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By Kevin HarrisAn Introduction to Python

What is Python?• Python is a portable, interpreted, object-oriented scripting language created byGuido van Rossum.• Its development started at the NationalResearch Institute for Mathematics andComputer Science in the Netherlands , andcontinues under the ownership of thePython Software Foundation.• Even though its name is commonlyassociated with the snake, it is actuallynamed after the British comedy troupe,Monty Python’s Flying Circus.

Why learn a scripting language?• Easier to learn than more traditionalcompiled languages like C/C++• Allows concepts to be prototypedfaster• Ideal for administrative tasks whichcan benefit from automation• Typically cross platform

Why Python?So, why use Python over otherscripting languages?• Well tested and widely used• Great documentation• Strong community support• Widely Cross-platform• Open sourced• Can be freely used and distributed, evenfor commercial purposes.

How do I install Python?• Simple… download it fromwww.python.org and install it.• Most Linux distributions already comewith it, but it may need updating.• Current version, as of thispresentation, is 2.4.2 Click here todownload this version for Windows.

How do I edit Python?• IDLE is a free GUI based text editor whichships with Python.• You could just write your scripts withNotepad, but IDLE has many helpfulfeatures like keyword highlighting.• Click your “Start” button and Find “Python2.4” on your “All Programs” menu. Thenselect and run “IDLE (Python GUI)”.• You can also right-click on any pythonscript (.py) and select “Edit with IDLE” toopen it.

“Hello World!”• The “Hello World!” example is a traditionamongst programmers, so lets start there…• Run IDLE and type in the followingprint “Hello World!”• Save your work and hit F5 to run it.• The script simply prints the string, “HelloWorld!” to the console.

Run Sample Script…• Load “hello_world.py” into IDLE and hit F5to run it.• NOTE: Scripts can also be executed bysimply double-clicking them, Unfortunately,they often run too quickly to read the outputso you’ll need to force a pause. I like touse a function called raw_input to pausethe script before it closes the console. Thenext script “hello_world_with_pause.py”,demonstrates this little hack.

The print function• The print function simply outputs a string value toour script’s console.print “Hello World!”• It’s very useful for communicating with users oroutputting important information.print “Game Over!”• We get this function for free from Python.• Later, we’ll learn to write our own functions.

Adding Comments• Comments allow us to add useful information toour scripts, which the Python interpreter willignore completely.• Each line for a comment must begin with thenumber sign ‘#’.# This is a programming tradition…print “Hello World!”• Do yourself a favor and use them!

Quoting Strings• Character strings are denoted using quotes.• You can use double quotes…print “Hello World!” # Fine.• Or, you can use single quotes…print ‘Hello World!’ # Also fine.• It doesn’t matter as long as you don’t mix themprint “Hello World!’ # Syntax Error!

Triple Quotes• You can even use triple quotes, whichmake quoting multi-line strings easier.print “““This is line one.This is line two.This is line three.Etc…”””

Run Sample Script…• Load “triple_quotes.py” into IDLE andhit F5 to run it.

Strings and Control Characters• There are several control characters which allowus to modify or change the way character stringsget printed by the print function.• They’re composed of a backslash followed by acharacter. Here are a few of the more importantones:n : New linet : Tabs : Backslash' : Single Quote" : Double Quote

Run Sample Script…• Load “control_characters.py” intoIDLE and hit F5 to run it.

Variables• Like all programming languages, Python variablesare similar to variables in Algebra.• They act as place holders or symbolicrepresentations for a value, which may or may notchange over time.• Here are six of the most important variable typesin Python:int Plain Integers ( 25 )long Long Integers ( 4294967296 )float Floating-point Numbers( 3.14159265358979 )bool Booleans ( True, False )str Strings ( “Hello World!” )list Sequenced List ( [25, 50, 75, 100])

Type-less Variables• Even though it supports variable types,Python is actually type-less, meaning youdo not have to specify the variable’s type touse it.• You can use a variable as a characterstring one moment and then overwrite itsvalue with a integer the next.• This makes it easier to learn and use thelanguage but being type-less opens thedoor to some hard to find bugs if you’re notcareful.• If you’re uncertain of a variable’s type, usethe type function to verify its type.

Rules for Naming Variables• You can use letters, digits, andunderscores when naming your variables.• But, you cannot start with a digit.var = 0 # Fine.var1 = 0 # Fine.var_1= 0 # Fine._var = 0 # Fine.1var = 0 # Syntax Error!

Rules for Naming Variables• Also, you can’t name your variablesafter any of Python’s reservedkeywords.and, del, for, is, raise, assert, elif,from, lambda, return, break, else,global, not, try, class, except, if, or,while, continue, exec, import, pass,yield, def, finally, in, print

Run Sample Script…• Load “variable_types.py” into IDLE and hitF5 to run it.• NOTE: Be careful when using Pythonvariables for floating-point numbers (i.e.3.14) Floats have limited precision, whichchanges from platform to platform. Thisbasically means there is a limit on how bigor small the number can be. Run the scriptand note how "pi" gets chopped off androunded up.

Numerical Precision• Integers• Generally 32 signed bits of precision• [2,147,483,647 .. –2,147,483,648]• or basically (-232, 232)• Example: 25• Long Integers• Unlimited precision or size• Format: <number>L• Example: 4294967296L• Floating-point• Platform dependant “double” precision• Example: 3.141592653589793

Type Conversion• The special constructor functions int, long, float, complex,and bool can be used to produce numbers of a specific type.• For example, if you have a variable that is being used as afloat, but you want to use it like an integer do this:myFloat = 25.12myInt = 25print myInt + int( myFloat )• With out the explicit conversion, Python will automaticallyupgrade your addition to floating-point addition, which youmay not want especially if your intention was to drop thedecimal places.

Type Conversion• There is also a special constructor function calledstr that converts numerical types into strings.myInt = 25myString = "The value of 'myInt' is "print myString + str( myInt )• You will use this function a lot when debugging!• Note how the addition operator was used to jointhe two strings together as one.

Run Sample Script…• Load “type_conversion.py” into IDLEand hit F5 to run it.

Arithmetic Operators• Arithmetic Operators allow us to performmathematical operations on two variables orvalues.• Each operator returns the result of the specifiedoperation.+ Addition- Subtraction* Multiplication/ Float Division** Exponentabs Absolute Value

Run Sample Script…• Load “arithmetic_operators.py” intoIDLE and hit F5 to run it.

Comparison Operators• Comparison Operators return a True orFalse value for the two variables or valuesbeing compared.< Less than<= Less than or equal to> Greater than>= Greater than or equal to== Is equal to!= Is not equal to

Run Sample Script…• Load “comparison_operators.py” intoIDLE and hit F5 to run it.

Boolean Operators• Python also supports three Boolean Operators,and, or, and not, which allow us to make use ofBoolean Logic in our scripts.• Below are the Truth Tables for and, or, and not.

Boolean OperatorsSuppose that… var1 = 10.• The and operator will return True if and only if bothcomparisons return True.print var1 == 10 and var1 < 5 (Prints False)• The or operator will return True if either of the comparisonsreturn True.print var1 == 20 or var1 > 5 (Prints True)• And the not operator simply negates or inverts thecomparison’s result.print not var1 == 10 (Prints False)

Run Sample Script…• Load “boolean_operators.py” intoIDLE and hit F5 to run it.

Special String Operators• It may seem odd but Python even supports a few operatorsfor strings.• Two strings can be joined (concatenation) using the +operator.print “Game ” + “Over!”Outputs “Game Over!” to the console.• A string can be repeated (repetition) by using the * operator.print “Bang! ” * 3Outputs “Bang! Bang! Bang! ” to the console.

Run Sample Script…• Load “string_operators.py” into IDLEand hit F5 to run it.

Flow Control• Flow Control allows a program orscript to alter its flow of executionbased on some condition or test.• The most important keywords forperforming Flow Control in Python areif, else, elif, for, and while.

If Statement• The most basic form of Flow Control is the ifstatement.# If the player’s health is less than or equal to 0 - killhim!if health <= 0:print “You’re dead!”• Note how the action to be taken by the ifstatement is indented or tabbed over. This is not astyle issue – it’s required.• Also, note how the if statement ends with a semi-colon.

Run Sample Script…• Load “if_statement.py” into IDLE andhit F5 to run it.

If-else Statement• The if-else statement allows us to pick one of twopossible actions instead of a all-or-nothing choice.health = 75if health <= 0:print “You're dead!”else:print “You're alive!”• Again, note how the if and else keywords and theiractions are indented. It’s very important to get thisright!

Run Sample Script…• Load “if_else_statement.py” into IDLEand hit F5 to run it.

If-elif-else Statement• The if-elif-else statement allows us to pick one ofseveral possible actions by chaining two or more ifstatements together.health = 24if health <= 0:print "You're dead!"elif health < 25:print "You're alive - but badly wounded!"else:print "You're alive!"

Run Sample Script…• Load “if_elif_else_statement.py” intoIDLE and hit F5 to run it.

while Statement• The while statement allows us to continuously repeat an action untilsome condition is satisfied.numRocketsToFire = 3rocketCount = 0while rocketCount < numRocketsToFire:# Increase the rocket counter by onerocketCount = rocketCount + 1print “Firing rocket #” + str( rocketCount )• This while statement will continue to ‘loop’ and print out a “Firingrocket” message until the variable “rocketCount” reaches thecurrent value of “numRocketsToFire”, which is 3.

Run Sample Script…• Load “while_statement.py” into IDLEand hit F5 to run it.

for Statement• The for statement allows us to repeat an action based on theiteration of a Sequenced List.weapons = [ “Pistol”, “Rifle”, “Grenade”, “Rocket Launcher” ]print “-- Weapon Inventory --”for x in weapons:print x• The for statement will loop once for every item in the list.• Note how we use the temporary variable ‘x’ to represent thecurrent item being worked with.

Run Sample Script…• Load “for_statement.py” into IDLE andhit F5 to run it.

break Keyword• The break keyword can be used to escape from while and forloops early.numbers = [100, 25, 125, 50, 150, 75, 175]for x in numbers:print x# As soon as we find 50 - stop the search!if x == 50:print "Found It!"break;• Instead of examining every list entry in “numbers”, The forloop above will be terminated as soon as the value 50 isfound.

Run Sample Script…• Load “for_break_statement.py” intoIDLE and hit F5 to run it.

continue Keyword• The continue keyword can be used to short-circuit or bypassparts of a while or for loop.numbers = [100, 25, 125, 50, 150, 75, 175]for x in numbers:# Skip all triple digit numbersif x >= 100:continue;print x• The for loop above only wants to print double digit numbers.It will simply continue on to the next iteration of the for loopif x is found to be a triple digit number.

Run Sample Script…• Load “for_continue_statement.py” intoIDLE and hit F5 to run it.

Functions• A function allows several Pythonstatements to be grouped together sothey can be called or executedrepeatedly from somewhere else inthe script.• We use the def keyword to define anew function.

Defining functions• Below, we define a new function called“printGameOver”, which simply prints out, “GameOver!”.def printGameOver():print “Game Over!”• Again, note how indenting is used to denote thefunction’s body and how a semi-colon is used toterminate the function’s definition.

Run Sample Script…• Load “function.py” into IDLE and hitF5 to run it.

Function arguments• Often functions are required toperform some task based oninformation passed in by the user.• These bits of Information are passedin using function arguments.• Function arguments are defined withinthe parentheses “()”, which are placedat the end of the function’s name.

Function arguments• Our new version of printGameOver, can now printout customizable, “Game Over!”, messages byusing our new argument called “playersName”.def printGameOver( playersName ):print “Game Over... ” + playersName + “!”• Now, when we call our function we can specifywhich player is being killed off.

Run Sample Script…• Load “function_arguments.py” intoIDLE and hit F5 to run it.

Default Argument Values• Once you add arguments to yourfunction, it will be illegal to call thatfunction with out passing the requiredarguments.• The only way around this it is tospecify a default argument value.

Default Argument Values• Below, the argument called “playersName”has been assigned the default value of,“Guest”.def printGameOver( playersName=“Guest” ):print “Game Over... ” + playersName + “!”• If we call the function and pass nothing, thestring value of “Guest” will be usedautomatically.

Run Sample Script…• Load “default_argument_values.py”into IDLE and hit F5 to run it.

Multiple Function arguments• If we want, we can define as many arguments as we like.def printGameOver( playersName, totalKills ):print “Game Over... ” + playersName + “!”print “Total Kills: ” + str( totalKills ) + “n”• Of course, we must make sure that we pass the arguments inthe correct order or Python will get confused.printGameOver( "camper_boy", 15 ) # Correct!printGameOver( 12, "killaHurtz" ) # Syntax Error!

Run Sample Script…• Load “multiple_arguments.py” intoIDLE and hit F5 to run it.

Keyword arguments• If you really want to change the order of how the argumentsget passed, you can use keyword arguments.def printGameOver( playersName, totalKills ):print “Game Over... ” + playersName + “!”print “Total Kills: ” + str( totalKills ) + “n”printGameOver( totalKills=12, playersName=“killaHurtz” )• Note how we use the argument’s name as a keyword whencalling the function.

Run Sample Script…• Load “keyword_arguments.py” intoIDLE and hit F5 to run it.

Function Return Values• A function can also output or return a value based on itswork.• The function below calculates and returns the average of alist of numbers.def average( numberList ):numCount = 0runningTotal = 0for n in numberList:numCount = numCount + 1runningTotal = runningTotal + nreturn runningTotal / numCount• Note how the list’s average is returned using the returnkeyword.

Assigning the return value• Here’s how the return value of our average function would beused…myNumbers = [5.0, 7.0, 8.0, 2.0]theAverage = average( myNumbers )print "The average of the list is " + str( theAverage ) + "."• Note how we assign the return value of average to ourvariable called “theAverage”.

Run Sample Script…• Load “function_return_value.py” intoIDLE and hit F5 to run it.

Multiple Return Values• A function can also output more than one value during the return.• This version of average not only returns the average of the listpassed in but it also returns a counter which tells us how manynumbers were in the list that was averaged.def average( numberList ):numCount = 0runningTotal = 0for n in numberList:numCount = numCount + 1runningTotal = runningTotal + nreturn runningTotal / numCount, numCount• Note how the return keyword now returns two values which areseparated by a comma.

Assigning the return values• Here’s how the multiple return value of our average functioncould be used…myNumbers = [5.0, 7.0, 8.0, 2.0]theAverage, numberCount = average( myNumbers )print "The average of the list is " + str( theAverage ) + "."print "The list contained " + str( numberCount ) + " numbers.“• Note how we assign the return values of average to ourvariables “theAverage” and “numberCount”.

Run Sample Script…• Load “multiple_return_values.py” intoIDLE and hit F5 to run it.

Conclusion• This concludes your introduction toPython.• You now know enough of the basicsto write useful Python scripts and toteach yourself some of the moreadvanced features of Python.• To learn more, consult thedocumentation that ships with Pythonor visit the online documentation.

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