New submission from Alexander Belopolsky email@example.com:
As discussed in "Breaking undocumented API" thread  on python-dev, a definition of "public names" is buried deep in the language reference manual:
""" The public names defined by a module are determined by checking the module’s namespace for a variable named __all__; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in __all__ are all considered public and are required to exist. If __all__ is not defined, the set of public names includes all names found in the module’s namespace which do not begin with an underscore character ('_'). __all__ should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module). """ 
It has been argued that this is not the authoritative definition and alternatives have been suggested such as "any name that does not begin with an underscore except imported modules." mportant for the users and developers of cpython and other python implementations to know what names they can rely upon to stay defined between releases, the rules for "public names" should be documented.
I agree that the library manual is a more appropriate place for such documentation. The definition should include the naming conventions and the set of promises that Python makes about public name availability in the future releases.
---------- assignee: docs@python components: Documentation messages: 121297 nosy: belopolsky, docs@python priority: normal severity: normal stage: needs patch status: open title: Document the rules for "public names" versions: Python 3.2
Alexander Belopolsky firstname.lastname@example.org added the comment:
Michael Foord suggested adding the following to developer documentation such as PEP 8.  I am not sure PEP 8 is the right place for it. In my opinion, PEP 8 is mostly about stylistic choices that don't have a major impact on the users. In other words, unless you are developing python itself, you are unlikely to consult PEP 8. The API stability rules, however affect every user who programmed in python long enough to see a major release. I don't have objections to Michael's definitions below, but if included in the library manual, they should be reworded to address the user rather than the developer of the API.
""" How about making this explicit (either pep 8 or our developer docs):
If a module or package defines __all__ that authoritatively defines the public interface. Modules with __all__ SHOULD still respect the naming conventions (leading underscore for private members) to avoid confusing users. Modules SHOULD NOT export private members in __all__.
Names imported into a module a never considered part of its public API unless documented to be so or included in __all__.
Methods / functions / classes and module attributes whose names begin with a leading underscore are private.
If a class name begins with a leading underscore none of its members are public, whether or not they begin with a leading underscore.
If a module name in a package begins with a leading underscore none of its members are public, whether or not they begin with a leading underscore.
If a module or package doesn't define __all__ then all names that don't start with a leading underscore are public.
All public members MUST be documented. Public functions, methods and classes SHOULD have docstrings. Private members may have docstrings. """ 
---------- nosy: +michael.foord
Ron Adam email@example.com added the comment:
You may also want to update help topics.
Identifiers (Names) *******************
An identifier occurring as an atom is a name. See section *Identifiers and keywords* for lexical definition and section *Naming and binding* for documentation of naming and binding.
When the name is bound to an object, evaluation of the atom yields that object. When a name is not bound, an attempt to evaluate it raises a ``NameError`` exception.
**Private name mangling:** When an identifier that textually occurs in a class definition begins with two or more underscore characters and does not end in two or more underscores, it is considered a *private name* of that class. Private names are transformed to a longer form before code is generated for them. The transformation inserts the class name in front of the name, with leading underscores removed, and a single underscore inserted in front of the class name. For example, the identifier ``__spam`` occurring in a class named ``Ham`` will be transformed to ``_Ham__spam``. This transformation is independent of the syntactical context in which the identifier is used. If the transformed name is extremely long (longer than 255 characters), implementation defined truncation may happen. If the class name consists only of underscores, no transformation is done.
Other topics that may be of interest.
IDENTIFIERS NAMESPACES PACKAGES PRIVATENAMES SPECIALATTRIBUTES SPECIALIDENTIFIERS SPECIALMETHODS
---------- nosy: +ron_adam
Changes by Ezio Melotti firstname.lastname@example.org:
---------- keywords: +easy type: -> enhancement versions: +Python 3.3, Python 3.4
Nick Coghlan added the comment:
PEP 8 now covers the developer side of things: http://www.python.org/dev/peps/pep-0008/#public-and-internal-interfaces
A user facing counterpart describing our backwards compatibility policy is still desirable. Updating PEP 5 wouldn't go astray, since that's *supposed* to serve that purpose :)
---------- nosy: +ncoghlan