An Introduction to Python

Blake Brogdon

What is Python?

Python is an interpreted, interactive, object-oriented programming language. (from python.org)

Portable Dynamic (type/language features) GPL-compatible Java-like or Pythonic code

History Written by Guido van Rossum

– CWI (Netherlands) 1991

– Released to USENET 1995

– Corporation for National Research Initiatives (Reston, VA) 2000

– BeOpen PythonLabs formed– Moved to Digital Creations (Zope Labs)

2001– PSF formed

Non-profit organization created specifically to own Python-related Intellectual Property (python.org)

The Interactive Environment

Unlike Java, Python can be run in an interactive shell– Statements can be evaluated on the fly

Methods do not need to be a part of a class A module is a file that contains Python code

– Function, Class or even just expressions

To use a module’s code call “import modname”– ex. import os

To access that module’s members, prepend the module’s name to the member– ex. print os.path

Basic Syntax

Indentation replaces brackets No semicolons Files can be called anything Unlike Java, Python uses colons http://www.hetland.org/python/instant-python.php Built-ins:

– True, False, None (null)

Everything is an object– Even functions and classnames

Lists Lists

– Like Java’s arrays– Ex

[1,2,3] [“Todd”, [“Blake”, “Pat”]] (lists can be nested) [“Blake”, 3] (can mix types in lists)

– Slicing Like indexing…but better Leads to very un-Java-like constructs

Dictionaries

Like Java/AP’s Hashtable class Examples

– {“a”:1, “b”:2, “c”:3}– {1:2, 3:4, 5:6}

Any combination of different types/lists

Functions No access specifier(public/private) No return type declaration No typing of arguments No brackets

Classes No access specifier (private/public) Constructor = __init__ All class methods take an implicit argument: self

– Ex: __init__(self)– self == this

Instantiation does not require a “new” Inheritance

– class Dog(Animal): – Call to super via explicit superclass init call

Animal.__init__(self)

Instance variable– Set with self.myVar = 1– Access with self.myVar

Interesting Features

Optional arguments List comprehensions Generators “and”/“or” tricks Lambda functions

List Comprehensions Replace multi-line loops with a single

statement

Output: [0, 2, 4, 6, 8] range(x) returns all the numbers from 0 to x

vs.

Generators Functions that can return a value and

maintain their state between calls Yield vs. Return

Output: 0 1 2 3 4 5 6 7 8 9 10

Optional Arguments

Keyword arguments– A value is not necessary– Use value supplied at function definition– Like Java’s overloaded methods– Flexibility

Output: 0 1 2 3 4 5 6 7 8 9 10

“And”/“Or” Tricks

“Empty” values evaluate to false– ex: 0, False, None, [], ()

(Most) Everything else is True A and B

– If A is True return it. – If A is False return b

A or B– If a is true, then return it. – If it isn't, then return b.

Lambda Functions An anonymous function

– So you don’t have to declare a separate, small function

Must be an expression (no ‘print’) Can have any number of args

Output: [1, 4, 9, 16, 25]

Note:– map(f, list) applies function f to each element in list– x**2 = x*x

How to Learn More help(m) prints help for m

– M can be a function, module, class or variable

dir(m) returns a list of all the public members of a module

obj.__doc__ is the documentation of a module, function or class

Links– http://rgruet.free.fr/PQR24/PQR2.4.html– http://hetland.org/python/instant-python– http://python.org/doc/2.4/tut/tut.html– http://python.org/download/

Case Study

Documentation printer for all of the functions within a module