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Guide to Programming with
Python
Chapter One
Getting Started: The Game Over Program
Objectives
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•
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Introduce Python
Demonstrate how to install Python
Explain how to print text to the screen
Describe comments and how to use them
Demonstrate Python’s development environment,
IDLE, using it to write, edit, run, and save programs
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Examining the Game Over Program
Figure 1.1: Game Over Program Output
The all-too familiar words from a computer game
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Examining the Game Over Program
(continued)
• “Hello World” program: By tradition, prints "Hello,
world!”
– Often used as first program
• 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
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•
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•
•
Powerful yet easy to use programming language
Developed by Guido van Rossum
First released in 1991
Named after comedy troupe Monty Python
An alarming number of references to spam, eggs,
and the number 42 in documentation
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Python Is Easy to Use
• High-level language: Separate from the low-level
processor operations; closer to human language
than machine language
• "Programming at the speed of thought"
• Increases productivity
– Python programs three to five times shorter than
Java
– Python programs five to ten times shorter than C++
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Python Is Easy to Use (continued)
• Python Program
print "Game Over!"
• C++ Program
#include <iostream>
int main()
{
std::cout << "Game Over!" << std::endl;
return 0;
}
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Python Is Powerful
• Used by large organizations
– NASA
– Google
– Microsoft
• Used in published games
– Battlefield 2
– Civilization IV
– Disney’s Toontown Online
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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
– 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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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
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Python Runs Everywhere
• Platform independent: Independent of the
specific computer operating system
• Python runs on
–
–
–
–
–
Windows
DOS
Mac OS
Linux
Many more
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Python Has a Strong Community
• As an approachable language, has approachable
community
• Python Tutor mailing list
– http://mail.python.org/mailman/listinfo/tutor
– Perfect for beginners
– No actual "tutors" or "students"
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Python Is Free and Open Source
• Open source: Publicly available; open source
software typically programmed by volunteers;
anyone can use source code without fee
• Can modify or even resell Python
• Embracing open-source ideals is part of what
makes Python successful
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Setting up Python on Windows
•
•
The book suggests using the CD-ROM it came
with, but that version of Python (2.3.5) is old now
Instead, follow the instructions on the class web
page:
1. Go to http://www.python.org/download/
2. Download the latest “standard” (aka “production”)
release installer (.msi file)
3. Double-click the installer program and follow its
instructions to install Python on your boot (C:) drive
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Setting up Python on Windows
Figure 1.2: Python 2.3.5 Installation Dialogue under Windows
Your computer is soon to be home to Python.
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Setting up Python on Other Operating
Systems
• Linux
– Python probably already installed
– Test: try running python at command prompt
– If not installed, go to http://www.python.org/download/ (you will
probably need to build from source)
• Mac OS 10.5.x
– 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
• Earlier Mac OS X and other systems
– If necessary, download appropriate version from Python web site
at http://www.python.org/download/
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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
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Programming in Interactive Mode
Figure 1.4: Python in interactive mode
Python awaits your command.
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Programming in Interactive Mode
(continued)
• 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
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Programming in Interactive Mode
(continued)
• At command prompt (>>>), type: print
• Python responds with: Game Over
Guide to Programming with Python
"Game Over"
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Programming in Interactive Mode
(continued)
Statement can display a string (actually, any
expression)
• String: Sequence of characters
• Statement: Single unit in programming language
that performs some action
• print
– print "Game Over"
• 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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Programming in Interactive Mode
(continued)
• 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
– Bug: Error in programming code
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Programming in Script Mode
Figure 1.5: Python in script mode
Your blank canvas awaits.
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Programming in Script Mode
(continued)
• 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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Programming in Script Mode
(continued)
• Write program
– In script window, type print "Game Over"
• Save program
– Select File, Save As, name game_over.py
– Always save before running
• Run Program
– Select Run, Run Module
– Results displayed in interactive window
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Programming in Script Mode
(continued)
Figure 1.6: Python after a script has been run
The results of running the Game Over program
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The Game Over Program
# Game Over
# Demonstrates the print command
print "Game Over"
raw_input("\n\nPress the enter key to exit.")
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The Game Over Program (continued)
• 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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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
• 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
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Guide to Programming with
Python
Chapter Two
Types, Variables, and Simple I/O: The Useless
Trivia Program
Objectives
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Use triple-quoted strings and escape sequences
Make programs do math
Store data in the computer’s memory
Use variables to access and manipulate that data
Get input from users to create interactive programs
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The Useless Trivia Program
Figure 2.1: Sample run of the Useless Trivia program
Whoa! Steve might think about a diet before he visits the sun.
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Using Quotes with Strings
• Can create a single string that's paragraphs long
• Can format text of string in a specific manner
• Can use quotes to create long string or to format
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The Game Over 2.0 Program
Figure 2.2: Sample run of the Game Over 2.0 program
Ah, the game is really over.
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Using Quotes
• Using quotes inside strings
– Define with either single (') or double quotes (")
• 'Game Over' or "Game Over"
– Define with one type, use other type in string
• "Program 'Game Over' 2.0"
• Triple-quoted strings can span multiple lines
"""
I am a
triple-quoted string
"""
• Line-continuation character
Guide to Programming with Python
\
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Using Escape Sequences with Strings
• Escape sequence: Set of characters that allow
you to insert special characters into a string
– Backslash followed by another character
– e.g. \n
– Simple to use
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The Fancy Credits Program
Figure 2.3: Sample run of the Fancy Credits program
So many people to thank, so many escape sequences
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Escape Sequences
• System bell
– print "\a"
• Tab
– print "\t\t\tFancy Credits"
• Backslash
– print "\t\t\t \\ \\ \\ \\ \\ \\ \\"
• Newline
– print "\nSpecial thanks goes out to:"
• Quote
– print "My hair stylist, Henry \'The Great\',
who never says \"can\'t\"."
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Escape Sequences (continued)
Table 2.1: Selected escape sequences
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Concatenating and Repeating Strings
• Can combine two separate strings into larger one
• Can repeat a single string multiple times
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The Silly Strings Program
Figure 2.4: Sample run of the Silly Strings program
Strings appear differently than in the program code.
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Concatenating Strings
• String concatenation: Joining together of two
strings to form a new string
• When used with string operands, + is the string
concatenation operator
– "concat" + "enate"
• Suppressing a Newline
– When used at the end of print statement, comma
suppresses newline
– print "No newline after this string",
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Repeating String
• Multiple concatenations
– When used with strings, * creates a new string by
concatenating a string a specified number of times
– Like “multiplying” a string
– "Pie" * 10 creates new string
"PiePiePiePiePiePiePiePiePiePie"
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Working with Numbers
• Can work with numbers as easily as with strings
• Need to represent numbers in programs
– Score in space shooter game
– Account balance in personal finance program
• Python can represent different types of numbers
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The Word Problems Program
Figure 2.5: Sample run of the Word Problems program
With Python, you can keep track of a pregnant hippo’s weight.
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Numeric Types
• Type: Represents the kind of value; determines
how the value can be used
• Two common numeric types
– Integers: Numbers without a fractional part
1, 0, 27, -100
– Floating-Point Numbers (or Floats): Numbers with
a fractional part
2.376, -99.1, 1.0
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Mathematical Operators
• Addition and Subtraction
– print 2000 - 100 + 50 displays 1950
• Integer Division
– print 24 / 6 displays 4
– But print 19 / 4 displays 4 as well
– Result of integer division always integer
• Floating-Point Division
– print 19.0 / 4 displays 4.75
– When at least one number is a float, result is a float
• Modulus (remainder of integer division)
– print 107 % 4 displays 3
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Mathematical Operators (continued)
Table 2.2: Mathematical operators with integers
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Mathematical Operators (continued)
Table 2.3: Mathematical operators with floats
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Understanding Variables
• Variable: Represents a value; provides way to get
at information in computer memory
• Variables allow you to store and manipulate
information
• You can create variables to organize and access
this information
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The Greeter Program
Figure 2.6: Sample run of the Greeter program
Using a variable for the name
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Creating Variables
• Assignment statement: Assigns a value to a
variable; creates variable if necessary
• name = "Larry"
– Stores string "Larry" in computer memory
– Creates variable name
– Assigns value so that name refers to "Larry"
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Using Variables
• Use variable where you want value it represents
• print name or print "Larry"
Both display Larry
• print "Hi, " + name or print "Hi, Larry"
Both display Hi, Larry
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Naming Variables
• Rules for legal variable names
– Can contain only numbers, letters, and underscores
– Can’t start with a number
– Can’t be a keyword
• Keyword: Built-in word with special meaning
• Legal Names
– velocity, player2, max_health
• Illegal Names
– ?again, 2nd_player, print
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Naming Variables (continued)
• Guidelines for good variable names
– Choose descriptive names; score instead of s
– Be consistent; high_score or highScore
– Follow traditions; Names that begin with underscore
have special meaning
– Keep the length in check
personal_checking_account_balance - too long?
– Self-documenting code: Code written so that it’s
easy to understand, independent of any comments
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Getting User Input
• Variables important for
– Getting user input
– Storing user input
– Manipulating user input
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The Personal Greeter Program
Figure 2.7: Sample run of the Personal Greeter program
name is assigned a value based on what the user enters.
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Getting User Input
• Function: A named collection of programming
code that can receive values, do some work, and
return values
• Argument: Value passed to a function
• Return value: Value returned from a function upon
completion
• Function is like a pizzeria
– Make a call
– Provide information (like pepperoni)
– Get something back (like a hot pepperoni pizza)
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Getting User Input (continued)
• raw_input()
function
– Prompts the user for text input
– Returns what the user entered as a string
• name = raw_input("Hi. What's your name? ")
– Uses argument "Hi. What's your name? " to
prompt user
– Returns what user entered as a string
– In assignment statement, name gets returned string
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Using String Methods
• String methods allow you to do many things,
including:
– Create new strings from old ones
– Create string that’s all-capital-letters version of
original
– Create new string from original, based on letter
substitutions
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The Quotation Manipulation Program
Figure 2.8: Sample run of the Quotation Manipulation program
The slightly low guess is printed several times with string methods.
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String Methods
• Method: A function that an object has
• Use dot notation to call (or invoke) a method
– Use variable name for object, followed by dot,
followed by method name and parentheses
– an_object.a_method()
• Strings have methods that can return new strings
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String Methods (continued)
• quote = "I think there is a world market for
maybe five computers."
– print quote.upper()
I THINK THERE IS A WORLD MARKET FOR MAYBE FIVE COMPUTERS.
– print quote.lower()
i think there is a world market for maybe five computers.
– print quote.title()
I Think There Is A World Market For Maybe Five Computers.
– print quote.replace("five", "millions of")
I think there is a world market for millions of computers.
• Original string unchanged
– print quote
I think there is a world market for maybe five computers.
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String Methods (continued)
Table 2.4: Useful string methods
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Using the Right Types
• Important to know which data types are available
• Equally important to know how to work with them
• If not, might end up with program that produces
unintended results
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The Trust Fund Buddy–Bad Program
Figure 2.9: Sample run of the Trust Fund Buddy-Bad program
The monthly total should be high, but not that high.
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Logical Errors
• Logical Error: An error that doesn’t cause a
program to crash, but instead produces unintended
results
• Program output that looks like very large number:
200001000017000500075001200068001000
• Remember, raw_input() returns a string, so
program is not adding numbers, but concatenating
strings
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Logical Errors (continued)
car = raw_input("Lamborghini Tune-Ups: ")
rent = raw_input("Manhattan Apartment: ")
jet = raw_input("Private Jet Rental: ")
gifts = raw_input("Gifts: ")
food = raw_input("Dining Out: ")
staff = raw_input("Staff (butlers, chef, driver, assistant): ")
guru = raw_input("Personal Guru and Coach: ")
games = raw_input("Computer Games: ")
total = car + rent + jet + gifts + food + staff + guru + games
• car, rent, jet, gifts, food, staff, guru, games
•
are strings
total is concatenation of all strings
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Converting Values
• Can convert one type of value to another
• Use built-in functions
• Solution to Trust Fund Buddy–Bad program
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The Trust Fund Buddy–Good Program
Figure 2.10: Sample run of the Trust Fund Buddy-Good program
Now the total is right.
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Converting Types
• int() function converts a value to an integer
car = raw_input("Lamborghini Tune-Ups: ")
car = int(car)
• Can nest multiple function calls
rent = int(raw_input("Manhattan Apartment: "))
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Converting Types (continued)
Table 2.5: Selected type conversion functions
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Augmented Assignment Operators
• Common to assign a value to a variable based on
its original value
– For example, increment value of variable
• Augmented assignment operators provide
condensed syntax
– Original: score = score + 1
– Augmented: score += 1
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Augmented Assignment Operators
(continued)
Table 2.6: Useful augmented assignment operators
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Printing Multiple Values
• To print multiple values in single print statement,
separate values by commas
– print "\nGrand Total: ", total
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Summary
• String can be defined with either single or double
quotes
• Tripled-quoted strings, defined by three opening
and closing quotes, can span multiple lines
• An escape sequence is a set of characters that
allow you to insert special characters into a string
• String concatenation is the joining together of two
strings to form a new string
• Integers, whole numbers with no decimal part, and
floats, numbers with a decimal part, are two
numeric types
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Summary (continued)
• Result of integer division is always an integer while
result of floating-point division is always a float
• A variable represents a value and provides way to
get at information in computer memory
• An assignment statement assigns a value to a
variable and creates variable if necessary
• A function is a named collection of programming
code that can receive values, do some work, and
return values
• The raw_input() function prompts the user for input
and returns what the user entered as a string
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Summary (continued)
•
•
•
•
A method is a function that an object has
Strings have methods that can return new strings
A logical error produces unintended results
Python has functions for converting values to an
integer, a float, or a string
• Augmented assignment operators provide
condensed syntax for changing the value of a
variable based on its original value
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