Programming in Python

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Programming in Python
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Why programming?

• Real data quantity
• No duplication navigation bar

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Why Python?

• Simple to pick up
• Batteries Included
• Popular for webserver programming

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Who is using it?

• Google (various projects)
• NASA (several projects)
• NYSE (one of only three languages "on the floor")
• Industrial Light Magic (everything)
• Yahoo! (Yahoo mail groups)
• RealNetworks (function and load testing)
• RedHat (Linux installation tools)
• LLNL, Fermilab (steering scientific applications)
• Zope Corporation (content management)
• ObjectDomain (embedded Jython in UML tool)
• Alice project at CMU (accessible 3D graphics)
• More success stories at www.pythonology.com

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docs.python.org

• Tutorials
• Language reference
• Library reference

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Exercises

• For those with little programming experience.
• http//www.upriss.org.uk/python/PythonCourse.html

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Your First Program Hello World

• gtgtgt print 'Hello World'
• Hello World

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Your First Program Hello World

• gtgtgt print 'Hello World'
• Hello World
• Putting it in a file
• hello.py
• print 'Hello World'
• Running a file
• python hello.py

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Play around!

• On your machine, type 'python'
• Or use an webbased version at
• http//try-python.mired.org

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Writing Code

• Use a proper editor.
• Not notepad/word
• Syntax highlighting
• Vi or Emacs
• Notepad and similar

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Writing Readable Code

• Use Comments
• Lines starting with a hash are ignored
• and can be used to explain your code, e.g.,
• Here, we issue multiple prints because
• that is all we can do
• print 'one line'
• print 'another line'
• print 'I hope I can do more than print soon'

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Programming in Python

• Expressions
• Variables
• Operators
• Strings
• Control Flow
• Functions
• Datastructures
• File I/O
• Objects

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Expressions

• A binary truth statement
• false or true
• 0 or anything but 0

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Examples
Expression Truth
1 true
53425 true
0 false




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Expressions

• Consist of
• operands
• constant values
• variables
• function calls
• operators
• arithmetic (plus, minus, ) written as (,
  -)
• comparative (less than, greater than,
  ) written as (lt, gt)
• logical (and, or, not) written as (and,
  or, not)

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Examples
Expression Truth
1 true
53425 true
0 false
1 0 true
1 1 false
1 lt 2 true
1 2 false
not 1 false
not 0 true
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Variables

• Use names to hold values
• x 2
• y x 2
• Like mathematics
• In computer systems,
• variables are containers
• y x is an assignment, not a mathematical
  relationship

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containers
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Variables

• myvalue 3.25
• message hello world
• Simple, eh?

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Variable types

• numbers
• x 3
• strings
• x some text

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Variable names

• Case sensitive
• x 3
• is not the same as
• X 3
• So
• x 3
• y X
• will not cause y to become equal to 3

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Operators

• Assignment
• i 1
• Arithmetic , -, , /
• i 1 1
• Comparison lt, gt, lt, gt, , !
• i lt 5
• Logical and, or, not
• i and j
• And more ...

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Expressions

• An expression can be broken down into operators
  and operands
• i lt 5

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Expressions

• but can be any combination of operands and
  operators
• a lt 3 and not b gt 5

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Expressions

• subparts are evaluated one by one
• (for a2, b6)
• a lt 3 and not b gt 5
• a lt 3 ?
• 2 lt 3 true
• true and not b gt 5

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Expression evaluation

• true and not b gt 5
• b gt 5 ?
• 6 gt 5 ? true
• true and not true
• not true? false
• true and false?
• false

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Expression evaluation

• what does not a lt b mean?
• (not a) lt b
• or
• not (a lt b)
• ?

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Expression evaluation

• when in doubt, add parentheses
• or lookup operator precedence on docs.python.org

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Strings

• 'this is a string'
• this is a string
• are identical. useful to avoid
• 'this isn't a string?'

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Example String Variables

• mymessage hello world
• print mymessage

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Multiline Strings

• triple quotes
• '''this
• is a
• string'''

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String Operations

• concatenation 'this is' 'a string'
• also 'these are now three strings ' 3
• strings consist of characters
• msg 'first second third'
• msg0
• msg04
• msg4
• msg-5

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Example

• x Hello
• print x World

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Control Flow

• Basics
• If
• While
• For
• Exception Handling

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CF Branching

• If temperature gt 35
• print 'it is hot!'
• elif temperature lt 20
• print 'it is cold!'
• else
• print 'ah, nice!'

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That WhiteSpace Thing

• Indentation has meaning
• if a
• print 'hello'
• print 'world'
• Is not the same as
• if a
• print 'hello'
• print 'world'

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CF Loops

• i 0
• while i lt 10
• print 'not done'
• i i 1
• print 'done'

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CF Bounded Loops
for i in range(0,10) print 'not done' print
'done'
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CF Break
for i in range(0,10) print 'not done' if j
0 break print 'done'
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CF Continue
for i in range(0,10) if j 0 continue prin
t 'not done' print 'done'
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CF Exception Handling
try print 1 'st try' except print that
didn't work
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Functions

• Structure your programs
• def sayHello()
• print 'hello world'
• sayHello()

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Functions why?

• a 5
• b 3
• c 2
• d 4
• a lt (b c) and d lt (b c)

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Functions structured programming

• def firstfunc()
• return b c
• a lt firstfunc() and d lt firstfunc()

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Function Parameters
def say(something) print Im saying
something say(I'm getting tired of Hello World)
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Default parameters
def say(something 'NOT Hello World') print
something say()
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Function Return Value

• def max(a, b)
• if a gt b
• return a
• else
• return b
• print max(4, 6)

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Exercises

• Write a program that prints hello! 10 times
• Write a program that calculates how many years,
  weeks and days have passed for a given variable
  totaldays (disregard leap years)
• Write a program that shows all divisors of a
  given variable number
• Write a program that calculates the first 10
  fibonacci numbers

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Hints

• Divisors example
• c a / b will round down a number, so
• c 4/2 will give c2
• c 5/2 will also give c2
• Nesting
• you can nest control flow, e.g.,
• while a lt 5
• if b 0

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Using Python
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Using Python
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Using Python
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Datastructures

• List
• Tuple
• Dictionary

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DS List

• mylist one, two, three, four
• like looking up characters in a string

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Exercises

• Create a list of all students in this class and
  print it
• Write a function that takes two parameters a
  list and a number.
• Have the function print the element in the list
  at index number

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DS More Lists

• Nested 1, 2, 2.5, 3
• Unordered 1, 3, 2
• Indexed mylist2
• Concatenation
• a 1, 2
• b 3
• c a b

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DS Tuple

• Immutable list
• (1, 2)
• More efficient than lists

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DS Dictionary

• List with not just values, but keyvalue pairs
• a name willem, cityamsterdam
• a0
• aname

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Exercise

• Create a dictionary of 10 countries and their
  capitals. Write a function that returns a capital
  when passed a country as parameter

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Advanced Topics

• Object Oriented Programming
• Libraries
• Servers
• Databases

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Object Oriented Programming

• structure readable and simple

objects
structured
functions
control flow
unstructured
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Intuition

• think of real world objects
• have properties
• can perform actions

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Example

• A car
• has a color
• can accelerate

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In programming

• actions are called methods (functions)
• properties are called attributes

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Objects and Classes

• classes are generic (types)
• objects are specific

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Example

• bike is a class
• my bike is an object

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Programming Classes

• class Car
• speed 0
• color blue
• def accelerate(self)
• self.speed self.speed 1
• def breakhard(self)
• self.speed 0

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Using Objects

• mycar Car()
• all objects of the same class share the same
  methods
• individual objects have individual state
• mycar red

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Inheritance

• Specialize classes
• a subclass inherits all methods and attributes
  from its superclass, but can change/add these

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Example

• class Bike
• speed 0
• def slowdown(self)
• self.speed self.speed 10
• class Willemsbike(Bike)
• def slowdown(self)
• self.speed self.speed - 1

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Encapsulation

• self. only usable from within an object
• (we'll skip the rest)

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Import minimize namespace clutter

• import time
• time.ctime()
• time is a module (really just a file)

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In practice in this course

• 1. If you use libraries you need to use objects
• time.ctime()
• 2. sometimes you may create a subclass
• class MyHandler(BaseHTTPServer.BaseHTTPRequestHand
  ler)

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Exercises

• Create the class fruit that has property color.
• Create the subclasses orange with orange color
  and
• mango with green color
• Create the class vehicle with
• properties speed, topspeed and acceleration
• methods accelerate and slowdown
• Create subclass truck with additional property
  load
• Create subclass car with higher topseed than
  truck
• Make sure you cannot accelerate beyond topspeed

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Libraries (or Batteries)

• File access

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Libraries (or Batteries)

• Don't reinvent the wheel

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Libraries (or Batteries)

• http//docs.python.org/library/

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Files

• myfile open(bike.py, r)
• i 1
• data myfile.readline()
• while data
• print line 1 data
• data myfile.readline()
• i i 1
• myfile.close

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Strings

• strings are objects!
• len(str)
• str.upper()
• str.capitalize()
• str.isupper()
• docs.python.org
• dont need to be imported, always there

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telnetlib

• open a raw TCP connection to another server
• running at HOST, PORT
• remember that webservers by default use port 80
• t telnetlib.Telnet(www.google.com, 80)
• read, write and close like files
• t.write(hello)
• reply t.read_all()

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datetime

• datetime.date()

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time

• time.sleep(5)
• time.clock()

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winsound

• .Beep(freq, duration)
• .PlaySound(file, flags)

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turtle graphics
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httplib

• HTTPConnection.request()
• HTTPConnection.getresponse()
• HTTPResponse.read(amt)

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webbrowser

• webbrowser.open(url, new0, autoraise1)

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HTMLParser
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email
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import

• Just once more
• Dont forget to use import, so
• import telnetlib
• t telnetlib.Telnet(host, port)

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Packages Beyond Libraries
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Assignment 1.b

• Deadline Monday 8PM
• Write a program that downloads webpages
• Use telnetlib
• Write your own HTTP request
• Display raw output on screen or
• Better save HTML to a file
• (NB need only 10 lines (but more is allowed))

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Assignment 1.b

• Tip
• HTTP requests are of the form
• GET ltpathgt HTTP/1.0\n\n

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Assignment 1.b

• Optional Advanced Assignment
• Write your own webserver
• derive a subclass from BaseHttpServer
• Incredibly Difficult Assignment
• derive from TCPServer

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Assignment 1.a Deadline TODAY

• Email me your assignment, dont put it in the
  directory
• Send all files in one email
• to wdb_at_few.vu.nl
• with subject Assignment 1.a ltyournamegt
• NB do the same with 1.b before Monday 8PM