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Modules and Packages
In programming, a module is a piece of software that has a specific functionality. For example, when building a ping pong game, one module may be responsible for the game logic, and another module draws the game on the screen. Each module consists of a different file, which may be edited separately.
Modules in Python are just Python files with a .py extension. The name of the module is the same as the file name. A Python module can have a set of functions, classes, or variables defined and implemented. The example above includes two files:
The Python script
game.py implements the game. It uses the function
draw_game from the file
or in other words, the
draw module that implements the logic for drawing the game on the screen.
Modules are imported from other modules using the
import command. In this example, the
game.py script may
look something like this:
# game.py # import the draw module import draw def play_game(): ... def main(): result = play_game() draw.draw_game(result) # this means that if this script is executed, then # main() will be executed if __name__ == '__main__': main()
draw module may look something like this:
# draw.py def draw_game(): ... def clear_screen(screen): ...
In this example, the
game module imports the
draw module, which enables it to use functions implemented
in that module. The
main function uses the local function
play_game to run the game, and then
draws the result of the game using a function implemented in the
draw module called
draw_game. To use
draw_game from the
draw module, we need to specify in which module the function is
implemented, using the dot operator. To reference the
draw_game function from the
we need to import the
draw module and then call
import draw directive runs, the Python interpreter looks for a file in the directory in which the script was executed with the module name and a
.py suffix. In this case it will look for
draw.py. If it is found, it will be imported. If it's not found, it will continue looking for built-in modules.
You may have noticed that when importing a module, a
.pyc file is created. This is a compiled Python file.
Python compiles files into Python bytecode so that it won't have to parse the files each time modules are loaded. If a
.pyc file exists, it gets loaded instead of the
.py file. This process is transparent to the user.
Importing module objects to the current namespace
A namespace is a system where every object is named and can be accessed in Python. We import the function
draw_game into the main script's namespace by using the
# game.py # import the draw module from draw import draw_game def main(): result = play_game() draw_game(result)
You may have noticed that in this example, the name of the module does not precede
draw_game, because we've specified the module name using the
The advantages of this notation is that you don't have to reference the module over and over. However, a namespace cannot have two objects with the same name, so the
import command may replace an existing object in the namespace.
Importing all objects from a module
You can use the
import * command to import all the objects in a module like this:
# game.py # import the draw module from draw import * def main(): result = play_game() draw_game(result)
This might be a bit risky as changes in the module may affect the module which imports it, but it is shorter, and doesn't require you to specify every object you want to import from the module.
Custom import name
Modules may be loaded under any name you want. This is useful when importing a module conditionally to use the same name in the rest of the code.
For example, if you have two
draw modules with slighty different names, you may do the following:
# game.py # import the draw module if visual_mode: # in visual mode, we draw using graphics import draw_visual as draw else: # in textual mode, we print out text import draw_textual as draw def main(): result = play_game() # this can either be visual or textual depending on visual_mode draw.draw_game(result)
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 again, so local variables inside the module act as a "singleton," meaning they are initialized only once.
You can then use this to initialize objects. For example:
# draw.py def draw_game(): # when clearing the screen we can use the main screen object initialized in this module clear_screen(main_screen) ... def clear_screen(screen): ... class Screen(): ... # initialize main_screen as a singleton main_screen = Screen()
Extending module load path
There are a couple of ways to tell the Python interpreter where to look for modules, aside from the
default local directory and built-in modules. You can use the environment variable
PYTHONPATH to specify additional directories to look for modules like this:
PYTHONPATH=/foo python game.py
game.py, and enables the script to load modules from the
foo directory, as well
as the local directory.
You may also use the
sys.path.append function. Execute it before running the
foo directory has been added to the list of paths where modules are looked for.
Exploring built-in modules
Check out the full list of built-in modules in the Python standard library here.
Two very important functions come in handy when exploring modules in Python - the
To import the module
urllib, which enables us to create read data from URLs, we
import the module:
# import the library import urllib # use it urllib.urlopen(...)
We can look for which functions are implemented in each module by using the
>>> 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 more about it with the
help function, using the Python interpreter:
Packages are namespaces containing multiple packages and modules. They're just directories, but with certain requirements.
Each package in Python is a directory which MUST contain a special file called
__init__.py. This file, which can be empty, indicates that the directory it's in is a Python package. That way it can be imported the same way as a module.
If we create a directory called
foo, which marks the package name, we can then create a module inside that
bar. Then we add the
__init__.py file inside the
To use the module
bar, we can import it in two ways:
from foo import bar
In the first example above, we have to use the
foo prefix whenever we access the module
bar. In the second example, we don't, because we've imported the module to our module's namespace.
__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"]
In this exercise, print an alphabetically sorted list of all the functions in the
re module containing the word
import re # Your code goes here find_members = 
import re # Your code goes here find_members =  for member in dir(re): if "find" in member: find_members.append(member) print(sorted(find_members))
test_object('find_members') success_msg('Great work!')
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