@public

This is a very simple decorator and function which populates a module’s __all__ and optionally the module globals. This provides both a pure-Python implementation and an optional C implementation.

Background

__all__ is great. It has both a functional and a documentation purpose.

The functional purpose is that it directly controls which module names are imported by the from <module> import * statement. In the absence of an __all__, when this statement is executed, every name in <module> that does not start with an underscore will be imported. This often leads to importing too many names into the module. That’s a good enough reason not to use from <module> import * with modules that don’t have an __all__.

In the presence of an __all__, only the names specified in this list are imported by the from <module> import * statement. This in essence gives the <module> author a way to explicitly state which names are for public consumption.

And that’s the second purpose of __all__; it serves as module documentation, explicitly naming the public objects it wants to export. You can print a module’s __all__ and get an explicit declaration of its public API.

The problem

__all__ has two problems.

First, it separates the declaration of a name’s public export semantics from the implementation of that name. Usually the __all__ is put at the top of the module, although this isn’t required, and in some cases it’s actively prohibited. So when you’re looking at the definition of a function or class in a module, you have to search for the __all__ definition to know whether the function or class is intended for public consumption.

This leads to the second problem, which is that it’s too easy for the __all__ to get out of sync with the module’s contents. Often a function or class is renamed, removed, or added without the __all__ being updated. Then it’s difficult to know what the module author’s intent was, and it can lead to an exception when a string appearing in __all__ doesn’t match an existing name in the module. Some tools like Sphinx will complain when names appear in __all__ don’t appear in the module. All of this points to the root problem; it should be easy to keep __all__ in sync!

The solution

This package provides a way to declare a name’s publicness right at the point of its declaration, and to infer the name to export from that definition. In this way, a module’s author never explicitly sets the __all__ so there’s no way for it to get out of sync.

This package, and Python issue 26632, propose just such a solution, in the form of a public builtin that can be used as either a decorator, or a callable.

You’ll usually use this as a decorator, for example:

@public
def foo():
    pass

or:

@public
class Bar:
    pass

If you were to print the __all__ after both of those code snippets, you’d see:

>>> print(__all__)
['foo', 'Bar']

Note that you do not need to initialize __all__ in the module, since public will do it for you. Of course, if your module already has an __all__, it will just append new names to the existing list.

The requirements to use the @public decorator are simple: the decorated thing must have a __name__ attribute. Since you’ll overwhelmingly use it to decorate functions and classes, this will always be the case.

There’s one other common use case that isn’t covered by the @public decorator. Sometimes you want to declare simple constants or instances as publicly available. You can’t use the @public decorator for two reasons: constants don’t have a __name__ and Python’s syntax doesn’t allow you to decorate such constructs.

To solve this use case, public is also a callable function accepting keyword arguments. An example makes this obvious:

public(SEVEN=7)
public(a_bar=Bar())

Now if you print the module’s __all__ you’ll see:

>>> print(__all__)
['foo', 'Bar', 'SEVEN', 'a_bar']

and as should be obvious, the module contains name bindings for these constants:

>>> print(SEVEN)
7
>>> print(a_bar)
<__main__.Bar object at ...>

Note: While you can use public() with multiple keyword arguments in a single call, the order of the resulting __all__ entries is undefined in Python versions earlier than 3.6, due to indeterminate dictionary sort order. If order matters to you, call public() multiple times each with a single keyword argument.

Extension module alternative

This package actually provides both a pure Python implementation and an optional C extension module. When you do the typical import, e.g.:

>>> from public import public

you’ll get the most efficient version available. Since the C implementation is entirely optional (albeit moderately more efficient), you’ll get that if it was built. If not, you’ll get the pure-Python implementation.

For all intents and purposes, the two versions are identical. You generally won’t notice the difference. If for some reason you want to force the pure-Python version just do:

>>> from public import py_public as public

Making @public a built-in

It can get rather tedious if you have to add the above import in every module where you want to use it. What if you could put public into Python’s builtins? Then it would be available in all your code for free:

>>> from public import install
>>> install()

and now you can just use @public without having to import anything in your other modules.

Again by default, this installs the most efficient version it can find, but if you wanted to force install the pure-Python version, just do:

>>> from public import py_install
>>> py_install()

Installation

Use the normal setup.py install or pip install commands to install this library. By default, the C extension is not built, in order to make it more portable to environments without a C compiler. If you want a version that’s a little more efficient than the pure-Python implementation, set the environment variable ATPUBLIC_BUILD_EXTENSION=1 when you build/install the module.

Caveats

There are some important usage restrictions you should be aware of:

  • Only use @public on top-level object. Specifically, don’t try to use @public on a class method name. While the declaration won’t fail, you will get an exception when you attempt to from <module> import * because the name pulled from __all__ won’t be in the module’s globals.

  • If you explicitly set __all__ in your module, be sure to set it to a list. Some style guides require __all__ to be a tuple, but since that’s immutable, as soon as @public tries to append to it, you will get an exception. Best practice is to not set __all__ explicitly; let @public do it!

  • If you still want __all__ to be immutable, put the following at the bottom of your module:

    __all__ = tuple(__all__)
    

Alternatives

This isn’t a unique approach to @public. Other implementations do exist. There are some subtle differences between this package and those others. This package:

  • uses keyword arguments to map names which don’t have an __name__ attribute;
  • can be used to bind names and values into a module’s globals;
  • provides both C and Python implementations;
  • can optionally put public in builtins.

Author

public is Copyright (C) 2016-2017 Barry Warsaw

Contact Barry:

Licensed under the terms of the Apache License 2.0. See LICENSE.txt for details.

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