TweetFollow Us on Twitter

Mac in the Shell-Learning Python on the Mac: Modules

Volume Number: 25
Issue Number: 03
Column Tag: Mac in the Shell

Learning Python on the Mac: Modules

Organize code into reusable entities

by Edward Marczak

Introduction

Python has an incredible amount to offer, and its concept of modules is near the top of the list of power. Modules bring a lot of functionality to the language, and offer the author the ability to reuse code in as many different applications as they wish. Since the start of our journey down the path of Python, we've covered basic data types, flow control and functions. Modules will be one of the last very high-level concepts we cover for a bit, but it's an important one. Like most Mac in the Shell columns, this one, too, is best read with a Terminal.app window open and your favorite text editor up and running. So, clear your brain, and let's dive in.

What is a Module?

Modules, really, are just files. Like many things in Python, some very powerful features come with little fanfare, and they 'just work.' By placing functions, variables and code into a standalone file, you create an organized, self-contained, reusable package known as a namespace. In the past two months, you've seen the import keyword, and just had to trust me that it did something useful.

In actuality, import reaches out into the file system, locates, reads and runs an external module. The imported file becomes an object in the importing script, and all definitions in the file become attributes of that object. For illustrative purposes, let's imagine this: top-level ("main") file a.py imports b.py. b.py also imports c.py and also imports some modules from the standard Python library of modules. This is shown in Figure 1.


Figure 1 - Python high-level program architecture.

In essence, the introductory Mac in the Shell articles introducing Python were incredibly simplified; it's rare that a Python program won't have the need to import a module. While we'll be concentrating on importing modules written in Python, do note that Python can actually import a variety of different module types.

How does python know where to find modules? The python interpreter walks a pre-defined set of locations to know where to look. After looking in the same directory as the importing program, an environment variable named PYTHONPATH is consulted. PYTHONPATH is a simple list of directories in which to search for modules. It is constructed just like the PYTHONDOCS variable, covered in the second article introducing Python (December 2008, issue 24.12). After searching PYTHONPATH, python will search among the standard library (location defined at compile time). Finally, python includes any paths listed in .pth files (a slightly advanced topic that we won't be covering this month).

Most people don't ever have to create a PYTHONPATH variable, as the default locations serve most needs. However, if you're sharing development among a few people and need to have a common location for specific modules, you'll need to point PYTHONPATH to your custom location. Like the PATH variable, PYTHONPATH is simply a colon-separated list of paths, searched in order (from left to right). For example, in the bash shell:

export PYTHONPATH="/usr/share/python/"

With this path set, python will additionally search for modules in the /usr/share/python directory.

All of this comes together for the python interpreter as sys.path. From a running program, or, via the interactive interpreter, import the built-in sys module and print sys.path. Here's an abridged listing:

>>> import sys
>>> print sys.path
['', '/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python25.zip',
'/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5']

Note that the path starts with an empty entry, which is how the program's directory is searched.

Let's start with a simple example to demonstrate modules.

Using Modules

Sticking with the a.py, b.py and c.py concept, let's define them for real. Follow along with these short listings.

Listing 1 - a.py

#!/usr/bin/env python
import b
if b.IsInAdmin('root'):
  print "root is in the admin group"
else:
  print "root is not in the admin group"

As stated earlier, the import statement reads a module file and executes it. So, the line import b reads the file b.py and executes it like any other Python program. The if statement tests the return value from an object imported from b. We can also say that the object is in b's namespace. Listing 2 reveals what b.py is doing.

Listing 2 - b.py

#!/usr/bin/env python
import c
def IsInAdmin(user):
  admins = c.GetAdminGroup();
  if user in admins:
    return True
  else:
    return False

b.py starts off with its own import: c.py. b.py defines one function, IsInAdmin(). Finally, look at c.py to see how it works its magic.

Listing 3 - c.py

#!/usr/bin/env python
import plistlib
def GetAdminGroup():
  admin_plist = plistlib.readPlist('/var/db/dslocal/nodes/Default/groups/admin.plist')
  return admin_plist['users']

c.py imports plistlib-something you don't have to define, as it's part of the standard modules shipped with OS X. plistlib allows the author an easy manner of reading and writing OS X's plist files (ASCII only, currently). Here, we'll directly grab the admin.plist file and return the contents from the users key.

Save all three of these files in the same directory, mark a.py executable (chmod 770 a.py), and run it via sudo (it needs root privileges as the admin.plist file is protected). The output looks simply like this:

$ sudo ./a.py 
root is in the admin group

Overall, that was fairly little work. Of course, in a real-world scenario, you wouldn't need to break it down as far as we did here. However, GetAdminGroup() might make a really nice addition to a larger OS X-specific library.

Imports and Namespaces

We've spoken a little about the concept of namespaces without formally defining it. A namespace is a mapping from names to objects. The important thing to realize is that there is no relation between names in different namespaces. Two modules can define x, and there will be no conflict. Internally to python, namespaces are implemented as dictionaries (remember those? Covered briefly in our introduction - November 2008, issue 24.11). To see this in action, run the interactive python shell (by typing python at the shell prompt) in the same directory as the modules we've been working on. A module's dictionary is hidden away in the __dict__ attribute. It responds to all standard dictionary messages. For example, to list the contents of the object, import our c.py module:

>>> import c
and then list the keys:
>>> c.__dict__.keys()
['GetAdminGroup', 'plistlib', '__builtins__', '__file__', '__name__', '__doc__']

This is the same list that you get already with the dir() function, covered in earlier columns. However, it does lead to some fairly (advanced) trickery and techniques.

The point of this is that an import statement will import objects into their own namespace. This name of a given namespace is derived from the filename. This means that module filenames must conform to good variable names. You can certainly create a module named while.py, but since "while" is also a built-in keyword, you won't be able to import it. Our c.py module has been imported into a namespace of 'c'.

This also means that you should be careful to name your module something useful, as you'll be referring to it in code... or will you? Well, that depends. It depends somewhat on how you import a module, and somewhat on style. I'll say it now: my personal preference is that you always import as we've been showing-by using the import keyword-and referencing the namespace in code. Some people consider this bulky, though. Look at our plistlib usage:

plistlib.readPlist()

Instead of simply calling readPlist, we have to qualify it. This prevents it from conflicting with any other function (object, really) of that name. Importing plistlib created a namespace for that module. However, there is a way to import into the current namespace, and that relies on the from keyword variant.

To import an entire module, or just a select part of it into the current namespace, you tell python, "from [module] import [what]". The "what" can reference a specific object in the module, or use a wildcard. To import all of c.py directly into the current namespace, b.py can issue:

from c.py import *

b.py could then make the following change:

admins = GetAdminGroup();

See? No need to qualify the function call. Also, be aware that a subset of a module can be imported by specifying it explicitly:

from plistlib import readPlist

While all of this may seem to lessen the burden on the python interpreter, it doesn't in reality. The problem with this is much more from a maintenance-for-the-author perspective. This is especially true when multiple people are working on a project, each by contributing a module. As you may guess, conflicting object names simply get crushed, and the last name defined 'wins.' You, the author, lose, of course. Additionally, since python reads the entire module file in any case, or, has a pre-compiled byte-code version, there are no real performance/time savings by importing a single object. You may change a module that is imported in this manner and pollute the namespace of the importing application without realizing it. Using a standard import, there will never be conflicting objects.

To Be Executed

I mentioned above that when a module is imported, it is run like any other Python application. This is true; it's how objects get created, just like in the top-level file. We can prove this, too. Consider the following code:

Listing 4 - msgtest.py

#!/usr/bin/env python
print "Running top-level python file"
import msgmod
print "Finishing up top-level file"

Listing 5 - msgmod.py

#!/usr/bin/env python print "Running msgmod.py module" Running msgtest.py yields the following output: Running top-level python file Running msgmod.py module Finishing up top-level file

Not surprising if you've been following along! The same goes for any assignment statements in a module. Of course, functions are read and turned into an object, but the contents of which are not executed until explicitly called. Sometimes, however, a file has value as both a top-level file and as something to be imported. As a top-level file, we want it to execute, but as an imported file, we just want it imported, ready at our beck and call. There's a Python-ism that addresses this. A built-in variable, __name__, is assigned in each module at runtime. When the top-level application is run, __name__ is "__main__". Instead of just defining statements at the top indent-level, every statement in the top-level file is put into a function (or class) definition. The only top level statement tests __name__, and call your main function as appropriate. For example, our a.py program would be altered like this:

#!/usr/bin/env python
import b
def main():
  if b.IsInAdmin('root'):
    print "root is in the admin group"
  else:
    print "root is not in the admin group"
if __name__ == '__main__':
  main()

This way, if we had other useful functions in a.py, it could be run directly, or imported as a module.

I will be using this idiom going forward.

Fin

For this month, that's enough. Modules are such a core concept in Python, it's worth it to make sure it's understood before moving on. Next month, we'll talk a little more about style, and then cover classes.

As an aside, I met many people at Macworld who were really exited by this column turning its attention to Python. I'd love to hear ideas from all readers, so, letters@mactech.com and emarczak@mactech.com are ready and waiting for mail.

Media of the month: "Synthetic Worlds: The Business and Culture of Online Games," by Edward Castronova. Interesting work that covers, mainly, the economy of virtual worlds (ok, MMOs). The book is a touch dated at this point, but overall, it should make the reader think about how much more our technology is touching and influencing our 'non-technological' world.

See you next month!


Ed Marczak is the Executive Editor of MacTech Magazine. He lives in New York with his wife, two daughters and various pets. He has been involved with technology since Atari sucked him in, and has followed Apple since the Apple I days. He spends his days on the Mac team at Google, and free time with his family and/or playing music. Ed is the author of the Apple Training Series book, "Advanced System Administration v10.5," and has written for MacTech since 2004.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

NetShade 6.3.1 - Browse privately using...
NetShade is an anonymous proxy and VPN app+service for Mac. Unblock your Internet through NetShade's high-speed proxy and VPN servers spanning seven countries. NetShade masks your IP address as you... Read more
Dragon Dictate 4.0.7 - Premium voice-rec...
With Dragon Dictate speech recognition software, you can use your voice to create and edit text or interact with your favorite Mac applications. Far more than just speech-to-text, Dragon Dictate lets... Read more
Persecond 1.0.2 - Timelapse video made e...
Persecond is the easy, fun way to create a beautiful timelapse video. Import an image sequence from any camera, trim the length of your video, adjust the speed and playback direction, and you’re done... Read more
GIMP 2.8.14p2 - Powerful, free image edi...
GIMP is a multi-platform photo manipulation tool. GIMP is an acronym for GNU Image Manipulation Program. The GIMP is suitable for a variety of image manipulation tasks, including photo retouching,... Read more
Sandvox 2.10.2 - Easily build eye-catchi...
Sandvox is for Mac users who want to create a professional looking website quickly and easily. With Sandvox, you don't need to be a Web genius to build a stylish, feature-rich, standards-compliant... Read more
LibreOffice 5.0.1.2 - Free, open-source...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
f.lux 36.1 - Adjusts the color of your d...
f.lux makes the color of your computer's display adapt to the time of day, warm at night and like sunlight during the day. Ever notice how people texting at night have that eerie blue glow? Or wake... Read more
VirtualBox 5.0.2 - x86 virtualization so...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more
File Juicer 4.43 - Extract images, video...
File Juicer is a drag-and-drop can opener and data archaeologist. Its specialty is to find and extract images, video, audio, or text from files which are hard to open in other ways. In computer... Read more
Apple MainStage 3.2 - Live performance t...
MainStage 3 makes it easy to bring to the stage all the same instruments and effects that you love in your recording. Everything from the Sound Library and Smart Controls you're familiar with from... Read more

ReBoard: Revolutionary Keyboard (Utilit...
ReBoard: Revolutionary Keyboard 1.0 Device: iOS Universal Category: Utilities Price: $1.99, Version: 1.0 (iTunes) Description: Do everything within the keyboard without switching apps! If you are in WhatsApp, how do you schedule a... | Read more »
Tiny Empire (Games)
Tiny Empire 1.1.3 Device: iOS Universal Category: Games Price: $2.99, Version: 1.1.3 (iTunes) Description: Launch cannonballs and blow tiny orcs into thousands of pieces in this intuitive fantasy-themed puzzle shooter! Embark on an... | Read more »
Astropad Mini (Productivity)
Astropad Mini 1.0 Device: iOS iPhone Category: Productivity Price: $4.99, Version: 1.0 (iTunes) Description: *** 50% off introductory price! ​*** Get the high-end experience of a Wacom tablet at a fraction of the price with Astropad... | Read more »
Emo Chorus (Music)
Emo Chorus 1.0.0 Device: iOS Universal Category: Music Price: $1.99, Version: 1.0.0 (iTunes) Description: Realistic Choir simulator ranging from simple Chorus emulation to full ensemble Choir with 128 members. ### introductory offer... | Read more »
Forest Spirit (Games)
Forest Spirit 1.0.5 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.5 (iTunes) Description: | Read more »
Ski Safari 2 (Games)
Ski Safari 2 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: The world's most fantastical, fun, family-friendly skiing game is back and better than ever! Play as Sven's sister Evana, share... | Read more »
Lara Croft GO (Games)
Lara Croft GO 1.0.47768 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.47768 (iTunes) Description: Lara Croft GO is a turn based puzzle-adventure set in a long-forgotten world. Explore the ruins of an ancient... | Read more »
Whispering Willows (Games)
Whispering Willows 1.23 Device: iOS Universal Category: Games Price: $4.99, Version: 1.23 (iTunes) Description: **LAUNCH SALE 50% OFF** - Whispering Willows is on sale for 50% off ($4.99) until September 9th. | Read more »
Calvino Noir (Games)
Calvino Noir 1.1 Device: iOS iPhone Category: Games Price: $3.99, Version: 1.1 (iTunes) Description: The film noir stealth game. Calvino Noir is the exploratory, sneaking adventure through the 1930s European criminal underworld.... | Read more »
Angel Sword (Games)
Angel Sword 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: Prepare to adventure in the most epic full scale multiplayer 3D RPG for mobile! Experience amazing detailed graphics in full HD.... | Read more »

Price Scanner via MacPrices.net

Big Grips Lift Handle For iPad Air and iPad A...
KEM Ventures, Inc. which pioneered the extra-large, super-protective iPad case market with the introduction of Big Grips Frame and Stand in 2011, is launching Big Grips Lift featuring a new super-... Read more
Samsung Launches Galaxy Tab S2, Its Most Powe...
Samsung Electronics America, Inc. has announced the U.S. release of the Galaxy Tab S2, its thinnest, lightest, ultra-fast tablet. Blending form and function, elegant design and multitasking power,... Read more
Tablet Screen Sizes Expanding as iPad Pro App...
Larger screen sizes are gaining favor as the tablet transforms into a productivity device, with shipments growing 185 percent year-over-year in 2015. According to a new Strategy Analytics’ Tablet... Read more
Today Only: Save US$50 on Adobe Elements 13;...
Keep the memories. lose the distractions. Summer’s winding down and it’s time to turn almost perfect shots into picture perfect memories with Elements 13. And get the power to edit both photos and... Read more
1.4GHz Mac mini on sale for $449, save $50
Best Buy has the 1.4GHz Mac mini on sale for $50 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Price for online orders only, in-store price may vary... Read more
12-inch 1.1GHz Gold MacBook on sale for $1149...
B&H Photo has the 12″ 1.1GHz Gold Retina MacBook on sale for $1149.99 including free shipping plus NY sales tax only. Their price is $150 off MSRP, and it’s the lowest price available for this... Read more
27-inch 3.3GHz 5K iMac on sale for $1849, sav...
Best Buy has the 27″ 3.3GHz 5K iMac on sale for $1849.99. Their price is $150 off MSRP, and it’s the lowest price available for this model. Choose free shipping or free local store pickup (if... Read more
Worldwide Tablet Shipments Expected to Declin...
Does Apple badly need a touchscreen convertible/hybrid laptop MacBook? Yes, judging from a new market forecast from the International Data Corporation (IDC) Worldwide Quarterly Tablet Tracker, which... Read more
Continued PC Shipment Shrinkage Expected Thro...
Worldwide PC shipments are expected to fall by -8.7 percent in 2015 and not stabilize until 2017, according to the latest International Data Corporation (IDC) Worldwide Quarterly PC Tracker data. The... Read more
Imminent iPhone 6s Announcement Leads To 103%...
NextWorth Solutions, with its online and in-store electronics trade-in programs including http://NextWorth.com, reports that it has experienced a 103 percent surge in quoted trade-in values over the... Read more

Jobs Board

*Apple* Music, Business Operations - Apple I...
Changing the world is all in a day039s work at Apple . If you love innovation, here039s your chance to make a career of it. You039ll work hard. But the job comes with Read more
WW *Apple* Retail Online Store: Customer In...
**Job Summary** The Apple Retail - Online Store is seeking an experienced web merchandising analytics professional to join the Customer Insights Team. The Web Read more
Senior Payments Security Manager - *Apple*...
**Job Summary** Apple , Inc. is looking for a highly motivated, innovative and hands-on senior payments security manager to join the Apple Pay security team. You will Read more
Software QA Engineer, *Apple* Pay Security...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
Business Process Engineer ( *Apple* Online S...
**Job Summary** The Apple Online Support Planning team is looking for an experienced Business Process engineer to lead key business process improvement initiatives Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.