Intro to Python Part 2

Intro to Python Part 2P. TenHoopen - WMLUGIntro to Python 2February 2009Slide 1

What is Python?Python is a cross-platform object-oriented programminglanguage invented by Guido van Rossum. It is an interpretedlanguage but there is support for compiling the programs.The most recent version is 3.0 (also known as Python 3000)which was released on December 3, 2008. It is incompatiblewith the 2.x releases.Python Home Page - http://www.python.org/Beginners Guide - http://wiki.python.org/moin/BeginnersGuide/NonProgrammersPython Language Reference - http://docs.python.org/reference/index.html#reference-indexGuido van Rossum Personal Page - http://www.python.org/~guido/Intro to Python 2February 2009Slide 2

Native Data TypesNative Python Data Types:●Dictionaries●Lists●TuplesIntro to Python 2February 2009Slide 3

DictionariesDictionaries are an unordered collection ofkey-value data pairs.It is unordered in the fact that as you additems, they are not necessarily appendedto the end.Keys need to be unique.Intro to Python 2February 2009Slide 4

Creating DictionariesCreating an empty dictionary:name={} # empty dictionaryExample:colors={}Intro to Python 2February 2009Slide 5

Creating DictionariesCreating a pre-popluated dictionary:name={"key":"value"}Example:hexColor={"white":"ffffff", "black":"000000","red":"ff0000"} # prepopulated dictionaryprint hexColor{'white': 'ffffff', 'black': '000000', 'red':'ff0000'}Intro to Python 2February 2009Slide 6

Adding ItemsAdding items to a dictionary:name["key"]="value"hexColor["blue"]="0000ff"print hexColor{'blue': '0000ff', 'white': 'ffffff','black': '000000', 'red': 'ff0000'}Notice that the item was inserted at the beginning,and not the end – hence unordered!Intro to Python 2February 2009Slide 7

Changing Item ValuesChanging values in a dictionary:Same syntax as adding but the value isupdated.hexColor["blue"]="blue"print hexColor{'blue': 'blue', 'white': 'ffffff','black': '000000', 'red': 'ff0000'}Intro to Python 2February 2009Slide 8

Deleting ItemsDeleting items from a dictionary:del name["key"]Example:del hexColor["blue"]print hexColor{'white': 'ffffff', 'black':'000000', 'red': 'ff0000'}Intro to Python 2February 2009Slide 9

Clearing All ItemsClearing all items from a dictionary:name.clear()Example:hexColor.clear()print hexColor{}Intro to Python 2February 2009Slide 10

ListsLists are an ordered collection of indexedvalues.The values can be strings or numbers andcan be duplicated.List indexes are zero-based.Intro to Python 2February 2009Slide 11

Creating New ListsCreating new lists:name[] # empty listExample:list1[]Intro to Python 2February 2009Slide 12

Creating New ListsCreating a prepopulated list:list2["white", "black", "42"]print list2['white', 'black', '42']Intro to Python 2February 2009Slide 13

Insert Item Into ListInserting items into a list:list2.insert(2, "purple")print list2['white', 'black', 'purple', '42']Intro to Python 2February 2009Slide 14

Appending ItemsAppending items to a list:list2.append("red")print list2['white', 'black', 'purple', '42','red']Intro to Python 2February 2009Slide 15

Deleting ItemsDeleting items from a list:list2.remove("white")This will remove the first occurance of thevalue "white" from the list.print list2['black', 'purple', '42', 'red']Intro to Python 2February 2009Slide 16

Using List ValuesUsing values from a list:name[index]Example:List2[1]'purple'Intro to Python 2February 2009Slide 17

Searching ListsSearching for values in a list:name.index("key")Example:list2.index("42")2Intro to Python 2February 2009Slide 18

Check A List For ValuesIf you try to search a list for a value thatisn't present, python will give an exception.To avoid this, you can check a list for acertain value:"black" in list2TrueYou get a True or False response back.Intro to Python 2February 2009Slide 19

ListYou can use the len function to get thenumber of items in a list:len(list2)4Intro to Python 2February 2009Slide 20

TuplesA tuple is an indexed immutable list.It can't be changed once it is created.Good for constant variables.Intro to Python 2February 2009Slide 21

Why Use TuplesTuples are faster than lists.Lists can be turned into tuples using thetuple function.Tuples can be turned into lists using thelist function.Intro to Python 2February 2009Slide 22

Creating TuplesCreating a new tuple:name=(“value1”, “value2”)Examples:t=("a", "b", "c")screensize=(800, 600)Intro to Python 2February 2009Slide 23

Checking A Tuple for ValueYou can check a tuple for a certain value:"b" in tTrueYou get a True or False response back.Intro to Python 2February 2009Slide 24

FunctionsFunctions are blocks of code that perform acertain function.The statements are indented inside thefunction.Intro to Python 2February 2009Slide 25

Defining A FunctionTo define a function use the def command:def functionName(parameters):The parameters are optional variablenames.Intro to Python 2February 2009Slide 26

DocumentingYou can document functions use a docstring.A doc string is a triple quotation markenclosed string. It can be multi-lined.Example:def function():“””This is a doc string.”””Intro to Python 2February 2009Slide 27

Return ValuesYou can return a value back from thefunction using the return command. Ifthat is not done, the function automaticallyreturns None which is the Python null value.def functionName(parameters):print “Hello from the function”return 1Intro to Python 2February 2009Slide 28

Variable ScopeThe scope of a variable is where the variable isaccessible. There is local and global scope.A variable defined within a function only isaccessible within that function, hence it has localscope.A variable can have global scope so that it isaccessible to all functions.Intro to Python 2February 2009Slide 29

Using Global VariablesSometimes you need to use variables thatwere declared outside of the function whichare normally inaccessible.Use the global command to access thevalues.global variableAIntro to Python 2February 2009Slide 30

Using FunctionsTo call a function:def sayHello():“””Function to print a message.”””print “Hello, how are you?”return 1sayHello()Hello, how are you?Intro to Python 2February 2009Slide 31

Function ParametersTo call a function that takes a parameter:def sayHello(passedName):“””Function to print a message.The passedName variable accepts the value passed to it fromthe calling statement. It has local scope in the function.”””print “Hello, %s how are you?” % passedNameExample 1 – Static string as a parameter:sayHello(“Homer”)Hello, Homer how are you?Example 2 – Variable as a parameter:myName=”Bart”sayHello(myName)Hello, Bart how are you?Intro to Python 2February 2009Slide 32

Putting It All TogetherDemoIntro to Python 2February 2009Slide 33

Putting It All Together#!/usr/bin/python"""FileStats.pyP. TenHoopen, 11/21/08Read the statistics of files from a specified directory and subdirectories.Display number of files, types and number of files, average size, smallest size,largest size"""# Import Modulesimport os# Global Variablesextensions = {}largestFileSz = 0largestFile = ""numFiles = 0sumFileSizes = 0.0# dictionary to hold extensions and counts# size of largest file# name of largest file# number of files found# total size of files used to calc average file sizeIntro to Python 2February 2009Slide 34

Putting It All Together# Functionsdef recordExtension(thisExtension):"""This function keeps track of the extensions that are found."""# add to dictionary if new, or update count if existingif thisExtension not in extensions:extensions[thisExtension] = 1else:extensions[thisExtension] += 1return 1Intro to Python 2February 2009Slide 35

Putting It All Togetherdef collectFileInfo(fileName):"""This function examines file statistics to keep track of largest file andtotal file sizes."""# use global variables defined aboveglobal largestFileSzglobal largestFileglobal sumFileSizes# collect file statisticsfileStats = os.stat(fileName)# get file sizesize = fileStats.st_size# keep size of all filessumFileSizes += size# is this the largest file found so far?if size > largestFileSz:largestFileSz = sizelargestFile = fileNamereturn 1Intro to Python 2February 2009Slide 36

Putting It All Togetherdef displayStats():"""This function displays the collected statistics."""print "\n\nResults:"print "\nThere were %d files found." % numFilesprint "\nCount \t File Ext"print "­­­­­ \t ­­­­­­­­"for extension in extensions:print "%d \t %s" % (extensions[extension], extension)print "\n"print "Largest file: %s at %d bytes" % (largestFile, largestFileSz)print "Total size of files is %d bytes" % (sumFileSizes)print "Average size of files is %.2f bytes" % (sumFileSizes / numFiles)print "\n"return 1Intro to Python 2February 2009Slide 37

Putting It All Togetherdef main():"""This function is called when the program starts. It analyzes thefiles specified in the topDir variable."""# variablestopDir = "."# directory to search, default to current directory# use global variables defined aboveglobal numFiles# get top directory to analyze from useruserDir = raw_input("Input the directory to analyze: ")if (len(userDir) > 0): # weak error checking!topDir = userDir # as long as the user enters something, useit otherwise use default value of "."print "\nAnalyzing files in '%s'..." % topDir# walk through the fileswalklist = os.walk(topDir)# os.walk returns {dirpath, dirnames, filenames}Intro to Python 2February 2009Slide 38

Putting It All Togetherfor root, dirs, files in walklist:# examine each file foundfor eachFile in files:numFiles += 1# look for a extension delimiter (.) in the file nameif "." in eachFile:# get the file extension using a string slice# extract all text after the periodextension = eachFile[eachFile.index(".")+1:]else:# no period delimiter so no extension can be determinedextension = "none"# keep track of extensions that are foundrecordExtension(extension)# get file name and pathfileName = os.path.join(root, eachFile)# examine files statscollectFileInfo(fileName)# display the results of the analysisdisplayStats()Return 1# This calls the 'main' function when this script is executed.if __name__ == '__main__': main()Intro to Python 2February 2009Slide 39

That's All FolksQuestions?Intro to Python 2February 2009Slide 40