TweetFollow Us on Twitter

Mac in the Shell: Learning Python on the Mac-Part 2

Volume Number: 24
Issue Number: 12
Column Tag: Mac in the Shell

Mac in the Shell: Learning Python on the Mac-Part 2

All about strings

by Edward Marczak


Last month, we began a journey to learn the Python programming language on the Mac. Although I'm assuming little to no programming experience, it should certainly enable experienced programmers to get up to speed quickly in Python as well. Last month started with the absolute basics: variables, objects, the interactive interpreter and the inevitable "Hello, World!" program. There was also a small homework assignment to keep your brain engaged with Python. Let's pick up where we left off.

Answer Key

Last month, I ended the column asking you to "write a program that creates two integer variables,"start"and "end" and one string, "text". Have the program print a slice of the string using the variables and a print statement that precedes the string with "The slice is: ". Here's a script that will do just that:

#!/usr/bin/env python
start = 8
end = 12
text = "This is some text"
print "The slice is:",text[start:end]

Simple, no? Well, this is certainly one way to handle it. "One way?" you ask. "How many ways can there be to write this basic code?" you may think. Well, that's why this entire column will focus on strings in Python, string manipulation and other string subtleties.

Before we continue, I'd like to make a distinction in my writing of this topic. When I refer to Python as "Python"-Capital "P"-I'm referring to Python in general: conceptually. When I use "python", I'm specifically referring to the Python runtime engine and language specification. If you notice this shift in the case throughout the article, that's the reason behind it.

OK. Onward.

Let Me Count the Ways

Like many other scripting languages, you may find that there are several ways to accomplish the same goal in Python. Sometimes the route you choose is purely stylistic. Sometimes the choice is "Pythonic"-instead of brute forcing a solution, Python may include some elegant, built-in way of dealing with your issue. Finally, there are just times where certain styles lend themselves to a particular situation better than other styles, so, you'll find yourself switching styles as needed.

Understanding strings in Python is important, as they are part of the collections class, and therefore respond to anything that a collection class will, as we saw with slices.

Strings in Python do take a little getting used to if you've used other scripting languages in the past. Let's get the basics out of the way.

Strings are formed using single, double or triple quotes. Single quotes and double quotes behave the same:

'This is a string'


"This is a string"

are treated the same. Both single and double quotes require escape sequences to represent special characters. Like any regular expression, Perl or PHP, special characters that you want represented literally, require a backslash escape. Examples include a quote within the outer quotes:

'Bill, it\'s time to go!'

and general special characters, such as newline:

print "Don't follow me too closely\n\n\n"
print "Is that far enough?"

Triple quotes-either ''' or """-have some special properties. Firstly, they can be multiline. Secondly, quotes are passed literally, but special characters are still recognized. The following will illustrate these points:

print """What on Earth is going on?
How am I spanning multiple lines? You think
this is funny?\n\nIt's "everyone's" opinion that
it isn't.

This outputs:

What on Earth is going on?
How am I spanning multiple lines? You think
this is funny?
It's "everyone's" opinion that
it isn't.

Notice that the newline characters were honored, but there was no need to escape the inner quotes.

Now, triple quotes are useful, but more often than not, you're going to need to intersperse the contents of variables or manipulate strings for output. There are a few ways to handle these scenarios. Let's start with the most simple: automatic string concatenation. To illustrate, we need to look at the print function. The Python print function automatically outputs a newline after printing its entire string of data. The commands:

print "Don't follow me too closely."
print "Is that far enough?"

will display:

Don't follow me too closely.
Is that far enough?

No explicit newline character needed. That's pretty straightforward. One way to get rid of the newline character is to use Python's automatic string concatenation feature. Python doesn't require any specific character to concatenate adjacent strings:

print "String 1" "String 2"

prints "String 1String 2". Easy enough, right? Using parentheses, you can extend this to work with multi-line code:

print ("Don't follow me too closely."
       "Is that far enough?")

A comma, which also concatenates strings, will suppress a newline character. It's also a way to add in variables. You may have noticed the homework assignment string used a comma:

print "The slice is:",text[start:end]

What you may not have noticed is that the comma also inserts a space. (There will be a space between the colon and the text in this example). Another way to concatenate strings is the addition symbol, which is overridden to work with text. This method does not insert a space:

print "The slice is: " + text[start:end]

Notice that we added a space ourselves before the final quote mark.

More useful in many ways is the format string. C programmers will recognize this immediately: Python uses the same string formatters as the printf functions in C. The easy introduction is this: Python's print function will substitute the contents of a variable into a string where it finds the string format specifier %s. Time for an example:

username = "bill"
print "Hello, %s" % username

This trivial example will print, "Hello, bill". In a larger program, we'd likely be fetching the username from some other location, such as a database. This style keeps code much more readable, especially when more variables are substituted in a single string. For example:

print "%s, you have %s credit remaining, " \
      "out of %s total." % (user['first'],

There are a few techniques pointed out here. First, you can use the backslash character to continue long lines. Use it where it makes your code more readable. The parentheses form a tuple-something we'll cover in detail next month. The values in the tuple are substituted into the string in order.

For the sake of completeness, there's one last form that is very useful, but won't make sense until we cover dictionaries. A dictionary is a Python data structure that offers a mapping of keys to values. Keys in a dictionary are unique. Due to this, a print format string will also accept a dictionary to map to:

print "%(first)s, you have %(credit)s credit remaining, " \
      "out of %(total)s total." % user

In this example, user is the dictionary-as in the previous example-and each format specifier contains the key in that dictionary to substitute the value of. This will be covered in future columns.

In order to bring this full circle, this shows another way we could have written the print statement in the homework assigment:

print "The slice is: %s" % text[start:end]

One thing I've glossed over a bit here: we're coercing all values into strings. To illustrate, look at this example:

print "The value is %s." % foo

Here, foo is an integer value, but we're placing it into a string. The Python interpreter will happily do the right thing here. But there are situations where you'll need to be a little more precise. In fact, the right way to handle this is to specify that the variable being substituted is an integer. Instead of playing fast and loose and treating all values as a string, "%s", you can specify integers with "%d":

print "The value is %d." % foo

Why does this eally matter, you may ask? Last month, we covered Python's various data types. String, Unicode, integer and float are all used for different purposes. When substituting a float value, you may want to specify the number of decimal places. Try this example:

myfloat = 5.9872345234
print "The number is %s." % myfloat
print "Reduced to 2 decimal places,",\
      "and rounded, it\'s %0.2f" % myfloat

The Python documentation contains a full list of format specifiers. Find it at:

If you install the documentation as instructed in the next section, this can be found locally at file:///Library/ Frameworks/Python.framework/Versions/2.5/Resources/English.lproj/Documentation/lib/typesseq-strings.html.

All You Need to do is Ask

One thing that I feel is important to teach is showing ways in which you can help yourself. Python was designed with this in mind, and supplies a built-in help system. In the interactive python shell, just type help(), or, look for help on a particular word:

>>> help('print')
Sorry, topic and keyword documentation is not available 
because the Python HTML documentation files could not be found.
If you have installed them, please set the environment
variable PYTHONDOCS to indicate their location.

Ah,'ll run into this under OS X. The documentation is, for some reason, not installed with the out-of-the-box Leopard installation. This is easily remedied, however. Download the PythonMac 2.5 distribution ( and mount the image, but do not run the installer. The installer on the image is a metapackage. Control-click on the included mpkg installer, and choose "Show Package Contents" from the resulting Finder menu. Navigate to Contents/Packages and double-click on PythonDocumentation-2.5.pkg to install the documentation. The docs are dropped onto the boot volume, but buried at /Library/Frameworks/Python.framework/Versions/2.5/Resources/English.lproj/Documentation. Even though they're present on disk, python will still not know where to locate them. Let's fix that. Of course, there needs to be a slight explanation first.

Python uses an environment variable named PYTHONDOCS to locate its documentation. As you may expect, PYTHONDOCS specifies a path in the same format as the shell PATH variable: an absolute path on the filesystem. To enable python to locate the documentation, we can create the PYTHONDOCS variable and add the OS X-specific location. In your shell, type and enter:

export PYTHONDOCS=/Library/Frameworks/Python.framework/Versions/2.5/Resources/English.lproj/Documentation

You need to export the variable so the subshell running the python interpreter inherits the value. Ideally, you should add this to a startup file so it's available each time you start a shell. Dropping it in your ~/.bash_profile file is recommended. There are also two very OS X-ish ways of dealing with this for those of you in a GUI environment. One is to simply open the GUI application from the shell that has the exported variable:

open /Users/Shared/Applications/

Just supply open with the full path to the application in question. This will allow the application access to any exported environment variables in that shell session. Since I'm in a shell pretty much full-time, this is my preferred method. Create an alias for the command you use if you do this often.

The second way is to add the environment variable to the user-global environment.plist, or, directly to a particular application's plist. Apple has excellent documentation available on just this topic, so there's no need for me to repeat it. The environment variable developer docs can be found at,. (If Apple ever moves this doc, hit up Google for "User Session Environment Variables").

If you're using TextMate, you can define shell variables in Preferences > Advanced > Shell Variables. Just add a variable named "PYTHONDOCS" with the path listed above, and you can avoid the entire edit-the-plist dance.

Once your environment variable is set, enter python again and ask for help:

>>> help()
Welcome to Python 2.5!  This is the online help utility.
(...output shortened for space considerations...)

Once in the help system, the python prompt will change to help>. You can look for general help on modules, topics or keywords just by typing "modules", "topics" or "keywords" (clever, eh?):

help> modules
Please wait a moment while I gather a list of all available modules...

If you know the module name or keyword, you can just bring up the information directly:

help> print
  6.6 The print statement

Finally, there's no need to enter the help system at all. You can call help with your query as an argument. For example:

>>> help('print')

To exit the help system, press ctrl-d, just like you're leaving the interactive python shell.

For pedantic among you, there's a small distinction to make here. The python interpreter initializes its own environment from the PYTHONDOCS environment variable. Once python is running, changing this variable will have no effect. Conversely, you can look up the current PYTHONDOCS path. From the interactive shell, type the following:

>>> import pydoc

python should spit back to you the current path inherited from PYTHONDOCS.


Between last month and this month, you now know everything practical about strings in Python. While there's a good road left to travel in Python itself, the good news is that so much of the work you'll typically do involves strings and collections that this is a great topic to understand. In next column, we'll tackle more. For now, practice with what you have learned so far.

Media of the month: Neuromancer by William Gibson. Every techie needs some William Gibson in their library. If you've already read Neuromancer, but haven't explored more, use this as an opportunity to pick up something more recent (Count Zero and Pattern Recognition come to mind).

Next month is Macworld. Please stop by the MacTech booth and say hello! Until then, keep scripting!

Ed Marczak knew even from the punch card and TTY days that technology was in his future. He finds all technology interesting, but chooses OS X when possible. When not computing, he spends time with his wife and two daughters.


Community Search:
MacTech Search:

Software Updates via MacUpdate

Square Rave guide - How to grab those te...
Square Rave is an awesome little music-oriented puzzle game that smacks of games like Lumines, but with its own unique sense of gameplay. To help wrap your head around the game, keep the following tips and tricks in mind. [Read more] | Read more »
Snowboard Party 2 (Games)
Snowboard Party 2 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Crowned the best snowboarding game available on the market, Snowboard Party is back to fulfill all your adrenaline needs in... | Read more »
The best games like Animal Crossing on m...
Animal Crossing amiibo Festival is out right now for the Wii U, reminding us of just how much fun that world can be. Or at least to go back and check in on our villages once in a while. [Read more] | Read more »
Between 2 Taps - Tap for Tap interview M...
Hello, and welcome back to Between 2 Taps, Tap for Tap’s Indie Dev interview series. [Read more] | Read more »
Facility 47 (Games)
Facility 47 1.0.1 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.1 (iTunes) Description: You wake up alone and freezing in an icy cell. You try the cell door but it’s locked, it seems that you are stuck with no... | Read more »
The best Photoshop alternative on iPad
Instagram and Lightroom are great and all, but sometimes people need to get extra creative with their image editing.Like, Photoshop creative. If you're one of these people, take a look at our pick for the best mobile Photoshop experience on iPad... | Read more »
The Walking Dead: No Man’s Land guide -...
A new update for The Walking Dead: No Man’s Land was released last week, making it the perfect time for you to head back to your base and take out some walkers. Here’s the lowdown on what’s new to the game, and how to take advantage. [Read more] | Read more »
Goat Rider guide - Tips and tricks to st...
We've all been there. One second, we're riding high on a crazed goat, and the next, we've been tossed off it like someone who's no good at goat ridin'. [Read more] | Read more »
Real Boxing 2 CREED: How to become a gre...
Just in time for Rocky fans who can’t wait to see CREED, the latest movie, we have the official tie-in game,Real Boxing 2 CREED. It builds on the success of its predecessor and there’s lots to take in so we at 148apps thought we’d run you through... | Read more »
CoinOp Heroes 2 guide - How to build an...
CoinOp Heroes 2 justlaunched and, like all clickers, it's dangerously addictive stuff. You have to furiously tap your screen to defeat wave after wave of foes and earn an insane amount of cash to spend on character upgrades and an army of minions... | Read more »

Price Scanner via

Best Buy Black Friday deals: Up to $200 off M...
Best Buy has posted their Black Friday sale prices for 2015. Save on MacBook Pros, MacBooks, MacBook Airs, iMacs, iPads, and Apple Watches. Choose free shipping or free local store pickup (if... Read more
Save $30-$40 on new Apple TVs after rebate
Adorama has new Apple TVs on sale for up to $40 off MSRP after mail-in rebate, good through December 15th. Shipping is free, and Adorama charges NY & NJ sales tax only: - 32GB Apple TV: $119.99... Read more
13-Inch Haswell MacBook Air At Two Years – Th...
The 13-inch mid-2013 “Haswell” MacBook Air I ordered in Apple’s November 2013 Black Friday sale was my first new Mac in four and a half years — the longest interval I’ve gone between system upgrades... Read more
Target Black Friday Early Access deals: $100...
Target is offering early access to their Black Friday deals on Apple products on their online store for today, the 25th, only. Choose free shipping or free local store pickup (if available): - Apple... Read more
BlackBerry Q3 Mobility Index Report Finds iOS...
BlackBerry has announced results of its thirteenth Good Mobility Index Report, showing that organizations are increasingly building custom secure apps. Among Good Powered by BlackBerry (formerly Good... Read more
Wednesday roundup of early Black Friday Mac s...
Save up to $500 on a new Mac with these early Black Friday deals from Apple resellers, currently the lowest prices available for these models: (1) B&H Photo has all new Macs on sale for up to $... Read more
iPod nano on sale for $119, $30 off MSRP
Walmart has the 16GB iPod nano (various colors) on sale for $119.20 on their online store for a limited time. That’s $30 off MSRP. Choose free shipping or free local store pickup (if available). Sale... Read more
Adorama Black Friday deals: Up to $400 off Ma...
Adorama has released their Black Friday deals for 2015. Save up to $400 on MacBook Pros, $200 on MacBooks and MacBook Airs, and $270 on iMacs. Use code RYBFDEAL during checkout to see these prices.... Read more
B&H Photo Deals: $200 off 12-inch 1.2GHz...
In addition to the B&H Photo Black Friday week sales we posted yesterday, B&H has lowered their price on two products to $200 off MSRP: - 12″ 1.2GHz Gray Retina MacBook: $1399 save $200 - 13... Read more
Best Buy Early Access: Today only, Up to $125...
Best Buy has iPad Air 2s on sale for up to $125 off MSRP and Apple Watch models on sale on their online store for up to $100 off MSRP with special codes through midnight CT tonight. Choose free... Read more

Jobs Board

Hardware Systems Integration Engineer - *App...
# Hardware Systems Integration Engineer - Apple Watch Job Number: 39380139 Santa Clara Valley, Califo ia, United States Posted: Apr. 23, 2015 Weekly Hours: **Job Read more
Sr. Technical/Project Manager, *Apple* Educ...
# Sr. Technical/Project Manager, Apple Education Job Number: 36588557 New York City, New York, United States Posted: Jul. 30, 2015 Weekly Hours: 40.00 **Job Summary** Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Merchant Operations Manager: *Apple* Pay -...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more than Read more
*Apple* Pay QA Manager - Apple Inc. (United...
Changing the world is all in a day's work at Apple . If you love innovation, here's your chance to make a career of it. You'll work hard. But the job comes with more than Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.