Modules allow organizing Python code into reusable files. The import statement accesses modules, with import module accessing all attributes and from module import function accessing specific functions. Packages are directories containing modules, allowing related modules to be grouped. The __name__ attribute determines if a module is run directly or imported, allowing main blocks to execute code when run directly but not on import.

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Python modules and packages
Pavan Verma
@YinYangPavan
Founder, P3 InfoTech Solutions Pvt. Ltd.
Python Programming Essentials

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Modules
A module is a file containing Python definitions,
functions, classes and statements with extension .py
Lets say we have a python file called area.py which
has a few functions in it and we want to use those
functions again in some other programs
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Modules (2)
import math
def circle_area(radius):
# pi r*r
return math.pi * radius * radius
def square_area(side):
# side * side
return side * side
def rectangle_area(width, height):
# width * height
return width * height
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area.py

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import
import area
print area.circle_area(5)
print area.square_area(20)
print area.rectangle_area(10, 20)
Output:
78.5398163397
400
200
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math1.py
The import statement gives
access to all attributes and
methods present in module
To access methods, attributes in
module access them using
module.method syntax

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import (2)
from area import circle_area
print circle_area(5)
Output:
78.5398163397
5
math1.py
from allows you to select
specific functions from the
module

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import (3)
from area import *
print circle_area(5)
print square_area(10)
print rectangle_area(25, 10)
Output:
78.5398163397
100
250
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math1.py
import * gives
access to all functions
in area module
You can use all the
functions in area
module directly

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Advantages of modules
 Putting code into modules is useful because of the
ability to import the module functionality
 A module allows you to logically organize your Python
code
 Grouping related code into a module makes the code
easier to understand and use
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Packages
 A Python package is simply a directory of Python module(s)
 Lets say we have another module called volumes.py
which has a few functions to compute volumes, now it
would make sense that area.py and volume.py are
both related and should be bundled in a package
 We are going to create a package geometry which will have
both modules (area.py and volume.py)
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Packages (2)
geometry
area volume
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• Create a directory named geometry and add both area.py and
volume.py in it
• Also create a file __init__.py in directory geometry
• The __init__.py files are required to make Python treat the
directories as containing packages

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Packages (3)
10
from geometry import area, volume
print area.rectangle_area(25, 10)
print volume.cube_volume(10)
Output:
250
1000
math1.py

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Advantages of packages
 Packages are a way of structuring Python’s module
namespace by using “dotted module names”. For
example, the module name A.B designates a
sub module named B in a package named A
 The use of dotted module names saves the authors of
multi-module packages like NumPy or the Python
Imaging Library from having to worry about each
other’s module names
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Locating modules
 When you import a module, the Python interpreter searches
for the module in the following sequences:
• The current directory
• If the module isn't found, Python then searches each
directory in the shell variable PYTHONPATH
• If all else fails, Python checks the default path. On UNIX, this
default path is normally
/usr/local/lib/python/
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main block
 Before python runs any module it sets up a bunch of
special variables of which one of them is __name__
 __name__ is set equal to "__main__" when the
module is run as standalone program
 If a module is imported the __name__ attribute for
that module is equal to its name
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main block (2)
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import math
def circle_area(radius):
return math.pi * radius * radius
def square_area(side):
return side * side
def rectangle_area(width, height):
return width * height
if __name__ == '__main__':
print square_area(5)
area.py
We have updated area.py
in geometry package a bit,
if __name__ == '__main__'
block has been added

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main block (3)
 The reason to add a block like
if __name__ == '__main__': to the module is that we want to
be able to test/run the module separately other than just importing it and
using it
 This is where __name__ comes into play when we run the module area.py
as a stand alone program like this
$ python area.py
the __name__ attribute becomes equal to __main__ hence the block
if __name__ == '__main__': is executed
 Now if you import this module in other programs, in that case the value of
attribute __name__ for area.py would be geometry.area hence the main
block in area.py does not get executed
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Summary
 Modules
 import
 Advantages of imports
 Packages
 Advantages of packages
 main block
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Resources
 Tutorial on modules http://net.tutsplus.com/tutorials/python-
tutorials/python-from-scratch-functions-and-modules-2/
 Packages Python docs
http://docs.python.org/2/tutorial/modules.html#packages
 __name__ explanation on stackoverflow
http://stackoverflow.com/questions/419163/what-does-if-name-main-do
 module’s __name__ attribute
http://ibiblio.org/g2swap/byteofpython/read/module-name.html
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