Python: Modules and Packages

Modules and Packages

Modules in Python are simply Python files with the .py extension, which implement a set of functions. Modules are imported from other modules using the import command.

To import a module, we use the import command. Check out the full list of built-in modules in the Python standard libraryhere.

The first time a module is loaded into a running Python script, it is initialized by executing the code in the module once. If another module in your code imports the same module again, it will not be loaded twice but once only – so local variables inside the module act as a “singleton” – they are initialized only once.

If we want to import the module urllib, which enables us to create read data from URLs, we simply import the module:

# import the library
import urllib

# use it
urllib.urlopen(...)

Exploring built-in modules

Two very important functions come in handy when exploring modules in Python – the dir andhelp functions.

We can look for which functions are implemented in each module by using the dir function:

>>> import urllib
>>> dir(urllib)
['ContentTooShortError', 'FancyURLopener', 'MAXFTPCACHE', 'URLopener', '__all__', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '__version__', '_ftperrors', '_get_proxies', '_get_proxy_settings', '_have_ssl', '_hexdig', '_hextochr', '_hostprog', '_is_unicode', '_localhost', '_noheaders', '_nportprog', '_passwdprog', '_portprog', '_queryprog', '_safe_map', '_safe_quoters', '_tagprog', '_thishost', '_typeprog', '_urlopener', '_userprog', '_valueprog', 'addbase', 'addclosehook', 'addinfo', 'addinfourl', 'always_safe', 'basejoin', 'c', 'ftpcache', 'ftperrors', 'ftpwrapper', 'getproxies', 'getproxies_environment', 'getproxies_macosx_sysconf', 'i', 'localhost', 'main', 'noheaders', 'os', 'pathname2url', 'proxy_bypass', 'proxy_bypass_environment', 'proxy_bypass_macosx_sysconf', 'quote', 'quote_plus', 'reporthook', 'socket', 'splitattr', 'splithost', 'splitnport', 'splitpasswd', 'splitport', 'splitquery', 'splittag', 'splittype', 'splituser', 'splitvalue', 'ssl', 'string', 'sys', 'test', 'test1', 'thishost', 'time', 'toBytes', 'unquote', 'unquote_plus', 'unwrap', 'url2pathname', 'urlcleanup', 'urlencode', 'urlopen', 'urlretrieve']

When we find the function in the module we want to use, we can read about it more using the help function, inside the Python interpreter:

help(urllib.urlopen)

Writing modules

Writing Python modules is very simple. To create a module of your own, simply create a new .py file with the module name, and then import it using the Python file name (without the .py extension) using the import command.

Writing packages

Packages are namespaces which contain multiple packages and modules themselves. They are simply directories, but with a twist.

Each package in Python is a directory which MUST contain a special file called __init__.py. This file can be empty, and it indicates that the directory it contains is a Python package, so it can be imported the same way a module can be imported.

If we create a directory called foo, which marks the package name, we can then create a module inside that package called bar. We also must not forget to add the __init__.py file inside thefoo directory.

To use the module bar, we can import it in two ways:

import foo.bar

or:

from foo import bar

In the first method, we must use the foo prefix whenever we access the module bar. In the second method, we don’t, because we import the module to our module’s namespace.

The __init__.py file can also decide which modules the package exports as the API, while keeping other modules internal, by overriding the __all__ variable, like so:

__init__.py:
 

__all__ = [“bar”]