OSC python programming pot

Python Programming

Science & Technology

Support Group

20-21 February 2007

Python Programming

Instructor

Peter G. Carswell

Supercomputer Resource Specialist

Ohio Supercomputer Center

[email protected]

(614) 292-1091

Python Programming 3

Table of Contents

• Introduction

• The “Not-So-Full” Monty

– Mechanics of using Python

– Variables, Data Types and Operators

– Programming statements

– Python Functions

– Using Python Modules

– Everything is an Object

– Classes and Objects

– Operator Overloading

– Constructors

Python Programming 4

What is Python?

• NOT an acronym (Thank goodness!). Named after Monty Python

• A compiled/interpreted mid-level language

– Not only used for scripting tasks

• Extremely useful for a huge variety of programming tasks (Modules)

• Easy to “glue” with other languages (C, C++, Java …)

• Under the hood: Object Oriented

• Commonly in use: Versions 2.3 and 2.4

– Use python –V to see the version

• Python is portable

• Python is free!

• Home Page: www.python.org

Python Programming 5

Basic Operation

• Python is both an interpreted and a compiled language

• When run, the program is first read and “compiled” in memory

– Not a true compilation to native machine instructions

– Python source code converted to byte code (platform-independent)

– Some optimizations are performed, e.g.

• eliminating unreachable code

• reducing constant expressions

• loading library definitions

• Second stage is line-by-line execution via the interpreter PVM (Python Virtual

Machine)

– analogous to Java VM

• Much faster than fully interpreted languages, such as the shell

• No object code

– But byte code saved in a file called prog.pyc

Python Programming

Running Python

Python Programming 7

To use Python: Interactive

• Type python in a shell window and start typing in commands at the Python

prompt >>>. The results of the commands will be seen immediately.

• Workshop examples will be for Python on Unix/Linux system.

[piv-login1]% python

Python 2.2.3 (#1, Feb 2 2005, 12:20:51)

[GCC 3.2.3 20030502 (Red Hat Linux 3.2.3-49)] on linux2

Type "help", "copyright", "credits" or "license" for more information.

>>>

>>> print "Hello World! (with respect)“ #Basic output command

Hello World! (with respect)

>>> a=34.5 # No variable type declaration

>>> a # To see contents of variable, type its name

34.5

>>> a*5.6

193.2

>>> z=a/33.3

>>> print '%7.2f' %(z) #Formatting Output

1.04

>>> ^D #How to exit the Python session

$

Python Programming 8

Ways to use Python: Shell Scripts

• A shell script is just a series of python commands entered line-by-line into a file.

By typing the name of the shell script after the python executable, all the

commands will be run in order.

• By convention files composed of Python commands have the suffix py. Let’s say

all the commands on the previous slide have been put into a file rabbit.py.

Here’s how it would be run:

Notice that only the text directly written out to stdout will appear on your monitor

$ python rabbit.py

Hello World! (with

respect)

1.04

$

Python Programming 9

Ways to use Python: Executable Scripts

• Say you want to run your Python script directly at the system prompt. (Just

like an operating system command). This requires two steps

– Make the first line in the rabbit.py file

#!<full pathname of Python executable>

– Second, give rabbit.py executable permission

$ chmod u+x rabbit.py

• Now the rabbit file looks like this:

• To run the commands in the rabbit.py file, just type its name at the system

prompt

#!/usr/local/bin/python

print "Hello World! (with respect)"

a=34.5

a

a*5.6

z=a/33.3

print '%7.2f' %(z)

$ rabbit.py

Hello World! (with respect)

1.04

Python Programming

Variables, Data Types and Operators

Python Programming 11

Variables

• Pick any name you want as long as it begins with a letter or underscore

• Don’t have to declare the variable name or type

• Variable “comes into existence” when it is assigned a value

• Case-sensitive

• Actually much more to variable assignments. Fascinating approach when

we get to objects ….

Python Programming 12

Data Types

• Python has five built-in, core data types. Listed here in the order in which they

will be covered

Numbers

Strings

Lists

Dictionaries

Tuples