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Michigan State University CSE 231 1

! Today: Program Development with Classes

(Punch & Enbody, Chapter 13)

! Next: More on Files and Exceptions

(Punch & Enbody, Chapter 14)

Lecture Topics

Michigan State University CS E 231 2

! Exams scores now on D2L

o Notify me of any inaccuracies

! Project # 9

o Scores sent via email

! Project #10

o Preliminary processing sent via email

o Scores sent via email end of week?

Announcements


! Due Monday, Dec 6th

!

Focuses on design, implementation, andtesting of a Python class to create and

manipulate times (UTC).

! Mostly the same methods as Project 10,

but a few changes – follow the Project 11

specs!

Computer Project #11


Three fundamental characteristics:

!

Encapsulation

! Polymorphism

! Inheritance

Object-Oriented Programming

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Encapsulation

!

Provides modularity so the new data type can be reusedin other contexts, raise level of abstraction in client code

!

Hides implementation details of the new data type to

improve readability

o Leading underscore signifies a “hidden” attribute (should not beused directly by client code)

!

Provides an interface to manipulate objects of the new

data type

o The methods and data members whose names do not begin with

underscore(s)

o All “overloaded” functions and operators


Polymorphism

! The new data type allows use of standardoperations/functions, as appropriate

! Examples:

o

construction

o

convert to string (for printing, display in shell)

o operator overloading:

• comparisons

•

addition, subtraction, multiplication, division, etc.

o len for new types of containers


Inheritance

! Create new data type (class) by using an existing

class as the basis for the new class

!

The new class inherits all attributes of the existingclass

! The new class can extend the existing class

o add new data members and methods

o

redefine existing data members and methods


Example

A company has built a set of classes to

display figures, such as circles, triangles

and rectangles.

A programmer is given the task of creating a

class to display labeled rectangles, which

are very similar to ordinary rectangles.

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Alternative #1

! The programmer starts from scratch and

creates an entirely new class to model

labeled rectangles.

! Bad approach: wastes time and effort

because it doesn’t utilize existing rectangle

class.


! Programmer copies source code for

rectangle class, modifies it to handle

labeled rectangles.

! Bad approach: most of the code is the

same, and now there are two independent

copies of the source code (difficult to

maintain over time).

Alternative #2


! Programmer alters source code forexisting rectangle class to handle labeled

rectangles.

! Bad approach: may break existingapplication programs (which depend onexisting class), may introduce errors intorectangle class (used to be error-free).

Alternative #3


Alternative #4

! Use inheritance: labeled rectangle class

inherits all attributes of rectangle class,

extends it to handle new attributes.

! Good approach: one copy of rectangle

class, no changes to rectangle class, allchanges isolated to labeled rectangle

class.

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Inheritance

! A class can have a relationship with other

classes: inherits from

! The “inherits from” relationships forms a

class hierarchy


Example:

Shape

Polygon

Rectangle

Square

CircleTriangle

Object


Classes related by a hierarchy

! Every new class has a parent class o shown in parenthesis in the class header

o the class ‘above’ the new class in the class hierarchy

! Any class can serve as the parent class for otherclasses, called its child classes.

! In Python, the root class of the class hierarchy iscalled object


class Shape (object):

...

class Circle (Shape):

...

class Polygon (Shape):

...

class Rectangle (Polygon):...

class Triangle (Polygon):...

class Square (Rectangle):

... Square

Rectangle Triangle

object

Shape

PolygonCircle

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Sub-classes and super-classes

! Each “descendant” of a class C is calleda sub-class of C:o

C’s children

o the children of C’s children

o

the children of the children of C’s children

o !

! Each “ancestor” of a class C is calleda super-class of C:o

C’s parent

o the parent of C’s parent

o ! Michigan State University CS E 231 18

Square

Polygon

CircleRectangle Triangle

object

Shape

s u p e r - c l a s s s

u b - c l a s s

The sub-class

relation isalso called

the “is a”

relation.


Inherited attributes

! Child class inherits all data attr ibutes (instance

variables) and function attributes (methods) of its

parent class

! Child class can introduce additional data

attributes and function attributes

! Child class can redefine existing data attributes

and function attributes


AttributesShape

Polygon

Rectangle

Square

CircleTriangle

tag __init__()

__str__()

__init__()area()

__init__() __str__()

area()perimeter()

__init__()

radius __init__()

__str__()area()

perimeter()

sides __init__()

__str__()perimeter()

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Namespace: collection of names and values

associated with those names (essentially a

dictionary)

Qualified name: name to the right of a “dot” (.)

signifying an attribute of an object

Scope Rules: determine the values associated

with names that can be legally referenced;

implement inheritance

Namespaces, Qualified names &

Scope Rules


Two naming styles

! Unqualified

o Variables: a = 10 + b

o

Functions: my_func( 50 )

! Qualified, used to access attributes of

o

Modules: string.punctuation

o

Classes: Date.__name[ self.__month ]

o Class instances:

A = Date(1, 1, 2014) A.get_iso()

Local: in current functionEnclosing: in outer functionGlobal: in global namespaceBuilt-in: in standard library


Scope rules for qualified names

Use LCB (implements inheritance)

! Local: look inside the local object

o

the obj referenced by the expression to theleft of the ‘dot’ preceding the qualified name

! Class: look inside the class definition

! Base: look inside the base classes

(start at the parent and work up to rootobject)


Example: date.py

Python tutor visualization

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Shape

Polygon

Rectangle

Square

CircleTriangle

tag __init__()

__str__()

__init__()area()

__init__() __str__()

area()perimeter()

__init__()

radius __init__()

__str__()area()

perimeter()

sides __init__()

__str__()perimeter()

Example: shapes.py


Global NS:

Shape !

Polygon !

Triangle !

Rectangle !

Square !

Circle !

tag ‘shape’

Shape inst.:

class Shape( object ):

def __init__( self, t="shape" ):"""create shape … """self.tag = t

. . .. . .

def main():

s = Shape(). . .

Main NS:

s


Global NS:

Shape !

Polygon !

Triangle !

Rectangle !

Square !

Circle !

tag ‘shape’

Shape inst.:


. . .

def __str__( self ):"""return str … """return self.tag

. . .

def main():

. . . print( "s prints as", s )

Prints:

s prints as shape

Main NS:

s


Global NS:

Shape !

Polygon !

Triangle !

Rectangle !

Square !

Circle !


def __init__( self, t="shape" ):"""create shape … ""”self.tag = t

. . .class Polygon( Shape ):

def __init__(self, ss, t="polygon"):"""create polygon … """Shape.__init__( self, t )self.sides = [float(s) for s in ss]

. . .

def main():. . .

p = Polygon([2,2,2,2,2])

tag ‘polygon’

sides [2.0,2.0,…]

Polygon inst.:

Main NS:

s !

P

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Global NS:

Shape !

Polygon !

Triangle !

Rectangle !

Square !

Circle !

tag ‘polygon’

sides [2.0,2.0,…]


def __str__( self ):return self.tag


def __str__( self ):res = Shape.__str__( self ) + " [" + \", ".join([str(l) for l in \self.sides]) + "]”

return res. . .

def main():. . .

print( "p prints as\n", p )

Prints: p prints as polygon [2.0, 2.0, 2.0,…]

Polygon inst.:

Main NS:

s !

P


Global NS:

Shape !

Polygon !

Triangle !

Rectangle !

Square !

Circle !

tag ‘polygon’

sides [2.0,2.0,…]

Polygon inst.:




def perimeter( self ):return sum( self.sides )

. . .

def main():. . .

print( "perim of p is", p.perimeter())

Prints: p prints as polygon [2.0, 2.0, …]

perim of p is 10.0

Main NS:

s !

P


Global NS:

Shape !

Polygon !

Triangle !

Rectangle !

Square !

Circle !

tag ‘triangle’

sides [2.0,2.0,3.0]


def __init__( self, t="shape" ):self.tag = t

. . .

class Polygon( Shape ):

def __init__(self, ss, t="polygon"):Shape.__init__( self, t )self.sides = [float(s) for s in ss]

. . .

class Triangle( Polygon ):

def __init__(self,s1=1,s2=1,s3=1, \t="triangle" ):

Polygon.__init__(self,[s1,s2,s3],t). . .

def main():. . .t = Triangle(2,2,3)

Main NS:

s !

P !

t

Triangle inst.:


Global NS:

Shape !

Polygon !

Triangle !

Rectangle !

Square !

Circle !



. . .

class Polygon( Shape ):

def __str__( self ):res = Shape.__str__( self ) + " [" + \", ".join([str(l) for l in \self.sides]) + "]”

return res. . .


. . .

def main():. . .

print( "t prints as\n", t )

Prints:

t prints asTriangle [2.0, 2.0, 3.0]

tag ‘triangle’

sides [2.0,2.0,3.0]

Main NS:

s !

P !

t

Triangle inst.:

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Main NS:

s !

P !

t

Global NS:

Shape !

Polygon !

Triangle !

Rectangle !

Square !

Circle !




def perimeter( self ):return sum( self.sides )

. . .


. . .

def main():. . .

print( "perim of t is", t.perimeter() )

Prints:


tag ‘triangle’

sides [2.0,2.0,3.0]

perim of t is 7.0

Triangle inst.:


Global NS:

Shape !

Polygon !

Triangle !

Rectangle !

Square !

Circle !


. . .

class Polygon( Shape ):. . .

class Triangle( Polygon ) :. . .

def area( self ):semip = sum( self.sides ) / 2.0

prod = semip*(semip-self.sides[0])* \(semip-self.sides[1])* \(semip-self.sides[2])

return math.sqrt( prod ). . .

def main():. . .

print( "area of t is", t.area() )

Prints:


tag ‘triangle’

sides [2.0,2.0,3.0]

Triangle inst.:

perim of t is 7.0

area of t is 1.98

Main NS:

s !

P !

t


Global NS:

Shape

Polygon

Triangle

Rectangle

Square

Circle

tag ‘shape’

shape inst.:

tag ‘polygon’

sides [2.0,2.0, 2.0, 2.0, 2.0]

polygon inst.:

tag ‘triangle’

sides [2.0,2.0,3.0]

triangle inst.:

tag ‘rectangle’

sides [2.0,3.0]

rectange inst.:

Main NS:

s

P

t

r


Global NS:

Shape

Polygon

Triangle

Rectangle

Square

Circle

tag ‘shape’

shape inst.:

tag ‘polygon’

sides [2.0,2.0, 2.0, 2.0, 2.0]

polygon inst.:

tag ‘triangle’

sides [2.0,2.0,3.0]

triangle inst.:

tag ‘rectangle’

sides [2.0,3.0]

rectange inst.:

tag ‘square’

sides [5.0,5.0]

square inst.:

Main NS:

s

P

t

r

sq

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Global NS:

Shape

Polygon

Triangle

Rectangle

Square

Circle

tag ‘shape’

shape inst.:

tag ‘polygon’

sides [2.0,2.0, 2.0, 2.0, 2.0]

polygon inst.:

tag ‘triangle’

sides [2.0,2.0,3.0]

triangle inst.:

tag ‘rectangle’

sides [2.0,3.0]

rectange inst.:

tag ‘square’

sides [5.0,5.0]

square inst.:

tag ‘circle’

radius 1

circle inst.:

Main NS:

s

P

t

r

sq

c


Namespaces are maps

! Every object has a namespace (dictionary)

! The object’s namespace is bound to a

special variable __dict__

o Collection of all local things (variables,

functions, etc.) in the object

o Accessed by name (the key)


Dictionary for class Date

__module__ __main__

__weakref__ <attribute '__weakref__' of 'Date' objects>

__dict__ <attribute '__dict__' of 'Date' objects>

__doc__ None

__init__ <function Date.__init__ at 0x02305108>

__repr__ <function Date.__repr__ at 0x0228A780>

__str__ <function Date.__str__ at 0x0228A738>

set_from_iso <function Date.set_from_iso at 0x0226FA98>

get_iso <function Date.get_iso at 0x0228AA08>

get_mdy <function Date.get_mdy at 0x02308AE0>

validate <function Date.validate at 0x0228A6F0>

mth_name ['Invalid', 'January', 'February',!, 'December']


Dictionary for instance A

month 1

day 1

year 2014

valid True

Dictionary for instance B

month 12

day 31

year 2014

valid True

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