TweetFollow Us on Twitter

Mac in the Shell: Python Text Parsing

Volume Number: 25
Issue Number: 07
Column Tag: Mac in the Shell

Mac in the Shell: Python Text Parsing

Automating entries through keyword searching

by Edward Marczak

Introduction

We've been covering Python basics over the last several columns. This month, we'll hit something with a little practicality: text processing. While computers are really good with numbers, people are really good with words. More often than not, input from people comes as text. Turns out that Python is pretty good at dealing with text processing and manipulation. Let's have a closer look, shall we?

More To The Story

OK, there's a little bit more to the story. I've dealt with e-mail systems and e-mail processing for a very long time. (Let's just say that I started with sendmail before it used m4, mmmmkay?). Oftentimes, though, we want a program dealing with incoming mail. This may be for the purposes of a mailing list, for auto-response or to parse the e-mail and then put relevant bits into a database.

E-mail is either really complex or really simple, depending on how you look at it. It's complex because it's got headers and encoding and parts. But it's simple, because it's all text. No matter what all of the pieces are, they're all just human readable text. Fortunately, there are many pre-built libraries that help deal with the complexity, allowing you, the script writer, to focus on the task at hand: processing the parts of the message body that you're interested in. Python's "batteries included" philosophy ensures that a good mail processing library ships as part of the core package.

How is any of this Mac-specific? Well, it isn't. Not directly. However, I just mentioned that Python has, by default-no extra installation required-a good e-mail processing library. Python ships standard with OS X. That's part of the equation solved. Then, there's the issue of receiving the mail in the first place.

Just about every contemporary mail system has a method of taking incoming mail and feeding it to a script. Postfix, which ships with OS X, is no exception. By default, an SMTP (Simple Mail Transfer Protocol) server wants to receive mail, decide if the mail is for a valid user on it's system, and to then drop that mail in the user's mailbox. That's it. But what about a list server? Well, you take the same SMTP server, but instead of delivering any mail to an end user's mailbox, you hand all mail off to the list server program. The list server program will determine who to deliver this mail to.

This is also similar to server-side anti-spam. All incoming mail is handed off to an anti-spam program. The mail is analyzed, potentially acted upon (read: dropped), and mail is then fed back into the SMTP server for final delivery.

We're not going to do anything so grand here today, but after finishing up, you'll have the groundwork. If you have an OS X machine acting as a mail relay and really want to test/use this, you're going to need to modify some postfix config files directly.

In /etc/postfix/transport, you'll need to first define a transport. Let's say your main mail server is called mail.example.com. If you want to divert mail to a script, have the mail sent to mproc.example.com, and add the following like to /etc/postfix/transport:

mproc.example.com mproc:

This says, "all mail that arrives for mproc.example.com send it to the transport named mproc." Once a transport is defined, we also need to tell postfix how to connect the dots between the transport named mproc and our script. That happens in /etc/postfix/master.cf. Add the following line to the end of the file:

mproc  unix  -       n       n       -       -       pipe
  flags=DRhu user=mproc argv=/usr/bin/mproc.py

This tells postfix that any mail arriving on the mproc transport should be piped to the mproc.py script. This is, of course, assuming that we store our script in /usr/bin as "mproc.py". Adjust as needed.

Of course, we're going to keep it simple: since the text will be piped into the script, it's easy to simulate. The pipe simply delivers the entire message on stdin.

A Text Processing Script

Again, we said that we're really focusing on processing e-mail as it arrives, so, we're going to look for input via stdin (which the pipe above does for us). Other text processing scripts may want to deal with text already in a file or elsewhere. I'll make sure to cover that in a future column, but that's not the goal of today's exercise. Despite 'keeping it simple,' we'll be covering a few new-to-us concepts.

Here's the assignment: currently, stock information arrives via e-mail where a dedicated person reads the mail and inputs the entries into a database. This person could clearly be doing better things, as this can be automated without changing the backend system that is sending the e-mail message (whether that's a person or a machine is immaterial for this article). These messages will have a strict format: category and value, separated by a colon. The body of a message would look like this:

Company: Cartier

Product: Watch

Model: Original Tank

Number: 12324A332

Price: $4,500

Available: Yes

However, there's a problem when parsing an e-mail message: it's never just the body that you receive. It's headers. And MIME parts. Oy. Fortunately, Python's email library has functions to deal with this.

I say, let's dive right in. Here's the code I'm using, which will be followed by an explanation of the program.

Listing 1: e-mail parsing program, epp.py

#!/usr/bin/env python
import email
import re
import sys
from email.Parser import Parser
# The keywords we're looking for
keys = ['Company', 'Product', 'Model', 'Number', 'Price', 'Available']
# Compile each keyword into a regular expression
keysre = {}
for i in keys:
  keysre[i] = re.compile(i)
# Read stdin into a single string
mystdin = sys.stdin.read()
# Create a parser object and parse the input
p = Parser()
ps = p.parsestr(mystdin)
# Examine each message part for an appripriate plain body
for i in ps.walk():
  if i.get_content_subtype() != "plain":
    continue
  plainbody = i.as_string()
# Break message into lines, based on newline char
plainbody = plainbody.splitlines()
for i in plainbody:
  # Look at each key for a match.
  for k in keys:
    if keysre[k].match(i):
      print i
sys.exit(0)

First thing to notice about the code is the relative brevity-37 lines in total. As usual, the first few lines simply get us set up: she-bang line and relevant imports, including the Python-supplied email module.

#!/usr/bin/env python
import email
import re
import sys
from email.Parser import Parser

There have been a few times in this column that I've mentioned the importance of regular expressions (RE). Python has good support for RE from the re module:

# The keywords we're looking for
keys = ['Company', 'Product', 'Model', 'Number', 'Price', 'Available']
# Compile each keyword into a regular expression
keysre = {}
for i in keys:
  keysre[i] = re.compile(i)

What is happening here is that we define a list of the keywords we're going to be looking for in the message body. Python regular expressions need to be compiled into an object, which is why we define the keysre dictionary. Of course, we could define these objects one at a time, but that's really inelegant and doesn't scale. In the loop, the dictionary is filled with keys that correspond to the words we're going to match, with a value of the compiled RE object.

# Read stdin into a single string
mystdin = sys.stdin.read()
# Create a parser object and parse the input
p = Parser()
ps = p.parsestr(mystdin)

The first part of this section is pretty simple: assign all of stdin to the variable mystdin. Part of the email library is the email parser object. This object allows an e-mail message, headers, MIME parts and all to be parsed, iterated over and picked apart. We're defining a new parser object and then loading the variable ps with a parsed version of the message that's arriving on stdin.

# Examine each message part for an appropriate plain body
for i in ps.walk():
  if i.get_content_subtype() != "plain":
    continue
  plainbody = i.as_string()

This section of the code hands us back the plain part of the message. MIME types are described in two parts, such as "text/html". We're only interested in the plain portion of the message if there are additional parts in the message. The conditional tests if the subpart is not plain. If it is not, we continue and go back to the top of the loop. If it is plain, we fall though and assign the entire subpart, as a string, to the variable plainbody.

# Break message into lines, based on newline char
plainbody = plainbody.splitlines()

The splitlines() string method returns a list, each element a line in the string, split by a separator-by default, the newline character. Now, we can examine each line in turn:

for i in plainbody:
  # Look at each key for a match.
  for k in keys:
    if keysre[k].match(i):
      print i

As we examine each line, an if statement tests for a match of our regular expressions by looping through the keysre dirctionary. If there's a match, we print it out. Naturally, we can take other action here besides printing it out, such as storing it internally, comparing it to some known value or even inserting it into a database. One thing you will likely want to do is to split the matching lines into key/value pairs. The string's split method does this very nicely. For example:

key, value = i.split(':')

The argument to split is the separator to split on. In our case, we know the lines are split by the colon character and that we're expecting back two values. The split method will happily split as many times as needed. In the case where you don't know how many values to expect, you may just want to assign to a list, like so:

values = i.split(':')

From there you can work out how many values were split and returned to you, and what to do with them.

Finally, we exit the program with a 'clean' exit code:

sys.exit(0)

Running the Program

If you don't happen to have any test e-mail sitting around, I've placed one on the MacTech ftp site, under this month's directory (ftp.mactech.com/src/mactech/volume25_2009/25.07.sit ). If you run your own mail server, you can actually just go grab a raw message from the mail spool-your own mail, mind you!

Since the instructions I gave in the first part allow postfix to send incoming mail through a pipe and to the application, we need a more convenient way to test. The command line makes this easy: just pipe it yourself. Don't forget to mark the program as executable:

chmod 770 epp.py

and then pipe away:

cat /path/to/mits_test_mail | ./epp.py

(or, substitute the ./ with the full path to the program, if needed). If you're using the test mail from the MacTech ftp site, you should see the output you expect: the values that we're matching on, with no headers, MIME clutter, etc. Take a look at the original test mail file to see just how much cruft is being left out.

Conclusion

This was a bit of a whirlwind tour of several concepts. I'd encourage you to bulk up an application like this by checking for error conditions and then taking appropriate action. Outside of that, though, it's pretty impressive at how few dedicated commands are needed to process a well-formed e-mail message. The rest are really just 'nuts and bolts' features of the language.

Media of the month: I'd like to think that everyone has some kind of music that they like. Something that reached them, or that reminds them of some period of time. Well, growing up in New York certainly left a musical stamp on me. I just finished "No Wave" by Marc Masters, and I just loved every second of it. I remember the NY scene around that time, but was certainly too young to fully appreciate it. I don't expect everyone to fully enjoy or 'get' No Wave. But sometimes, the best way to enjoy music is by reading about it. So think of the music that inspires you and find the reading material that points out its inspiration. Thanks to Bruce Gerson for inspiring the topic this month.

Next month, we'll expand on some of the concepts covered here and dig deeper into the well that Python has to offer.


Ed Marczak is the Executive Editor of MacTech Magazine. He has written for MacTech since 2004.

 
AAPL
$108.00
Apple Inc.
+1.02
MSFT
$46.95
Microsoft Corpora
+0.90
GOOG
$559.08
Google Inc.
+8.77

MacTech Search:
Community Search:

Software Updates via MacUpdate

Vitamin-R 2.20b1 - Personal productivity...
Vitamin-R creates the optimal conditions for your brain to work at its best by structuring your work into short bursts of distraction-free, highly focused activity alternating with opportunities for... Read more
Dropbox 2.10.44 - Cloud synchronization...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keep them up-to-date between systems... Read more
Sandvox 2.9.2 - Easily build eye-catchin...
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
Cocktail 8.0.1 - General maintenance and...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LibreOffice 4.3.3.2 - Free Open Source o...
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
VMware Fusion 7.0.1 - Run Windows apps a...
VMware Fusion allows you to create a Virtual Machine on your Mac and run Windows (including Windows 8.1) and Windows software on your Mac. Run your favorite Windows applications alongside Mac... Read more
OneNote 15.3.2 - Free digital notebook f...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
Audio Hijack Pro 2.11.4 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Iridient Developer 3.0.0 beta 3 - Powerf...
Iridient Developer (was RAW Developer) is a powerful image conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more
TextWrangler 4.5.11 - Free general purpo...
TextWrangler is the powerful general purpose text editor, and Unix and server administrator's tool. Oh, and also, like the best things in life, it's free. TextWrangler is the "little brother" to... Read more

Latest Forum Discussions

See All

Monster Flash Review
Monster Flash Review By Jordan Minor on October 31st, 2014 Our Rating: :: ALONE IN THE DARKUniversal App - Designed for iPhone and iPad Solid shooting and a surprising amount of spooky tension make Monster Flash a great portable... | Read more »
Retry Review
Retry Review By Rob Thomas on October 31st, 2014 Our Rating: :: SOARING HIGHUniversal App - Designed for iPhone and iPad Flappy who? Let Retry wash all those bad bird-related memories away on a cool retro-flavored flight… right... | Read more »
Dementia: Book of the Dead Review
Dementia: Book of the Dead Review By Lee Hamlet on October 31st, 2014 Our Rating: :: A TOUGH READUniversal App - Designed for iPhone and iPad A witch hunter is sent after a demonic book in the spooky but short-lived Dementia: Book... | Read more »
Card Dungeon, the Semi-Board Game Roguel...
Card Dungeon, the Semi-Board Game Roguelike, Has Been Renovated Posted by Jessica Fisher on October 31st, 2014 [ permalink ] | Read more »
Logitech Protection + Power iPhone5/5S C...
Made by: Logitech Price: $99.99 Hardware/iOS Integration Rating: 3 out of 5 stars Usability Rating: 0.5 out of 5 stars Reuse Value Rating: 0.75 out of 5 stars Build Quality Rating: 0.75 out of 5 stars Overall Rating: 1.25 out of 5 stars | Read more »
This Is Not a Test Goes Free, Permanentl...
This Is Not a Test Goes Free, Permanently Posted by Jessica Fisher on October 31st, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Swap Heroes Review
Swap Heroes Review By Campbell Bird on October 31st, 2014 Our Rating: :: STRATEGIC SWAPPINGUniversal App - Designed for iPhone and iPad Rotate a cast of heroes to fend of waves of monsters in this difficult, puzzle rpg.   | Read more »
Night Sky Pro™ (Reference)
Night Sky Pro™ 3.0.1 Device: iOS Universal Category: Reference Price: $2.99, Version: 3.0.1 (iTunes) Description: Night Sky Pro™Wonder No More™ Night Sky Pro™ is the ultimate stargazing experience. From the creators of the original... | Read more »
Audio Defence : Zombie Arena (Games)
Audio Defence : Zombie Arena 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: A zombie shooter audio game. Made from gut-wrenching 3D binaural sound, for a new kind of weird immersion. You... | Read more »
RPG Asdivine Hearts (Games)
RPG Asdivine Hearts 1.1.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.1.0 (iTunes) Description: SPECIAL PRICE50% OFF (USD 7.99 -> USD 3.99)!!! Travel alongside four companions and a cat in the adventure of a... | Read more »

Price Scanner via MacPrices.net

Apple now offering refurbished 2014 13-inch R...
The Apple Store is now offering Apple Certified Refurbished 2014 13″ Retina MacBook Pros for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple Regains Momentum As Windows Stutters An...
The latest smartphone sales data from Kantar Worldpanel ComTech, for the three months to March 2014, shows Apple performing strongly in the first quarter of the year, with sales bouncing back in... Read more
Worldwide Smartphone Shipments Increase 25.2%...
New smartphone releases and an increased emphasis on emerging markets drove global smartphone shipments above 300 million units for the second consecutive quarter, according to preliminary data from... Read more
Apple now offering refurbished 2014 15-inch M...
The Apple Store is now offering Apple Certified Refurbished 2014 15″ Retina MacBook Pros for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Apple drops prices on refurbished 2013 Retina...
The Apple Store has dropped prices on 2013 Apple Certified Refurbished 13″ and 15″ Retina MacBook Pros, with Retina models now available starting at $999. Apple’s one-year warranty is standard, and... Read more
New 2.8GHz Mac mini on sale for $949, save $5...
Abt Electronics has the new 2.8GHz Mac mini in stock and on sale for $949.05 including free shipping. Their price is $50 off MSRP, and it’s the lowest price available for this model from any reseller... Read more
Sale! 3.7GHz Quad Core Mac Pro available for...
 B&H Photo has the 3.7GHz Quad Core Mac Pro on sale for $2649 including free shipping plus NY sales tax only. Their price is $350 off MSRP, and it’s the lowest price for this model from any... Read more
Mujjo Steps Up The Game With Refined Touchscr...
Netherlands based Mujjo have just launched their Refined Touchscreen Gloves, stepping up their game. The gloves feature a updated elegant design that takes these knitted gloves to the next level. A... Read more
Sale! Preorder the new 27-inch 5K iMac for $2...
 Abt Electronics has the new 27″ 3.5GHz 5K iMac on sale and available for preorder for $2374.05 including free shipping. Their price is $125 off MSRP, and it’s the lowest price available for this... Read more
Simplex Solutions Inc. Brings Secure Web Surf...
New York based Simplex Solutions Inc. has announced the release and immediate availability of Private Browser 1.0, its revolutionary new secure web browser developed for iPhone, iPad and iPod touch... Read more

Jobs Board

Position Opening at *Apple* - Apple (United...
…Summary** As a Specialist, you help create the energy and excitement around Apple products, providing the right solutions and getting products into customers' hands. You Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** Being a Business Manager at an Apple Store means you're the catalyst for businesses to discover and leverage the power, ease, and flexibility of Apple Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As more and more people discover Apple , they visit our stores seeking ways to incorporate our products into their lives. It's your job, as a Store Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** At the Apple Store, you connect business professionals and entrepreneurs with the tools they need in order to put Apple solutions to work in their Read more
Solutions Specialist with *Apple* Knowledge...
Company Description: We are an Apple Authorized Sales and Service Provider. We have been selling and servicing Apple computers in the Fairfield County area for over Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.