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Packages

In this part of the Python programming tutorial, we will talk about packages.

A package is a collection of modules which have a common purpose. Technically a package is a directory which must have one special file called __init__.py.

There are several ways to manage Python code:

• functions
• classes
• modules
• packages

When we deal with large projects containing hundreds or thousands of modules, using packages is crucial. For example, we could put all database related modules in a database package, user interface code in ui package etc.

  
read.py
constants/
          __init__.py
          names.py 

In our current working directory we have a constants directory and a read.py script.

The constants directory has two files. __init__.py file makes constants a Python package. The names.py is an ordinary module.

from names import names

print "initializing constants package"

The __init__.py file is initialized when the package is imported. Here we can control, what object will be available to the module, which imports the package.

#!/usr/bin/python

# names.py

names = ('Jack', 'Jessica', 'Robert', 'Lucy', 'Tom')

The names.py module has a tuple of 5 names.

#!/usr/bin/python

# read.py

import constants

print constants.names

Finally, the read.py script imports the constants package. We print the names tuple from the package.

$ ./read.py 
initializing constants package
('Jack', 'Jessica', 'Robert', 'Lucy', 'Tom')

---

We can also create subpackages. To access subpackages, we use the dot operator.

read.py
constants/
          __init__.py
          names.py 
          numbers/
                   __init__.py
                   integers.py

This is the new hierarchy. We have a subpackage called numbers.

from integers import integers

The __init__.py file in the numbers package has this one line.

#!/usr/bin/python

# integers.py


integers = (2, 3, 45, 6, 7, 8, 9)

The integers module defines a tuple of 7 integers. This tuple will be accessed from the read.py script.

#!/usr/bin/python

# read.py

import constants
import constants.numbers as int

print constants.names
print int.integers

The read.py script is modified a bit.

$ ./read.py 
initializing constants package
('Jack', 'Jessica', 'Robert', 'Lucy', 'Tom')
(2, 3, 45, 6, 7, 8, 9)

---

from package import * construct in a __init__.py file could cause problems on some older operating systems. Therefore a special variable __all__ has been introduced. This variable controls what objects will be imported from a package.

__all__ = ["names"]

print "initializing constants package"

Here we modify the __init__.py file in the constants directory.

#!/usr/bin/python

# read.py

from constants import names
import constants.numbers as int

print names.names
print int.integers

We also change the read.py script accordingly.

$ ./read.py
initializing constants package
('Jack', 'Jessica', 'Robert', 'Lucy', 'Tom')
(2, 3, 45, 6, 7, 8, 9)

The output is the same.

In this chapter, we have covered packages in Python.