[[PageOutline]]

= Python in WebKit =

Below is an overview of WebKit's use of Python.

== Basics ==

 * WebKit's Python scripts require Python 2.5 or higher to run.
 * [http://trac.webkit.org/browser/trunk/WebKitTools/Scripts/test-webkitpy Scripts/test-webkitpy] unit tests the Python code.  It also cleans orphan *.pyc files from webkitpy (i.e. .pyc files without a corresponding .py file).  You can also do this manually (for all *.pyc files in webkitpy) using the following command:
{{{
find WebKitTools/Scripts -wholename '*.pyc' | xargs rm
}}}

== Code Structure ==

 * Most of the Python code is in [http://trac.webkit.org/browser/trunk/WebKitTools/Scripts/webkitpy WebKitTools/Scripts/webkitpy]. 
 * [http://trac.webkit.org/browser/trunk/WebKitTools/Scripts WebKitTools/Scripts] also contains end-user Python scripts (which usually import from webkitpy).
 * Generally, we try to keep as much of the Python code in webkitpy as possible since this allows the code to be organized more nicely (for unit tests to be in companion files, etc).
 * Unit test files are in correspondence with modules.  For example, if module.py is the name of a module, its unit test file would be module_unittest.py and would lie in the same directory.
 * The root-level folders in webkitpy/ generally correspond to end-user scripts in WebKitTools/Scripts.  For example--
  * check-webkit-style -> webkitpy/style/
  * new-run-webkit-tests -> webkitpy/layout_tests/
  * webkit-patch -> webkit/tool/
 * Exceptions to the rule above are--
  * webkitpy/common/: code shared by multiple root folders
  * webkitpy/python24/: code that needs to work under Python 2.4 (currently just the version-checking code)
  * [http://trac.webkit.org/browser/trunk/WebKitTools/Scripts/webkitpy webkitpy/thirdparty/]: all third-party code in webkitpy

== Style ==

Informally, we try to follow [http://www.python.org/dev/peps/pep-0008/ PEP8].  Eventually we may make this official.

For the time being, we are not following the 79 character line length limit (or any line length limit for that matter).

For discussion purposes, here are some additional guidelines to consider:

 * Prefer single quotes to double quotes.  Use double quotes only if the string contains single quotes (or if you are using "triple double quotes").  This makes cutting and pasting from the console easier since Python itself behaves this way when rendering string values to the console.
 * Order `from` and `import` statements within a group by the name of the leading module instead of putting all `from` statements before `import` statements:
{{{
# Correct:
import logging
from optparse import OptionParser
import sys

# Incorrect:
from optparse import OptionParser
import logging
import sys
}}}
 This keeps the ordering of lines the same when changing a `from` statement to an `import` statement, and vice versa.  This also makes it easer to see if both `from` and `import` statements are used to import from the same package (since the lines will be adjacent).  By "within a group," we mean in the PEP8 sense of grouping standard library imports separately from local application imports, etc.

== Upgrading from Python 2.3 or Python 2.4 ==

[Much of these instructions are Mac-specific.]

If you are upgrading from Python 2.3 or 2.4, you should upgrade to Python 2.6 rather than 2.5.

You can tell what version of Python you are currently using by typing--
{{{
python -V
}}}

Before trying to install a new version, check whether your machine already has other versions installed.  From a Mac, you can try reading the man page--
{{{
man python
}}}
For example, Snow Leopard comes with Python 2.5, 2.6, and 3.0.  The man page provides instructions on how to switch your system between these system-installed versions.

If you need to install a new version not on your machine, you can use [http://guide.macports.org/ MacPorts] to do this.  MacPorts allows you to install Python versions alongside your system versions without interfering with them.  After installing MacPorts, simply type (for example)--
{{{
sudo port install python26
}}}

You should probably also install python_select using MacPorts--
{{{
sudo port install python_select
}}}

The python_select command allows you to quickly go back and forth between Python versions, like so--
{{{
> python -V
Python 2.6.4
> sudo python_select python24
Selecting version "python24" for python
> python -V
Python 2.4.6
}}}

To find out what versions of Python you can switch to using python_select, type--
{{{
python_select -l
}}}

== Consequences of not using Python 2.6 ==

Below is a list of some consequences of not using Python 2.6 (and things we should revisit in our code base should we ever upgrade to 2.6):

 * We can't use os.path.relpath(): [http://docs.python.org/library/os.path.html#os.path.relpath]
 * We can't use ZipFile.extractall(): [http://docs.python.org/library/zipfile.html#zipfile.ZipFile.extractall] 
 * We need to use an import statement to use the "with" keyword: http://docs.python.org/whatsnew/2.5.html#pep-343-the-with-statement