Packages

Describing Packages

  • Packages allow a collection of modules to be grouped under a common package name
  • This technique helps resolve namespace conflicts between module names used in different applications

  • A package is defined by doing the following:

    • Creating a directory of the same name as the package
    • Creating the file __init__.py in that directory
  • For example, a package might be organized as follows:
Graphics/
├── __init__.py
├── Primitive/
│   ├── lines.py
│   ├── fill.py
│   └── text.py
│       ...
├── Graph2d/
│   ├── __init__.py
│   └── plot2d.py
│       ...
├── Graph3d/
│   ├── __init__.py
│   └── plot3d.py
│       ...
└── Formats/
    ├── __init__.py
    └── gif.py

Relative Imports

  • In Python 3, import assumes an absolute path
  • Relative imports can be used to load submodules contained in different directories of the same package
  • A relative import more clearly states your intentions
  • A .. moves out one directory level and drops down into a different package directory
  • The following is an example:
# plot2d.py
from ..Primitives import lines
  • Relative imports can be used to load all the submodules automatically:
# Graphics/__init__.py
from . import Primitive, Graph2d, Graph3d

Reasons for using __init__.py

  1. Used so modules outside of the package can import modules within the package

    • This is only true for Python 3.2 and older versions
    • This is not necessary for Python 3.3 and newer versions

  2. Used for initialization

    • Specifically, it is used for initializing and code that needs to be run beforehand
    • In other words, any module importing the package will always run anything in __init__.py first
    • This is true even for modules outside of the package

  3. Used for convenience

    • Modules outside of the package can call any function within the package
    • For example, we can do from package import inner_function
    • Meaning, we don't need to do from package.inner_script import inner_function

Setting up Packages

  • A setup.py file makes it easier to install your package on some machine
  • The following is an example setup.py file:
>>> from setuptools import setup

>>> setup(
...     name='foo',
...     version='1.0',
...     description='A useful module',
...     author='Man Foo',
...     author_email='foomail@foo.com',
...     packages=['foo'],  # same as name
...     install_requires=['bar', 'greek']) # external 
...                                        # packages as 
...                                        # dependencies

References

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