Guide to Programming with Python

Chapter OneGetting Started: The Game Over Program

2

Fm: http://xkcd.com/353/

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Objectives Introduce Python

– History– High-level language (computer program &

algorithms)– Interpreted language

Get started with Python– Python installation – Statements (e.g., print text to the screen)– Comments & application

Demonstrate Python’s development environment, IDLE: iterative / script modes

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Hello World / Game Starts

“Hello World” program: By tradition, prints "Hello, world!” – Often used as first program– print “Hello world”

Console window: Provides a text-based interface to Windows operating system

Terminal application: Provides a text-based interface to Mac OS X and Linux operating systems

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Introducing Python Powerful yet easy to use programming language

(platform independent) Developed by Guido van Rossum First released in 1991 Named after comedy troupe Monty Python Getting more popular

– Python is free and open source– Has strong community

High-level programming Language Interpreted language Object oriented

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Algorithms & Computer programs An algorithm is a set of well-defined instructions for

accomplishing a task (e.g., S’mores algorithm for making s'mores)

When we write computer program, we are generally implementing a method (an algorithm) devised previously to solve some problem.

A computer program is a sequence of instructions that are executed by a CPU

Computer programs can be written in high-level (e.g., Python, Perl, C, C++, Java), or primitive programming languages

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High-level Programming Language High-level programming languages separate from

the low-level processor operations; closer to human language than machine language (machine code or machine language, a system of instructions and data executed directly by a computer's central processing unit, CPU.)

This greater abstraction and hiding of details is generally intended to make the language user-friendly ("Programming at the speed of thought”), but maybe less efficient (but it is not a big problem in many cases with powerful personal computers)

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Python Is a “Glue” Language

Can be integrated with other languages– C/C++– Java

Use existing code Leverage strengths of other languages

– Extra speed that C or C++ offers

Python is Interpreted Language A python program is just a text file; How can a computer

know what to do? Different models of execution for modern high-level languages

Interpreted – Interpreted languages are read and then executed

directly, with no compilation stage.– Examples: Python, Perl, Ruby, Java

Compiled – Compiled languages are transformed into an executable

form before running.– Examples: C (and C++)

Comparison of “Hello World” in Python, Perl, and C

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Setting up Python Windows Linux

– Python probably already installed– Test: try running python at command prompt

Mac OS 10.5.x and up– Lion (10.7) Python 2.7.2 installed; IDLE 2.7.2 (

http://www.python.org/idle/), Tk version 8.5– Leopard (10.5.x) already has Python 2.5.1 installed, but you need

to install IDLE.app following instructions at http://wiki.python.org/moin/MacPython/Leopard

If not installed, go to http://www.python.org/download/

Remember: your computer needs a python “interpreter” to understand python codes

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Introducing IDLE

Integrated Development Environment (IDE): Application that helps software developers write programs– Like a word processor for your code

IDE that ships with Python Has two “modes”: Interactive and Script (see

demo)

But using which editing program (IDLE, or whatever) is really personal taste.

Interactive and Script Mode

Iterative mode: You tell Python what to do, and it will do it immediately

Script mode: You write, edit, load, and save python programs (just like you write a document in Word processor, and any other types of text processors). But definitely you do not want to write your Python codes with Word!

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Programming in Interactive Mode Great for immediate feedback

– Test a simple idea– Remember how something works

Open Python in interactive mode– In Windows, from the Start menu, choose Programs, Python

<version>, IDLE (Python GUI) On STC Lab machines

– Windows: Will be in Start menu > All Programs > Departmentally Sponsored > Informatics

– Mac: Type python in /Applications/Utilities/Terminal.app or run IDLE.app from the Developer Tools folder in the Dock

Command prompt >>>

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Jargon Statement: Single unit in programming language

that performs some action– print "Game Over”

print Statement can display a string (actually, any expression)

String: Sequence of characters Expression: Something which has a value or that

can be evaluated to a single value– "Game Over"– 7 + 2

Code: Sequence of programming statements

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Jargon Syntax highlighting: Displaying programming

code in different colors or fonts, according to the category of each item

Errors– Computers take everything literally– primt "Game Over" produces an Error Message: SyntaxError: invalid syntax

– Syntax error: Error in the rules of usage; often a typo (versus logic error)

– Bug: Error in programming code

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Programming in Script Mode

Great for programs you want to run later– Write, edit, save, and load programs– Like word processor for your programs

• Find and replace

• Cut and paste

Open a script window– In interactive window, select File menu, New

Window

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The Game Over Program

Comment: Note in source code meant only for programmers; ignored by computer– Start comment with #– Use opening block of comments

Blank Lines– Also (generally) ignored by computer– Use for readability; keep related code together

Console Window– Final line keeps console window open

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Python Is Object-Oriented

Object-oriented programming (OOP): Methodology that defines problems in terms of objects that send messages to each other– dir(1) – In a game, a Missile object could send a Ship

object a message to Explode OOP not required, unlike Java and C#

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Summary

Python is a high-level, object-oriented programming language that’s powerful yet easy to use

Python can interface with other programming languages

IDLE is Python’s standard IDE IDLE has an interactive mode that offers immediate

response to Python code IDLE has a script mode that allows programmers to

write, edit, load, save, and run their programs

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Summary (continued)

A string is a sequence of characters A statement is a single unit of programming that

performs some action The print statement displays strings on the screen An expression is something which has a value or

that can be evaluated to a single value Syntax highlighting is displaying programming code

in different colors or fonts, according to the category of each item

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Summary (continued)

A syntax error is a violation of the grammar of a programming language; often caused by a typo (we will see logic error)

A bug is an error in programming code A comment is a note in source code meant only for

programmers; ignored by computer  Comments start with # You should use an opening block of comments in

your programs to identify the programmer, the creation date, and the program’s purpose