Python Functions Basics

Python packages

Packages are the way to structure module namespaces.
We generally keep python modules or files in different directories to structure our program, not keeping everything in a single directory
A directory of python code is called a package.

A simple example

Let us create a python package with some python modules inside it
FIrst of all we need to create a directory called bar.
The name of the directory bar is the name of the package.
Let us create a module foo.py and a special file __init__.py inside it.
NOTE: __init__.py is file need to present inside a directory to make it a python package if you are using python versions < 3.3 (including all 2.X)
__init__.py can be blank

bar
|-- __init__.py
|-- foo.py

foo.py

def printme():
    print("Python packages tutorial")

Here bar is the python package and foo is the module inside package
Python package can have many subdirectories inside it which can be called as python sub packages.
Every sub package should have on __init__.py within it (for < v. 3.3)

Import python modules inside package

Any module and its attributes inside a package can be imported using import or from... import keywords using dotted notations
Let us import the module foo and its definitions present in bar
main.py

import bar.foo
bar.foo.printme() # prints - Python packages tutorial

We can import the module directly using from..import
main.py

from bar import foo
foo.printme() # prints - Python packages tutorial

We can also directly import printme() into current namespace as follows
main.py

from bar.foo import printme
printme() # prints - Python packages tutorial

When we import a module inside a package, python also tries to search package and module within module search paths.
We have to set the proper module search path before importing ( same as discussed in the previous chapter)
For example if the package bar is present inside directory "C:\\code" and we are in a different directory, then we have to set "C:/code" to PYTHONPATH or append to sys.path before import
On console

C:>set PYTHONPATH=C:\\code
C:>python main.py
Python packages tutorial

Or at runtime
main.py

impot sys
sys.append('C:\\code') # setting the search path to sys.append
import bar.foo
bar.foo.printme()

C:>python main.py
Python packages tutorial

More complex example

Let us create a sound package as follows

sound
|-- filters
|   |-- __init__.py
|   |-- equalizer.py
|   |-- karaoke.py
|-- effects
|   |-- __init__.py
|   |-- echo.py
|   |-- reverse.py
|-- formats
|   |-- __init__.py
|   |-- mp3.py
|   |-- mp4.py
|
| -- __init__.py

The sound package has 3 more sub packages here filters, effects, formats.
All the 3 sub packages has a __init__.py within it, qualifying it to be a python package (not required for > v 3.3+)
We can import these python modules as follows

from sound.filters import equalizer
from sound.filters import karaoke
from sound.effects import echo
from sound.effects import reverse
from sound.formats import mp3
from sound.formats import mp4

All the sub package inside the a package can be imported with dotted notation as above