Guide to Programming with Python 2
Objectives
• 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
Guide to Programming with Python 3
Examining the Game Over Program
Figure 1.1: Game Over Program OutputThe all-too familiar words from a computer game
Guide to Programming with Python 4
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
• 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++
Guide to Programming with Python 7
Python Is Easy to Use (continued)
• Python Programprint "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
Guide to Programming with Python 15
Setting up Python on Windows
Figure 1.2: Python 2.3.5 Installation Dialogue under WindowsYour computer is soon to be home to Python.
Guide to Programming with Python 16
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/
Guide to Programming with Python 17
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
Guide to Programming with Python 18
Programming in Interactive Mode
Figure 1.4: Python in interactive modePython awaits your command.
Guide to Programming with Python 19
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
Guide to Programming with Python 20
Programming in Interactive Mode (continued)
• At command prompt (>>>), type: print "Game Over"• Python responds with: Game Over
Guide to Programming with Python 21
Programming in Interactive Mode (continued)
• print Statement can display a string (actually, any expression)
• String: Sequence of characters• Statement: Single unit in programming language
that performs some action– 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
Guide to Programming with Python 22
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
Guide to Programming with Python 24
Programming in Script Mode
Figure 1.5: Python in script modeYour blank canvas awaits.
Guide to Programming with Python 25
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
Guide to Programming with Python 26
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
Guide to Programming with Python 27
Programming in Script Mode (continued)
Figure 1.6: Python after a script has been runThe results of running the Game Over program
Guide to Programming with Python 28
The Game Over Program
# Game Over
# Demonstrates the print command
print "Game Over"
raw_input("\n\nPress the enter key to exit.")
Guide to Programming with Python 29
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
Guide to Programming with Python 31
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
Guide to Programming with Python 32
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
Guide to Programming with Python
Chapter TwoTypes, Variables, and Simple I/O: The Useless
Trivia Program
Guide to Programming with Python 34
Objectives
• 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
Guide to Programming with Python 37
The Game Over 2.0 Program
Figure 2.2: Sample run of the Game Over 2.0 program
Ah, the game is really over.
Guide to Programming with Python 38
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 atriple-quoted string """
• Line-continuation character \
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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
Guide to Programming with Python 40
The Fancy Credits Program
Figure 2.3: Sample run of the Fancy Credits program
So many people to thank, so many escape sequences
Guide to Programming with Python 41
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\"."
Guide to Programming with Python 42
Escape Sequences (continued)
Table 2.1: Selected escape sequences
Guide to Programming with Python 43
Concatenating and Repeating Strings
• Can combine two separate strings into larger one
• Can repeat a single string multiple times
Guide to Programming with Python 44
The Silly Strings Program
Figure 2.4: Sample run of the Silly Strings program
Strings appear differently than in the program code.
Guide to Programming with Python 45
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"
Guide to Programming with Python 47
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
Guide to Programming with Python 52
Mathematical Operators (continued)
Table 2.3: Mathematical operators with floats
Guide to Programming with Python 53
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
Guide to Programming with Python 60
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 quoteI think there is a world market for maybe five computers.
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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
Guide to Programming with Python 71
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
Guide to Programming with Python 72
Converting Values
• Can convert one type of value to another
• Use built-in functions
• Solution to Trust Fund Buddy–Bad program
Guide to Programming with Python 73
The Trust Fund Buddy–Good Program
Figure 2.10: Sample run of the Trust Fund Buddy-Good program
Now the total is right.
Guide to Programming with Python 74
Converting Types
• int() function converts a value to an integercar = raw_input("Lamborghini Tune-Ups: ")
car = int(car)
• Can nest multiple function callsrent = int(raw_input("Manhattan Apartment: "))
Guide to Programming with Python 75
Converting Types (continued)
Table 2.5: Selected type conversion functions
Guide to Programming with Python 76
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
Guide to Programming with Python 77
Augmented Assignment Operators (continued)
Table 2.6: Useful augmented assignment operators
Guide to Programming with Python 78
Printing Multiple Values
• To print multiple values in single print statement, separate values by commas– print "\nGrand Total: ", total
Guide to Programming with Python 79
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
Guide to Programming with Python 81
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