# 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 `twisted.spread`...) but when choosing class names, be as precise as possible. Write code with a dictionary and thesaurus open on the table next to you.

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.

## Testing

Unit tests are written using the PyUnit framework. Many examples are in the `twisted.test` package, and all tests should be integrated through the main test suite builder in `twisted.test.test_all`.

Acceptance tests are all automated by the bin/accepttests script currently. (TODO: real acceptance tests strategy!)

## Whitespace

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

## 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.

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.

Note that this rule is a change from previous versions of Twisted, so a lot of code does not yet adhere to it.

By default, objects from modules in Twisted ought to be imported directly into the namespace of the module that uses them, like this: `from twisted.python.defer import Deferred`

In some cases, however, Twisted takes advantage of the dynamic binding of Python in order to provide an interface through a module that may be imported -- we call these "volatile" modules, because their internals are subject to change without warning. In these cases, it is desireable to import the module itself into the user's namespace, so that things bound to it will be looked up on each call. For obvious reasons, classes in volatile modules should never be subclassed.

Volatile modules will have a __volatile__ attribute set to 1, and should always have a docstring snippet like the one that follows:

    """
    NOTE: this is a __volatile__ module.  This means that if you are using it
    externally, you should know that its contents may change at any time.  In
    general, all this means is that you must not use it like this::

        from twisted.package.module import Class
        Class()

    but instead like this::

        from twisted.package import module
        module.Class()
    """

## 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,

    # parent/
    # --- __init__.py ---
    import child

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

## Docstrings

Wherever possible, docstrings should be used to describe the purpose of methods, functions, classes, and modules. In cases where it's desirable to avoid documenting thoroughly -- for example, and evolving interface -- insert a placeholder docstring ("UNDOCUMENTED" is preferred), so that the auto-generated API documentation will not pick up an extraneous comment as the documentation for that module/class/function.

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.

Docstrings should be written in StructuredText format; more documentation is available on the HappyDoc website.

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.
- Code examples in docstrings should be prefixed by the | character. This will prevent IM-Python from regarding sample code as real functions, methods, and classes.

For example,

    def foo2bar(f):
        """I am a function to convert foos to bars.

        I should be used when you have a foo but you want a bar; note that this is a
        non-destructive operation.  If I can\'t convert the foo to a bar I will raise a
        FooException().

        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.

## 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 include the name of the module they are a part of. Examples:

- twisted.reality.thing.Thing
- twisted.web.Handler
- twisted.spread.pb.ViewPoint
- twisted.parser.patterns.Pattern

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.

## 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.

## 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.

## 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.

## 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. I use the Twisted Web server on the main TML website, and aside from being good PR, this 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.

Last modified: Sun Mar 17 00:30:20 EST 2002