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Python Style Guide

This document describes expected practices when writing Python code. There are occasions when you can break these rules, but be prepared to justify doing so when your code gets reviewed.

Existing Conventions

There are well-established conventions in the Python community, and in general we should follow these. General Python conventions, and required reading:

Note that our standards differ slightly from PEP-8 in some cases.

Coding standards other projects use:

Although Launchpad is still written in Python 2, there are several things you can do to improve compatibility with Python 3, making an eventual port easier. There are some good guidelines in the Ubuntu wiki about Python 3.

Whitespace and Wrapping

Multiline braces

There are lots of cases where you might have a list, tuple or dictionary literal that spans multiple lines. In these cases, you should consider formatting these literals as follows. This format makes changes to the list clearer to read in a diff. Note the trailing comma on the last element.

   1     mydict = {
   2         'first': 1,
   3         'second': 2,
   4         'third': 3,
   5         }
   7     mylist = [
   8         'this is the first line',
   9         'this is the second line',
  10         'this is the third line',
  11         ]


Consistency with existing code is the top priority. We follow PEP-8 with the following exceptions:

Private names are private

You should never call a non-public attribute or method from another class. In other words, if class A has a method _foo(), don't call it from anywhere outside class A.


Docstrings should be valid reST (with all the painful indentation rules that implies) so that tools such as pydoctor can be used to automatically generate API documentation.

You should use field names as defined in the epydoc documentation but with reST syntax.

Using `name` outputs a link to the documentation of the named object, if pydoctor can figure out what it is. For an example of pydoctor's output, see

Here is comprehensive example. Parameter descriptions are a good idea but not mandatory. Describe in as much or as little detail as necessary.

   1 def example2(a, b):
   2     """Perform some calculation.
   4     It is a **very** complicated calculation.
   6     :param a: The number of gadget you think this
   7               function should frobnozzle.
   8     :type a: ``int``
   9     :param b: The name of the thing.
  10     :type b: ``str``
  11     :return: The answer!
  12     :rtype: ``str``.
  13     :raise ZeroDivisionError: when ``a`` is 0.
  14     """


Each module should look like this:

   1 # Copyright 2009-2011 Canonical Ltd.  All rights reserved.
   3 """Module docstring goes here."""
   5 __metaclass__ = type
   6 __all__ = [
   7     ...
   8     ]

The file has most of this already, so save yourself time by copying that when starting a new module. The "..." should be filled in with a list of public names in the module.

Note that although PEP-8 says to "put any relevant __all__ specification after the imports", Launchpad code should have the __all__ before the imports. This makes it easy to see what a module contains and exports, and avoids the problem that differing amounts of imports among files means that the __all__ list is in a different place each time. Given that we have the Import Fascist utility (see Imports), we use __all__ more often than in general Python code. __all__ should be formatted like imports.



There are restrictions on which imports can happen in Launchpad. Namely:

These restrictions are enforced by the Import Fascist, which will cause your tests not to pass if you don't abide by the rules.

Imports should be fully qualified. Good:

   1 # foo/
   2 import foo.baz


   1 # foo/
   2 import baz

I.e. if imports foo.baz, it should say import foo.baz, not import baz.

Multiline imports

Sometimes import lines must span multiple lines, either because the package path is very long or because there are multiple names inside the module that you want to import.

Never use backslashes in import statements! Use parenthesized imports:

   1 from foo import (
   2    That, 
   3    TheOther, 
   4    This,
   5 )

Like other lists, imports should list one item per line. The exception is if only one symbol is being imported from a given module.

   1 from import CheckBoxMatrixWidget

But if you import two or more, then each item needs to be on a line by itself. Note the trailing comma on the last import and that the closing paren is on a line by itself.

   1 from import (
   2     CheckBoxMatrixWidget,
   3     LaunchpadRadioWidget,
   4     )

Also, 'make lint' is a very good tool for helping you maintain your imports.

Import scope

We encourage importing names from the location they are defined in. This seems to work better with large complex components.

Circular imports

With the increased use of native Storm APIs, you may encounter more circular import situations. For example, a MailingList method may need a reference to the EmailAddress class for a query, and vice versa. The classic way to solve this is to put one of the imports inside a method instead of at module global scope (a "nested import").

Short of adopting something like Zope's lazy imports (which has issues of its own), you can't avoid this, so here are some tips to make it less painful.

   1         def doFooWithBar(self, ...):
   2             # Import this here to avoid circular imports.
   3             from import Bar
   4             # ...
   5             return store.find((Foo, Bar), ...)

Circular imports and webservice exports

One of the largest sources of pain from circular imports is caused when you need to export an interface on the webservice. Generally, the only way around this is to specify generic types (like the plain old Interface) at declaration time and then later patch the webservice's data structures at the bottom of the interface file.

Fortunately there are some helper functions to make this less painful, in lib/lp/services/webservice/ These are simple functions where you can some info about your exported class/method/parameters and they do the rest for you.

For example:

   1     from import (
   2         patch_entry_return_type, patch_collection_return_type)
   3     patch_collection_return_type(
   4         IArchive, 'getComponentsForQueueAdmin', IArchivePermission)
   5     patch_entry_return_type(
   6         IArchive, 'newPackageUploader', IArchivePermission)


Properties are expected to be cheap operations. It is surprising if a property is not cheap operation. For expensive operations use a method, usually named getFoo(). Using cachedproperty provides a work-around but it should not be overused.

Truth conditionals

Remember that False, None, [], and 0 are not the same although they all evaluate to False in a boolean context. If this matters in your code, be sure to check explicitly for either of them.

Also, checking the length may be an expensive operation. Casting to bool may avoid this if the object specializes by implementing nonzero.

Chaining method calls

Since in some cases (e.g. class methods and other objects that rely on descriptor get() behaviour) it's not possible to use the old style of chaining method calls (SuperClass.method(self, ...)), we should always use the super() builtin when we want that.

/!\ The exception to this rule is when we have class hierarchies outside of our control that are known not to use super() and that we want to use for diamond-shaped inheritance.

Use of lambda, and operator.attrgetter

Prefer operator.attrgetter to lambda. Remember that giving functions names makes the code that calls, passes and returns them easier to debug.

Use of open()

When opening a file do one of:

  1. Use an encoding (via in Python 2, or open(..., encoding="...") in Python 3), or

  2. Use "b" in the mode to signify reading/writing bytes.

Discussion of text and binary modes

Python 2 does implicit encoding and decoding between unicode and byte strings. This is convenient, but means we can get away with being vague when reading and writing.

Python 3, however, will not let us be vague:

$ python3
>>> open("foo", "w").write(b'bar')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: must be str, not bytes
>>> open("foo", "wb").write('bar')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' does not support the buffer interface

Python 3's open() takes a new encoding parameter - much like in Python 2 - with the following behaviour:

Equals "don't count on it". Getting into the habit of using binary mode now - or - ought to help us avoid this pitfall when porting to Python 3.

Python 3's open() function also enables universal newlines in text mode by default, for writing too; Python 2 does not support universal newlines when writing. Using binary mode ought to help us avoid this pitfall too.

Use of hasattr

Use safe_hasattr from lazr.restful.utils instead of the built-in hasattr function because the latter swallows exceptions.


We use two database ORM (object-relational mapper) APIs in Launchpad, the older and deprecated SQLObject API and the new and improved Storm API. All new code should use the Storm API, and you are encourages to convert existing code to Storm as part of your tech-debt payments.


Field attributes

When you need to add ID attributes to your database class, use field_id as the attribute name instead of fieldID.

Multi-line SQL

SQL doesn't care about whitespace, so use triple quotes for large SQL queries or fragments, e.g.:

   1     query = """
   2         SELECT,, Person.displayname
   3         FROM TeamParticipation
   4         INNER JOIN Person ON =
   5         WHERE TeamParticipation.person = %s
   6         """ % sqlvalues(personID)

This also easy to cut-and-paste into psql for interactive testing, unlike if you use several lines of single quoted strings.

Creating temporary files

We should use the most convenient method of the tempfile module, never taint '/tmp/' or any other 'supposed to be there' path.

Despite of being developed and deployed on Ubuntu systems, turning it into restriction might not be a good idea.

When using tempfile.mkstemp remember it returns an open file-descriptor which has to be closed or bound to the open file, otherwise they will leak and eventually hit the default Linux limit (1024).

There are 2 good variations according to the scope of the temporary file.

   1     fd, filename = mkstemp()
   2     os.close(fd)
   3     ...
   4     act_on_filename(filename)


   1     fd, filename = mkstemp()
   2     temp_file = os.fdopen(fd, 'w')
   3     ...
   4     temp_file.write('foo')
   5     temp_file.close()

Never use:

   1     fd, filename = mkstemp()
   2     temp_file = open(filename)
   3     temp_file.write('foo')
   4     temp_file.close()
   5     # BOOM! 'fd' leaked.

It's also important to mention that in testing context, specially if you are using the lp.testing.TestCase (or one of its specializations) you can simply create a brand new temporary directory (using mkdtemp). Create as many files you need within it and register a cleanup function to purge the temporary directory recursively.

   1 class TestFoo(TestCase):
   2 ...
   3     def test_foo(self):
   4         tempdir = mkdtemp()
   5         self.addCleanup(shutils.rmtree, tempdir)
   6         ...
   7         do_something(os.path.join(tempdir, 'test.log'))
   8         ...

Configuration hints


To make wrapping and tabs fit the above standard, you can add the following to your .vimrc:

autocmd BufNewFile,BufRead *.py set tw=78 ts=4 sts=4 sw=4 et

To make trailing whitespace visible:

set list
set listchars=tab:>.,trail:-

This will also make it obvious if you accidentally introduce a tab.

To make long lines show up:

match Error /\%>79v.\+/

For an even more in-depth Vim configuration, have a look at UltimateVimPythonSetup for a complete vim file you can copy to your local setup.


There are actually two Emacs Python modes. Emacs comes with python.el which (IMO) has some quirks and does not seem to be as popular among hardcore Python programmers. python-mode.el comes with XEmacs and is supported by a group of hardcore Python programmers. Even though it's an add-on, it works with Emacs just fine.

PythonStyleGuide (last edited 2015-03-09 14:49:04 by cjwatson)