
:LastChangedDate: $LastChangedDate$
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Twisted Coding Standard
=======================






Naming
------


    
Try to choose names which are both easy to remember and
meaningful. Some silliness is OK at the module naming level
(see :api:`twisted.spread <twisted.spread>` ...) but when
choosing class names, be as precise as possible.

    


Try to avoid overloaded terms. This rule is often broken,
since it is incredibly difficult, as most normal words have
already been taken by some other software. More importantly,
try to avoid meaningless words. In particular, words like
"handler" , "processor" , "engine" , "manager" 
and "component" don't really indicate what something does,
only that it does *something* .

    


Use American spelling in both names and docstrings.  For compound
technical terms such as 'filesystem', use a non-hyphenated spelling in
both docstrings and code in order to avoid unnecessary
capitalization.

    



Testing
-------


    

Overview
~~~~~~~~


    
Twisted development should always be
`test-driven <http://en.wikipedia.org/wiki/Test-driven_development>`_ .  The complete test suite in the head of the SVN trunk is required to
be passing on `supported platforms <http://buildbot.twistedmatrix.com/supported>`_ at all times.  Regressions in the test suite
are addressed by reverting whatever revisions introduced them.  For
complete documentation about testing Twisted itself, refer to the
:doc:`Test Standard <test-standard>` .  What follows is
intended to be a synopsis of the most important points.

    



Test Suite
~~~~~~~~~~


    
The Twisted test suite is spread across many subpackages of the
``twisted`` package.  Many older tests are in
``twisted.test`` .  Others can be found at places such as
``twisted.web.test`` (for ``twisted.web`` tests)
or ``twisted.internet.test`` (for ``twisted.internet`` 
tests). The latter arrangement, ``twisted.somepackage.test`` ,
is preferred for new tests except when a test module already exists in
``twisted.test`` .


    



Parts of the Twisted test suite may serve as good examples of how to
write tests for Twisted or for Twisted-based libraries (newer parts of
the test suite are generally better examples than older parts - check
when the code you are looking at was written before you use it as an
example of what you should write).  The names of test modules should
begin with ``test_`` so that they are automatically discoverable by
test runners such as Trial.  Twisted's unit tests are written using
:api:`twisted.trial <twisted.trial>` , an xUnit library which has been
extensively customized for use in testing Twisted and Twisted-based
libraries.

    


Implementation (ie, non-test) source files should begin with a
``test-case-name`` tag which gives the name of any test
modules or packages which exercise them.  This lets tools discover a
subset of the entire test suite which they can run first to find tests
which might be broken by a particular change.

    


It is strongly suggested that developers learn to use Emacs, and use
the ``twisted-dev.el`` file included in
`twisted-emacs <http://launchpad.net/twisted-emacs>`_ 
to bind the F9 key to "run unit tests" and bang on it
frequently.  Support for other editors is unavailable at this time but
we would love to provide it.

    


To run the whole Twisted test without using emacs, use trial:

    



.. code-block:: console

    
    $ bin/trial twisted



    
To run an individual test module, such as
``twisted/mail/test/test_pop3.py`` , specify the module
name:

    



.. code-block:: console

    
    $ bin/trial twisted.mail.test.test_pop3



    
To run the tests associated with a particular implementation file,
such as ``twisted/mail/pop3.py`` , use the
``testmodule`` option:

    



.. code-block:: console

    
    $ bin/trial twisted/mail/pop3.py



    
All unit test methods should have docstrings specifying at a high
level the intent of the test. That is, a description that users of the
method would understand.

    


If you modify, or write a new, HOWTO, please read the `Lore <http://twistedmatrix.com/trac/wiki/TwistedLore>`_ 
documentation to learn how to format the docs.

    



Copyright Header
----------------


    
Whenever a new file is added to the repository, add the following
license header at the top of the file:





.. code-block:: python

    
    # Copyright (c) Twisted Matrix Laboratories.
    # See LICENSE for details.



    
When you update existing files, if there is no copyright header, add
one.

    



Whitespace
----------


    
Indentation is 4 spaces per indent.
Tabs are not allowed.
It is preferred that every block appears on a new line, so that control structure indentation is always visible.

    


Lines are flowed at 79 columns. They must not have trailing
whitespace. Long lines must be wrapped using implied line continuation
inside parentheses; backslashes aren't allowed. To handle long import
lines, please repeat the import like this:





.. code-block:: python

    
    from very.long.package import foo, bar, baz
    from very.long.package import qux, quux, quuux



    
Top-level classes and functions must be separated with 3 blank lines,
and class-level functions with 2 blank lines. The control-L (i.e. ^L) form
feed character must not be used.

    



Modules
-------


    
Modules must be named in all lower-case, preferably short,
single words. If a module name contains multiple words, they
may be separated by underscores or not separated at all.

    


Modules must have a copyright message, a docstring and a
reference to a test module that contains the bulk of its tests.
Use this template:

    



:download:`new_module_template.py <../listings/new_module_template.py>`

.. literalinclude:: ../listings/new_module_template.py


In most cases, modules should contain more than one class,
function, or method; if a module contains only one object,
consider refactoring to include more related functionality in
that module.

    


Depending on the situation, it is acceptable to have imports that
look like this:


.. code-block:: python

    from twisted.internet.defer import Deferred


or like this:


.. code-block:: python

    from twisted.internet import defer


That is, modules should import *modules* or *classes and functions* , but not *packages* .

    


Wildcard import syntax may not be used by code in Twisted.  These
imports lead to code which is difficult to read and maintain by
introducing complexity which strains human readers and automated tools
alike.  If you find yourself with many imports to make from a single
module and wish to save typing, consider importing the module itself,
rather than its attributes.

    


*Relative imports* (or *sibling imports* ) may not be
used by code in Twisted.  Relative imports allow certain circularities
to be introduced which can ultimately lead to unimportable modules or
duplicate instances of a single module.  Relative imports also make the
task of refactoring more difficult.

    


In case of local names conflicts due to import, use the ``as`` 
syntax, for example:


.. code-block:: python

    from twisted.trial import util as trial_util



    


The encoding must always be ASCII, so no coding cookie is necessary.

    



Packages
--------


    
Package names should follow the same conventions as module
names. All modules must be encapsulated in some package. Nested
packages may be used to further organize related modules.

    


``__init__.py`` must never contain anything other than a
docstring and (optionally) an ``__all__`` attribute. Packages are
not modules and should be treated differently. This rule may be
broken to preserve backwards compatibility if a module is made
into a nested package as part of a refactoring.

    


If you wish to promote code from a module to a package, for
example, to break a large module out into several smaller
files, the accepted way to do this is to promote from within
the module. For example,





.. code-block:: python

    
    # parent/
    # --- __init__.py ---
    import child
    
    # --- child.py ---
    import parent
    class Foo:
        pass
    parent.Foo = Foo



    
Every package should be added to the list in
``setup.py`` .

    


Packages must not depend circularly upon each other.  To simplify
maintaining this state, packages must also not import each other
circularly.  While this applies to all packages within Twisted, one
``twisted.python`` deserves particular attention, as it may
not depend on any other Twisted package.

    



String Formatting Operations
----------------------------


    
When using `string formatting    operations <http://docs.python.org/lib/typesseq-strings.html>`_ like ``formatString % values`` you should always
use a tuple if you're using non-mapping ``values`` . This is to
avoid unexpected behavior when you think you're passing in a single value,
but the value is unexpectedly a tuple, e.g.:





.. code-block:: python

    
    def foo(x):
        return "Hi %s\n" % x



    
The example shows you can pass in ``foo("foo")`` or
``foo(3)`` fine, but if you pass in ``foo((1,2))`` ,
it raises a ``TypeError`` . You should use this instead:





.. code-block:: python

    
    def foo(x):
        return "Hi %s\n" % (x,)



    

Docstrings
----------


    
Docstrings should always be used to describe the purpose of methods,
functions, classes, and modules.  Moreover, all methods, functions, classes,
and modules must have docstrings.  In addition to documenting the purpose of
the object, the docstring must document all of parameters or attributes of
the object.

    


When documenting a class with one or more attributes which are
initialized directly from the value of a ``__init__`` argument by
the same name (or differing only in that the attribute is private), it is
sufficient to document the ``__init__`` parameter (in
the ``__init__`` docstring).  For example:





.. code-block:: python

    
    class Ninja(object):
        """
        @ivar speed: See L{__init__}
        @ivar _stealth: See C{stealth} parameter of L{__init__}
        """
        def __init__(self, speed, stealth):
            """
            @param speed: The maximum rate at which this ninja can travel (m/s)
            @type speed: C{int} or C{float}
    
            @param stealth: This ninja's ability to avoid being noticed in its
                activities, as a percentage modifier.
            @type: C{int}
            """
            self.speed = speed
            self._stealth = stealth



    
It is not necessary to have a second copy of the documentation for the
attribute in the class docstring, only a reference to the method
(typically ``__init__`` which does document the
attribute's meaning.  Of course, if there is any interesting additional
behavior about the attribute that does not apply to
the ``__init__`` argument, that behavior should be
documented in the class docstring.

    


Docstrings are *never* to be used to provide semantic
information about an object; this rule may be violated if the
code in question is to be used in a system where this is a
requirement (such as Zope).

    


Docstrings should be indented to the level of the code they
are documenting.

    


Docstrings should be triple-quoted. The opening and the closing of the
docstrings should be on a line by themselves. For example:

.. code-block:: python

    
    class Ninja(object):
        """
        A L{Ninja} is a warrior specializing in various unorthodox arts of war.
        """
        def attack(self, someone):
            """
            Attack C{someone} with this L{Ninja}'s shuriken.
            """




    


Docstrings should be written in epytext format; more
documentation is available in the
`Epytext Markup Language documentation <http://epydoc.sourceforge.net/manual-epytext.html>`_ .

    


Additionally, to accommodate emacs users, single quotes of the type of
the docstring's triple-quote should be escaped. This will prevent font-lock from
accidentally fontifying large portions of the file as a string.

    


For example,




.. code-block:: python

    
    def foo2bar(f):
        """
        Convert L{foo}s to L{bar}s.
    
        A function that should be used when you have a C{foo} but you want a
        C{bar}; note that this is a non-destructive operation.  If this method
        can't convert the C{foo} to a C{bar} it will raise a L{FooException}.
    
        @param f: C{foo}
        @type f: str
    
        For example::
    
            import wombat
            def sample(something):
                f = something.getFoo()
                f.doFooThing()
                b = wombat.foo2bar(f)
                b.doBarThing()
                return b
    
        """
        # Optionally, actual code can go here.



    

Comments
--------


    
Comments marked with XXX or TODO must contain a reference to the
associated ticket.

    



Versioning
----------


    
The API documentation should be marked up with version information.
When a new API is added the class should be marked with the epytext
``@since:`` field including the version number when
the change was introduced, eg. ``@since: 8.1`` .

    



Scripts
-------


    
For each "script" , that is, a program you expect a Twisted user
to run from the command-line, the following things must be done:

    



#. Write a module in :api:`twisted.scripts <twisted.scripts>` 
   which contains a callable global named ``run`` . This
   will be called by the command line part with no arguments (it
   will usually read ``sys.argv`` ). Feel free to write more
   functions or classes in this module, if you feel they are useful
   to others.
#. Create a file which contains a shebang line for Python. For Twisted
   Core, this file should be placed in the ``bin/`` directory; for
   example, ``bin/twistd`` . For sub-projects, it should be placed
   in ``bin/<subproject>`` ; for example, the key-generation tool
   for the Conch sub-project is in ``bin/conch/ckeygen`` .
   
   .. code-block:: python
   
   
       #!/usr/bin/env python
   

To make sure that the script is portable across different UNIX like
operating systems we use the ``/usr/bin/env`` command. The env
command allows you to run a program in a modified environment. That way
you don't have to search for a program via the ``PATH`` environment
variable. This makes the script more portable but note that it is not a
foolproof method. Always make sure that ``/usr/bin/env`` exists or
use a softlink/symbolic link to point it to the correct path. Python's
distutils will rewrite the shebang line upon installation so this policy
only covers the source files in version control.

      

#. For core scripts, add this Twisted running-from-SVN header:
   
   .. code-block:: python
   
   
       import sys
       try:
           import _preamble
       except ImportError:
           sys.clear_exc()
   
   
   
   Or for sub-project scripts, add a modified version which also adjusts ``sys.path`` :
   
   .. code-block:: python
   
   
       import sys, os
       extra = os.path.dirname(os.path.dirname(sys.argv[0]))
       sys.path.insert(0, extra)
       try:
           import _preamble
       except ImportError:
           sys.clear_exc()
       sys.path.remove(extra)
   
#. And end with:
   
   .. code-block:: python
   
   
       from twisted.scripts.yourmodule import run
       run()
   
#. Write a manpage and add it to the ``man`` folder
   of a subproject's ``doc`` folder. On Debian systems
   you can find a skeleton example of a manpage in
   ``/usr/share/doc/man-db/examples/manpage.example`` .



This will insure your program will work correctly for users of SVN,
Windows releases and Debian packages.

    



Examples
--------


    
For example scripts you expect a Twisted user
to run from the command-line, add this Python shebang line at the top
of the file:




.. code-block:: python

    
    #!/usr/bin/env python



    

Standard Library Extension Modules
----------------------------------


    
When using the extension version of a module for which there is also
a Python version, place the import statement inside a try/except block,
and import the Python version if the import fails.  This allows code to
work on platforms where the extension version is not available.  For
example:


.. code-block:: python

    
    try:
        import cPickle as pickle
    except ImportError:
        import pickle



Use the "as" syntax of the import statement as well, to set
the name of the extension module to the name of the Python module.

    


Some modules don't exist across all supported Python versions. For
example, Python 2.3's ``sets`` module was deprecated in Python 2.6
in favor of the ``set`` and ``frozenset`` builtins.
:api:`twisted.python.compat <twisted.python.compat>` would be the place to add
``set`` and ``frozenset`` implementations that work across
Python versions.

    



Classes
-------


    
Classes are to be named in mixed case, with the first letter
capitalized; each word separated by having its first letter
capitalized. Acronyms should be capitalized in their entirety.
Class names should not be prefixed with the name of the module they are
in. Examples of classes meeting this criteria:

    




- twisted.spread.pb.ViewPoint
- twisted.parser.patterns.Pattern


    


Examples of classes **not** meeting this criteria:

    




- event.EventHandler
- main.MainGadget


    


An effort should be made to prevent class names from clashing
with each other between modules, to reduce the need for
qualification when importing. For example, a Service subclass
for Forums might be named twisted.forum.service.ForumService,
and a Service subclass for Words might be
twisted.words.service.WordsService. Since neither of these
modules are volatile *(see above)* the classes may be
imported directly into the user's namespace and not cause
confusion.

    



New-style Classes
~~~~~~~~~~~~~~~~~


    
Classes and instances in Python come in two flavors: old-style or
classic, and new-style. Up to Python 2.1, old-style classes were the
only flavour available to the user, new-style classes were introduced
in Python 2.2 to unify classes and types. All classes added to Twisted
should be written as new-style classes. If ``x`` 
is an instance of a new-style class, then ``type(x)`` 
is the same as ``x.__class__`` .

    



Methods
-------


    
Methods should be in mixed case, with the first letter lower
case, each word separated by having its first letter
capitalized. For example, ``someMethodName`` ,
``method`` .

    


Sometimes, a class will dispatch to a specialized sort of
method using its name; for example, twisted.reflect.Accessor.
In those cases, the type of method should be a prefix in all
lower-case with a trailing underscore, so method names will
have an underscore in them. For example, ``get_someAttribute`` .
Underscores in method names in twisted code are therefore
expected to have some semantic associated with them.

    


Some methods, in particular ``addCallback`` and its
cousins return self to allow for chaining calls.  In this case,
wrap the chain in parenthesis, and start each chained call on
a separate line, for example:





.. code-block:: python

    
    return (foo()
            .addCallback(bar)
            .addCallback(thud)
            .addCallback(wozers))



    

Callback Arguments
------------------


    
There are several methods whose purpose is to help the user set up
callback functions, for example :api:`twisted.internet.defer.Deferred.addCallback <Deferred.addCallback>` or the
reactor's :api:`twisted.internet.base.ReactorBase.callLater <callLater>` method. To make
access to the callback as transparent as possible, most of these methods
use ``**kwargs`` to capture arbitrary arguments
that are destined for the user's callback. This allows the call to the
setup function to look very much like the eventual call to the target
callback function.

    


In these methods, take care to not have other argument names that will
"steal" the user's callback's arguments. When sensible, prefix these
"internal" argument names with an underscore. For example, :api:`twisted.spread.pb.RemoteReference.callRemote <RemoteReference.callRemote>` is
meant to be called like this:





.. code-block:: python

    
    myref.callRemote("addUser", "bob", "555-1212")
    
    # on the remote end, the following method is invoked:
    def addUser(name, phone):
        ...



    
where "addUser" is the remote method name. The user might also
choose to call it with named parameters like this:





.. code-block:: python

    
    myref.callRemote("addUser", name="bob", phone="555-1212")



    
In this case, ``callRemote`` (and any code that uses the
``**kwargs`` syntax) must be careful to not use
"name" , "phone" , or any other name that might overlap with
a user-provided named parameter.  Therefore, ``callRemote`` is
implemented with the following signature:





.. code-block:: python

    
    class SomeClass(object):
        def callRemote(self, _name, *args, **kw):
            ...



    
Do whatever you can to reduce user confusion. It may also be
appropriate to ``assert`` that the kwargs
dictionary does not contain parameters with names that will eventually
cause problems.


    



Special Methods
---------------


    
The augmented assignment protocol, defined by ``__iadd__`` and other
similarly named methods, can be used to allow objects to be modified in
place or to rebind names if an object is immutable -- both through use
of the same operator.  This can lead to confusing code, which in turn
leads to buggy code.  For this reason, methods of the augmented
assignment protocol should not be used in Twisted.

    



Functions
---------


    
Functions should be named similiarly to methods.

    


Functions or methods which are responding to events to
complete a callback or errback should be named ``_cbMethodName`` or
``_ebMethodName`` , in order to distinguish them from normal
methods.

    



Attributes
----------


    
Attributes should be named similarly to functions and
methods. Attributes should be named descriptively; attribute
names like ``mode`` , ``type`` , and
``buf`` are generally discouraged. Instead, use
``displayMode`` , ``playerType`` , or
``inputBuffer`` .

    


Do not use Python's "private" attribute syntax; prefix
non-public attributes with a single leading underscore. Since
several classes have the same name in Twisted, and they are
distinguished by which package they come from, Python's
double-underscore name mangling will not work reliably in some
cases. Also, name-mangled private variables are more difficult
to address when unit testing or persisting a class.

    


An attribute (or function, method or class) should be
considered private when one or more of the following conditions
are true:

    




- The attribute represents intermediate state which is not
  always kept up-to-date.
- Referring to the contents of the attribute or otherwise
  maintaining a reference to it may cause resources to
  leak.
- Assigning to the attribute will break internal
  assumptions.
- The attribute is part of a known-to-be-sub-optimal
  interface and will certainly be removed in a future
  release.


    



Python 3
--------


    
Twisted is being ported to Python 3.  Be sure to
follow `the    guidelines for Python 3 compatibility. <http://twistedmatrix.com/trac/wiki/Plan/Python3#Reviewerchecklist>`_ Check on them often as they may
change as lessons are learned during the porting effort.

    



Database
--------


    
Database tables will be named with plural nouns.

    


Database columns will be named with underscores between
words, all lower case, since most databases do not distinguish
between case.

    


Any attribute, method argument, or method name that
corresponds *directly* to a column in the database will
be named exactly the same as that column, regardless of other
coding conventions surrounding that circumstance.

    


All SQL keywords should be in upper case.

    



C Code
------


    
Wherever possible, C code should be optional, and the
default python implementation should be maintained in tandem
with it. C code should be strict ANSI C, and
**must** build using GCC as well as Visual Studio
for Windows, and really shouldn't have any problems with other
compilers either. Don't do anything tricky.

    


C code should only be used for efficiency, not for binding
to external libraries. If your particular code is not
frequently run, write it in Python. If you require the use of
an external library, develop a separate, external bindings
package and make your twisted code depend on it.

    



Commit Messages
---------------


    
The commit messages are being distributed in a myriad of ways. Because
of that, you need to observe a few simple rules when writing a commit
message.

    


The first line of the message is being used as both the subject of
the commit email and the announcement on #twisted. Therefore, it should
be short (aim for < 80 characters) and descriptive -- and must be
able to stand alone (it is best if it is a complete sentence). The rest
of the e-mail should be separated with *hard line breaks* into
short lines (< 70 characters). This is free-format, so you can do
whatever you like here.

    


Commit messages should be about *what* , not *how* : we can
get how from SVN diff. Explain reasons for commits, and what they
affect.

    


Each commit should be a single logical change, which is internally
consistent. If you can't summarize your changes in one short line, this
is probably a sign that they should be broken into multiple checkins.

    



Source Control
--------------


    
Twisted currently uses Subversion for source control.  All
development **should** occur using branches; when a task is
considered complete another Twisted developer may review it and if no
problems are found, it may be merged into trunk. The Twisted wiki has `a start <http://twistedmatrix.com/trac/wiki/TwistedDevelopment>`_ .
Branches **must** be used for major development.  Branches
should be managed using `Combinator <http://divmod.org/trac/wiki/DivmodCombinator>`_ (but
if you can manage them in some other way without anyone noticing, knock
yourself out).

    


Certain features of Subversion should be avoided.

    




- 
  
  
  Do not set the ``svn:ignore`` property on any
  file or directory.  What you wish to ignore, others may wish to examine.
  What others may wish you ignore, *you* may wish you examine.
  ``svn:ignore`` will affect everyone who uses
  the repository, and so it is not the right mechanism to express personal
  preferences.
  
  
  
  
  If you wish to ignore certain files use the ``global-ignores`` feature of ``~/.subversion/config`` , for example:
  
  
  
  
  
  .. code-block:: console
  
  
      [miscellany]
      global-ignores = dropin.cache *.pyc *.pyo *.o *.lo *.la #*# .*.rej *.rej .*~
  
  
  


    



Fallback
--------


    
In case of conventions not enforced in this document, the reference
documents to use in fallback is
`PEP 8 <http://www.python.org/dev/peps/pep-0008/>`_ for Python
code and `PEP 7 <http://www.python.org/dev/peps/pep-0007/>`_ for
C code. For example, the paragraph **Whitespace in Expressions and Statements** in PEP 8 describes what should be done in Twisted
code.

    



Recommendations
---------------


    
These things aren't necessarily standardizeable (in that
code can't be easily checked for compliance) but are a good
idea to keep in mind while working on Twisted.

    


If you're going to work on a fragment of the Twisted
codebase, please consider finding a way that you would *use* 
such a fragment in daily life. Using a Twisted Web server on your
website  encourages you to actively maintain and improve your code,
as the little everyday issues with using it become apparent.

    


Twisted is a **big** codebase! If you're
refactoring something, please make sure to recursively grep for
the names of functions you're changing. You may be surprised to
learn where something is called. Especially if you are moving
or renaming a function, class, method, or module, make sure
that it won't instantly break other code.

  

