has improved or not. Last but not least, the code is given an overall mark, based on the number an severity of the warnings and errors. This has proven to be very motivating for programmers. 2.2. Installation 2.2.1. Dependancies Pylint requires the latest logilab-astng and logilab-common packages. It should be compatible with any python version greater than 2.2.0 (python 2.2 users will have to install the optik package). 2.2.2. Distributions The source tarball is available at ftp://ftp.logilab.fr/pub/pylint. You may apt-get a debian package by adding deb ftp://ftp.logilab.org/pub/debian unstable/ to your /etc/apt/sources.list file. Pylint is also available in the standard Debian distribution (but add our public debian repository anyway if you want to get the latest releases and upgrades earlier) Contributed RPM packages for pylint and logilab-common are available at ftp://ftp.nest.pld-linux.org/test. Pylint is also available in Gentoo, Fedora 4, Ubuntu, FreeBSD, Darwin (and maybe other, if si drop us a note please!). 2.2.3. Source distribution installation From the source distribution, extract the tarball, go to the extracted directory and simply run python setup.py install You'll have to install dependancies in a similar way. Windows users may get valuable information about pylint installation on this page. 2.2.4. Note for Windows users On Windows, once you have installed pylint, the command line usage is pylint.bat [options] module_or_package But this will only work if pylint.bat is either in the current directory, or on your system path. (setup.py install install python.bat to the Scripts subdirectory of your Python installation -- e.g. C:Python24Scripts.) You can do any of the following to solve this: change to the appropriate directory before running pylint.bat add the Scripts directory to your path statement in your autoexec.bat file (this file is found in the root directory of your boot-drive) create a 'redirect' batch file in a directory actually on your systems path To effect (2), simply append the appropriate directory name to the PATH= statement in autoexec.bat. Be sure to use the Windows directory separator of ';' between entries. Then, once you have rebooted (this is necessary so that the new path statement will take effect when autoexec.bat is run), you will be able to invoke PyLint with pylint.bat on the command line. (3) is the best solution. Once done, you can call pylint at the command line without the .bat, just as do non-Windows users by typing: pylint [options] module_or_package To effect option (3), simply create a plain text file pylint.bat with the single line: C:\PythonDirectory\Scripts\pylint.bat (where PythonDirectory is replaced by the actual Python installation directory on your system -- e.g. C:Python24Scriptspylint.bat). 2.3. Invoking pylint Pylint is meant to be called from the command line. The usage is pylint [options] module_or_package You should give pylint the name of a Python package or module. Pylint will import this package or module, so you should pay attention to your PYTHONPATH, since it is a common error to analyze an installed version of a module instead of the development version. It is also possible to analyze python files, with a few restriction. The thing to keep in mind is that pylint will try to convert the file name to a module name, and only be able to process the file if it succeeds. pylint mymodule.py should always work since the current working directory is automatically added on top of the python path pylint directory/mymodule.py will work if "directory" is a python package (i.e. has an __init__.py file) or if "directory" is in the python path. For more details on this see the FAQ_. You can also start a thin gui around pylint (require TkInter) by typing pylint-gui This should open a window where you can enter the name of the package or module to check, at pylint messages will be displayed in the user interface. 2.4. Pylint output The default format for the output is raw text. But passing pylint the --html option will produce an HTML document. There are several sections in pylint's output. 2.4.1. Source code analysis section For each python module, pylint will first display a few '*' characters followed by the name of the module. Then, a number of messages with the following format: MESSAGE_TYPE: LINE_NUM:[OBJECT:] MESSAGE You can get another output format, useful since it's recognized by most editors or other development tools using the --parseable=y option. The message type can be: [R]efactor for a "good practice" metric violation [C]onvention for coding standard violation [W]arning for stylistic problems, or minor programming issues [E]rror for important programming issues (i.e. most probably bug) [F]atal for errors which prevented further processing Sometimes the line of code which caused the error is displayed with a caret pointing to the error. This may be generalized in future versions of pylint. Example (extracted from a run of pylint on itself...): ************* Module pylint.checkers.format W: 50: Too long line (86/80) W:108: Operator not followed by a space print >>sys.stderr, 'Unable to match %r', line ^ W:141: Too long line (81/80) W: 74:searchall: Unreachable code W:171:FormatChecker.process_tokens: Redefining built-in (type) W:150:FormatChecker.process_tokens: Too many local variables (20/15) W:150:FormatChecker.process_tokens: Too many branchs (13/12) 2.4.2. Reports section Following the analysis message, pylint will display a set of reports, each one focusing on a particular aspect of the project, such as number of messages by categories, modules dependancies... For instance, the metrics report displays summaries gathered from the current run. the number of processed modules for each module, the percentage of errors and warnings the total number of errors and warnings percentage of classes, functions and modules with docstrings, and a comparison from the previous run percentage of classes, functions and modules with correct name (according the the coding standard), and a comparison from the previous run a list of external dependencies found in the code, and where they appear Also, a global evaluation for the code is computed, and an optional witty comment is displayed (if --comment=y was specified on the command line). 2.5. Command line options First of all, we have two basic (but useful) options.
Option | Description
--version | show program's version number and exit
-h , --help | show help about the command line options
Pylint is architectured around several checkers. By default all checkers are enabled. You can disable a specific checker by specifying --enable-<checker>=n, or disable all checkers using --disable-all and afterwards enable specific checkers with --enable-<checker>=y. See the list of available features for a description of provided checkers with their functionalities. Each checker has some specific options, which can take either a yes/no value, an integer, a python regular expression, or a comma separated list of values (which are generally used to override a regular expression in special cases). For a full list of options, use --help Specifying all the options suitable for your setup and coding standards can be tedious, so it is possible to use a rc file to specify the default values. Pylint looks for /etc/pylintrc and ~/.pylintrc. The --generate-rcfile option will generate a commented configuration file according to the current configuration on standard output and exit. You can put other options before this one to use them in the configuration, or start with the default values and hand tune the configuration. Other useful global options include:
Option | Description
--zope | Initialize Zope products before starting
--ignore=file | Add <file> (may be a directory) to the black list. It should be a base name, not a path. You may set this option multiple times.
--statistics=y_or_n | Compute statistics on collected data.
--persistent=y_or_n | Pickle collected data for later comparisons.
--comment=y_or_n | Add a comment according to your evaluation note.
--parseable=y_or_n | Use a parseable output format.
--html=y_or_n | Use HTML as output format instead of text.
--enable-msg=msgids | Enable the given messages.
--disable-msg=msgids | Disable the given messages.
--enable-msg-cat=cats | Enable all messages in the given categories.
--disable-msg-cat=cats | Disable all messages in the given categories.
--errors-only | Enable only checkers from the error category.
2.6. Daily pylint usage What pylint says is not to be taken as gospel. While getting as few false positives for errors as possible is a goal for us -- and python makes it hard enough, it is not the case for warnings. Quoting Alexandre My usage pattern for pylint is to generally run pylint -e quite often to get stupid errors flagged before launching an application (or before comitting). I generally run pylint with all the bells and whistles activated some time before a release, when I want to cleanup the code. And when I do that I simply ignore tons of the false warnings (and I can do that without being driven mad by this dumb program which is not smart enough to understand the dynamicity of Python because I only run it once or twice a week in this mode) Quoting Marteen Ter Huurne In our project we just accepted that we have to make some modifications in our code to please PyLint: stick to more naming conventions (unused variables ending in underscores, mix-in class names ending in "Mixin") making all abstract methods explicit (rather than just not defining them in the superclass) for messages which are useful in general, but not in a specific case: add "# pylint: disable-msg=X0123" comments for PyLint bugs: add "#pylint: disable-msg=X0123" comments for PyLint limitations: add "#pylint: disable-msg=X0123" comments (for instance Twisted's modules create a lot of definitions dynamically so PyLint does not know about them) The effort is worth it, since PyLint helps us a lot in keeping the code clean and finding errors early. Although most errors found by PyLint would also be found by the regression tests, by fixing them before committing, we save time. And our regression tests do not cover all code either, just the most complex parts. 2.7. Bug reports, feedback You think you have found a bug in Pylint? Well, this may be the case since Pylint is under development. Please take the time to send a bug report to python-projects@logilab.org if you've not found it already reported on the tracker page. This mailing list is also a nice place to discuss Pylint issues, see below for more information about pylint's related lists. You can check for already reported bugs, planned features on pylint's tracker web page: http://www.logilab.org/project/name/pylint Notice that if you don't find something you have expected in pylint's tracker page, it may be on the tracker page of one of its dependancies, namely astng and common: http://www.logilab.org/project/name/logilab-astng http://www.logilab.org/project/name/logilab-common 2.8. Mailing lists Use the python-projects@logilab.org mailing list for anything related to Pylint. This is in most cases better than sending an email directly to the author, since others will benefit from the exchange, and you'll be more likely answered by someone subscribed to the list. This is a moderated mailing list, so if you're not subscribed email you send will have to be validated first before actually being sent on the list. You can subscribe to this mailing list at http://lists.logilab.org/mailman/listinfo/python-projects Archives are available at http://lists.logilab.org/pipermail/python-projects/ If you prefer speaking french instead of english, you can use the generic forum-fr@logilab.org mailing list: (un)subscribe: http://lists.logilab.org/mailman/listinfo/forum-fr archives: http://lists.logilab.org/pipermail/forum-fr Notice though that this list has a very low traffic since most pylint related discussions are done on the python-projects mailing list. 3. Advanced usage 3.1. Base configuration To be written... 3.2. Environment To be written... 3.3. Messages control An example available from the examples directory: """pylint option block-disable-msg""" __revision__ = None class Foo(object): """block-disable-msg test""" def __init__(self): pass def meth1(self, arg): """this issues a message""" print self def meth2(self, arg): """and this one not""" # pylint: disable-msg=W0613 print self\ + "foo" def meth3(self): """test one line disabling""" # no error print self.bla # pylint: disable-msg=E1101 # error print self.blop def meth4(self): """test re-enabling""" # pylint: disable-msg=E1101 # no error print self.bla print self.blop # pylint: enable-msg=E1101 # error print self.blip def meth5(self): """test IF sub-block re-enabling""" # pylint: disable-msg=E1101 # no error print self.bla if self.blop: # pylint: enable-msg=E1101 # error print self.blip else: # no error print self.blip # no error print self.blip def meth6(self): """test TRY/EXCEPT sub-block re-enabling""" # pylint: disable-msg=E1101 # no error print self.bla try: # pylint: enable-msg=E1101 # error print self.blip except UndefinedName: # pylint: disable-msg=E0602 # no error print self.blip # no error print self.blip def meth7(self): """test one line block opening disabling""" if self.blop: # pylint: disable-msg=E1101 # error print self.blip else: # error print self.blip # error print self.blip def meth8(self): """test late disabling""" # error print self.blip # pylint: disable-msg=E1101 # no error print self.bla print self.blop 4. About analysis 4.1. Pylint heuristics To be written... 4.2. About astng inference To be written... 5. Enhancing Pylint 5.1. Writing your own checker You can find some simple examples in the examples directory of the distribution (custom.py and custom_raw.py). I'll try to quickly explain the essentials here. First, there are two kinds of checkers : * raw checkers, which are analysing each module as a raw file stream * ast checkers, which are working on an ast representation of the module The ast representation used is an extension of the one provided with the standard python distribution in the compiler package. The extension adds additional information and methods on the tree nodes to ease navigation and code introspection. An AST checker is a visitor, and should implement visit_<lowered class name> leave_<lowered class name> methods for the nodes it's interested in. To get description of the different classes used in an ast tree, look at the compiler.ast documentation. Checkers are ordered by priority. For each module, pylint's engine: give the module source file as a stream to raw checkers get an ast representation for the module make a depth first descent of the tree, calling visit_<> on each AST checker when entering a node, and living_<> on the back traversal Notice that the source code is probably the best source of documentation, it should be clear and well documented. Don't hesitate to ask for any information on the python-projects mailing list. 5.2. Contribute ! All our software is developped using the mercurial version control system. This is a very cool distributed vcs and its usage is very similar to other ones such as cvs or subversion (though the distributed feature introduced some different usage patterns). See mercurial home page for installation on your computer and basic usage. Note that it's very easy to send us patches using hg email command ;). You can get the in-development pylint source code from our public mercurial repository: http://www.logilab.org/src/pylint The same is true for pylint dependancies (if you use pylint code from the repository, you should usually use code from the repository as well for astng and common): http://www.logilab.org/src/logilab/astng http://www.logilab.org/src/logilab/common 6. Other information 6.1. IDE integration Pylint is integrated in the following editors/IDEs: emacs (of course) eric3 eclipse (using the pydev plugin, see also http://msdl.cs.mcgill.ca/MSDL/people/denis/meetings/pythonDev) To use pylint from within vim, see http://www.gonzo.kiev.ua/projects/pylint.vim To use pylint from within komodo, see http://mateusz.loskot.net/2006/01/15/running-pylint-from-komodo/ To use pylint from within gedit, see http://live.gnome.org/Gedit/PylintPlugin 6.2. Some projects using Pylint The following projects are known to use pylint to help develop better code: OSAF Chandler (http://www.osafoundation.org/) Xen (http://www.xensource.com/) CPS (http://www.nuxeo.org) ERP5 (http://www.erp5.org/) pyxmpp (http://pyxmpp.jabberstudio.org/) mercurial eXe (http://exelearning.org/) PrimaGIS (http://www.primagis.org) python-cdd (http://projetos.ossystems.com.br/python-cdd/) CDSWare (http://cdsware.cern.ch/) ASE (http://dcwww.camp.dtu.dk/campos/ASE/intro.html) RunJob (http://projects.fnal.gov/runjob/) Slugathon (http://slugathon.python-hosting.com/) Topographica (http://topographica.org/Home/index.html) (at least they intend to do so) http://browsershots.org many more... Also notice that the CheeseCake kwalitee reporting tool uses pylint to analyze the source code. 7. Frequently Asked Questions / Usage tips for PyLint Question: Is it possible to give file as argument to pylint, instead of module ? Answer: pylint expects the name of a package or module as argument. As a convenience, you can give to it a file name if it's possible to guess a module name from the file's path, using the python path. Some examples : "pylint mymodule.py" should always works since the current working directory is automatically added on top of the python path "pylint directory/mymodule.py" will work if "directory" is a python package (i.e. has an __init__.py file) or if "directory" is in the python path. "pylint /whatever/directory/mymodule.py" will work if either: "/whatever/directory" is in the python path your cwd is "/whatever/directory" "directory" is a python package and "/whatever" is in the python path "directory" is a python package and your cwd is "/whatever" and so on... Question: I'm using psyobj from psyco and get a lot of spurious "unused variables messages". Is it normal ? Answer: Yes. That's actually due to a bug in psyco, making the locals() function for objects inheriting from psyobj returning an empty dictionary. For the moment, the only way to fix this is to use the PYLINT_IMPORT environment variable to not use psyco during pylint checking. Sample code import os try: if os.environ.has_key('PYLINT_IMPORT'): raise ImportError() from psyco.classes import psyobj except ImportError: class psyobj: pass NOTICE: this problem should not occurs with pylint >= 0.5 since from this version pylint is not looking anymore for information in living objects (i.e. it doesn't anymore import analysed modules) Question: I've a function / method which is a callback where I do not have any control on received argument, and pylint is complaining about unused arguments. What can I do to avoid those warnings ? Answer: prefix (ui) the callback's name by cb_, as in cb_onclick(...). By doing so arguments usage won't be checked. Another solution is to use one of the name defined in the "dummy-variables" configuration variable for unused argument ("_" and "dummy" by default). Question: When is pylint considering a class as an interface ? Answer: A class is considered as an interface if there is a class named "Interface" somewhere in it ancestor's tree. Question: When is pylint considering that a class is implementing a given interface ? Answer: Pylint is using the Zope 2 interfaces conventions, and so is considering that a class is implementing interfaces listed in its __implements__ attribute. Question: When is pylint considering a class as an abstract class ? Answer: A class is considered as an abstract class if at least one of its methods is doing nothing but raising NotImplementedError Question: Is there some way to disable some message for a particular module only ? Answer: Yes, you can disable or enable (globally disabled) message at the module level by adding the corresponding option in a comment at the top of the file: # pylint: disable-msg=W0401, E0202 # pylint: enable-msg=C0302 Question: I have a mixin class relying on attributes of the mixed class, and I would like to not have the "access to undefined member" message on this class. Is it possible ? Answer: Yes :o) To do so you have to set the ignore-mixin-members option to "yes" (this is the default value) and to name your mixin class with a name which ends with "mixin" (whatever case) Question: Is it possible to locally disable a particular message for a block of code or for a single line of code ? Answer: Yes, this feature has been added in pylint 0.11. This may be done by adding "#pylint: disable-msg=W0123,E4567" at the desired block level or at the end of the desired line of code Question: Where is the persistent data stored to make comparison between two successive runs ? Answer: Analysis data are stored as pickle file in a directory which is localized using the following rules: value of the PYLINTHOME environment variable if set ".pylint.d" subdirectory of the user's home directory if it is found (not always findable on Windows platforms) ".pylint.d" directory in the current directory Question: How can I know the option name (for pylintrc) corresponding to a specific command line option ? Answer: You can always generate a sample pylintrc file with --generate-rcfile Every option present on the command line before this will be included in the rc file For example: pylint --disable-msg=W0702,C0103 --class-rgx='[A-Z][a-z]+' --generate-rcfile Question: pychecker has no problem finding the imports and reporting on problems with them, while pylint seems unable to deal with the same imports. Why ? Answer: pychecker and pylint use different approaches. pychecker imports the modules and rummages around in the result, hence it sees my mangled sys.path. pylint doesn't import any of the candidate modules and thus doesn't include any of import's side effects (good and bad). It traverses an AST representation of the code. 8. Docutils System Messages Unknown target name: "faq".