Tuesday, September 25, 2007

Why Zope community use namespace packages ?

Zope project and the community in general use lots of namespaces
packages. Though, we have some non-namespace packages like
ZODB3,ZConfig etc. Zope community has created many namespaces
for packages like `zope`, `zope.app`, `zc`, `z3c`, `lovely` etc. Here
`zc` stands for Zope Corporation, `z3c` for Zope 3 Community and
lovely for Lovely Systems' packages.

Python is a language with namespace support at many levels.
Remember, the last line of Zen of Python reads like this: "Namespaces
are one honking great idea -- let's do more of those!"
Take it positively, don't interpret it more ;)

A namespace package will not have any method,class or any other
objects defined in that level. So a normal namespace package will be
only having an empty `__init__.py` file. Eggs and setuptools provides
some new advantages for the distribution of namespace packages. So,
normally a namespace package's `__init__.py` file will contain
something like this:

import pkg_resources

That's all you required to put in your namespace package's
`__init__.py` file. But you also will be required to add one more
keyword argument for `setup` function in your `setup.py` script like


You can even have nested namespace packages, in that case you have to
add it like this:

namespace_packages=['zope', 'zope.app'],

Remember, Zen of Python also says: "Flat is better than nested." .
Even though Zope use `zope.app` nested namespace package, Zope
community discourage nested namespace packages. The `zope.app`
namespace may be consider as a mistake of Zope project.

The `zope` and `zope.app` were the first namespace packages used by
Zope. I still remember the appraisal I got from Jim Fulton after
implementing his proposal for making `zope.app` a pure namespace

Here I will list some advantages of namespace packages. Feel free to
add/explain anything you found. :)

  1. Better grouping for projects/community/companies
  2. Better name for packages and don't worry about a name conflict
  3. Re-use package name in different namespaces
  4. Easy distribution as eggs

I think the last point requires bit explanation. Consider two
packages in same namespace, `zope.interface` and `zope.testbrowser`.
In Python, package name is tied to directory structure. So, in normal
distutils based distribution both `interface` and `testbrowser` should
be under `zope` directory. But setuptools and eggs allows you to
install both separately and still use both.

Let's save some names for new generation smart Python programmers !
Don't pollute top-level names, use namespace packages !!