Introduction to the Python Programming Language

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Last updated: 2004.05.20

Python is a nice little scripting language that is well designed. That means that its features and syntax actually make sense. Here are some notes on using Python. Additional information can be found at Python.org, the home of Python.

One of the coolest little Python software out there is the edna MP3 server. It really shows what can be done in Python. Edna also contains my first open source software work, even if it is a tiny tiny bit of a patch to the software !

What's cool about Python

It supports strings, tuples and lists (sequence) as basic data types
It uses source code indentation to define program blocks.
This seems like a really bad idea to start, but it ends up enforcing standardize well formatted source file.
You can say "if 1 < x < 50:"
You can say "a = b = 3"
You can say "a,b = b,a"
Slicing a string (or a list)
Optional and default arguments in function

Some articles that will help you learn Python

Guido van Rossum's home page. Not very up to date, but his list of publications and presentations are useful.
The useless python! site. It is suppose to be retiring, so the index page is not useful. Use the old index page here instead.
David Metz: Text processing in Python: Tips for beginners
Or run it on your Palm Pilot
Python 2.0 Quick Language Reference
The book: How to Think Like a Computer Scientist , which is available online and free as well as in printed form. I recommend this book with some reservation -- The book present Python systematically, broken down by programming language elements, which is how I tend to learn a new language, hence the ' like a computer scientist' part.
However, the authors 'thinking' part, is rather amateurish. Any book that talks about the need of 'debugging' because "Programming is a complex process, and because it is done by human beings, it often leads to errors. For whimsical reasons, programming errors are called bugs and the process of tracking them down and correcting them is called debugging.",
or present recursing as "Following the flow of execution is one way to read programs, but it can quickly become labyrinthine. An alternative is what we call the "leap of faith." When you come to a function call, instead of following the flow of execution, you assume that the function works correctly and returns the appropriate value." make me cringe.
read What's wrong with Perl (and right with Python)

The Language

This is a quick summary of language features:

None or Null

There is a single global object, call None, that is useful for indicating null or nothing.

Data Type Conversions

int(string) -> int
float(string) -> float
str(int) -> str
str(unicode_str)->str
list(imatable_tuple)->list

Loop

Q: How do I write for x = 0 to 9?

A: for x in range(0,10): print x

Lists

List are represented by square brackets:

[1,2,3,6,7] is a list of integers
[ (a,1), (c,3)] is a list of tuples

List Initialization

To create/initialize a list, use the repeat operator on lists.
For example, initialize a list of length 10, with zeros:

L1 = [0] * 10

List Concatination

L1.extend(L2)
is the same as:
L1[len(L1):] = L2

List Copying, cloning, aliasing

L1 = [1,2,3]
L2 = L1[:]

note:
L2 = L1 just makes L2 an alias of L1, i.e. L1 and L2 are references pointing to the same actual list object -- dangerous.
prove that by doing
print id(L1), id(L2)
you will see both id's are the same.

Use tuples if your objects can be immutable !

List Sorting

L1.sort()
sorts it in place.

Lambda Function

The lambda function is a great tool for writing simple function in line. Typically you can use this with the map and filter functions. [tbw]

Map and Filter

The filter function let you apply a functional filter to a list. The map function let you transform a list. [tbw]

Slicing

a = '123456'
a[start:end]
a[0] = '1' # note: starts from zero
a[5] = '6'
a[0:6] = '123456' # note: end is non inclusive
Cool stuff: negative values can be used in both start and end index:
a[0:-1] = '12345'
Leaving out index means beginning or end
a[:-1] = '12345'

Tuples vs Sequences

Tuples are immutable.

File Processing

Reading a File

Reading text file is slow if you read the file line by line. The prefered method is to use the readlines() method to read the entire file into memory:

f = open("path/to/filename")
   lines = f.readlines()
   for line in lines:
       # do something with line
       line = line[:-1] # remove newline at the end

If the file is too large, you can use the optional size hint argument to tell readlines to read complete lines from the file, but not to use up all memory:

Dictionary

A dictionary is setup using the {} braces. dictionary = {'key1':'data1', 'key2','data2' }

Database

One big problem with Python is that it is not as easy to setup to connect to real databases (like Oracle) then P**l. So let's focus on something that Python does well -- a simple object database using the shelve.py module. Or use the more elaborated ZODB, part of the ZOPE web application platform. ZODB is available as a standalone package for use outside of ZOPE.

Shelve.py

This module lets you store Python objects in a disk based database. It uses the underlying anydbm.py module to store the data, after converting the object to a string using pickle.py.

Cool Python Apps

Quixote -- web application framework, light and easy to learn, better than zope I think.

Be careful with default arguments in function

i = 9

def f(n=i):
      print 'in f(), n=',n

f(1)
f()
i=2
f()

This will produce:

in f(), n= 1
in f(), n= 9
in f(), n= 9  <=== not 2

This is because the default argument referencing the global "i" is only evaluated once when the parser parses the function definition. You have to set the default to None and test for None to get the more logical behaviour:

i = 9

def f(n=None):
      if n==None:
           n=i
      print 'in f(), n=',n

f(1)
f()
i=2
f()