Running Linux, 5th Edition - Matthias Kalle Dalheimer [422]
If you normally develop software in C, C++, or Java, and from time to time you want to do some scripting, you might find that Perl's syntax is too different from what you are normally used to—for example, you need to type a dollar in front of a variable:
foreach $user ...
Before we look into a bit more detail at what Python is, let us suggest that whether you choose to program in Perl or Python is largely a matter of "religion," just as it is a matter of "religion" whether you use Emacs or vi, or whether you use KDE or GNOME. Perl and Python both fill the gap between real languages such as C, C++, and Java, and scripting languages such as the language built into bash, tcsh or zsh.
In contrast to Perl, Python was designed from the beginning to be a real programming language, with many of the constructs inspired from C. This does undoubtedly mean that Python programs are easier to read than Perl ones, even though they might come out slightly longer.
Python is an object-oriented language, but you do not need to program in an object-oriented fashion if you do not want to. This makes it possible for you to start your scripting without worrying about object orientation; as you go along and your script gets longer and more complicated, you can easily convert it to use objects.
Python scripts are interpreted, which means that you do not need to wait for a long compilation process to take place. Python programs are internally byte-compiled on the fly, which ensures that they still run at an acceptable speed. In normal daily use, you don't really notice all this, except for the fact that when you write a .py file, Python will create a .pyc file.
Python has lists, tuples, strings, and associative arrays (or, in Python lingo, dictionaries) built into the syntax of the language, which makes working with these types very easy.
Python comes with an extensive library, similar in power to what we saw previously for Perl. See http://www.python.org/doc/current/lib/lib.html for a complete library reference.
Parsing Output from the Last Command Using Python
Most programming languages are best explained using examples, so let's look at a Python version of the last log statistics script we previously developed using Perl. Your first impression of the script might very well be that it is way longer than the Perl script. Remember that we are forcing Python to "compete" here in the area where Perl is most powerful. To compensate, we find that this script is more straightforward to read.
Also notice the indentation. Whereas indentation is optional in most other languages and just makes the code more readable, it is required in Python and is one of its characterizing features.
1 #!/usr/bin/python
2
3 import sys, re, string
4
5 minutes = { }
6 count = { }
7 line = sys.stdin.readline()
8 while line:
9 match = re.match( "^(\S*)\s*.*\(([0-9]+):([0-9]+)\)\s*$", line )
10 if match:
11 user = match.group(1)
12 time = string.atoi(match.group(2))*60 + string.atoi(match.group(3))
13 if not count.has_key( user ):
14 minutes[ user ] = 0
15 count[ user ] = 0
16 minutes[ user ] += time
17 count[user] += 1
18 line = sys.stdin.readline()
19
20 for user in count.keys():
21 hour = `minutes[user]/60`
22 min = minutes[user] % 60
23 if min < 10:
24 minute = "0" + `min`
25 else:
26 minute = `min`
27 print "User " + user + ", total login time " + \
28 hour + ":" + minute + \
29 ", total logins " + `count[user]`
The script should be self-explanatory, with a few exceptions. On line 3 we import the libraries we want to use. Having imported string, for instance, we may use it as in line 12, where we use the method atoi from the library string.
On lines 5 and 6 we initialize two dictionaries. In contrast to Perl, we need to initialize them before we can assign values to them. Line 7 reads a line from standard input. When no more lines can be read, the readline method returns None, which is the equivalent to a null pointer.