Transcript The Scala Experience Safe Programming Can be Fun!

Programming for
Engineers in
Python
Lecture 6: More Object Oriented
Programming
Autumn 2011-12
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Lecture 5 Highlights
• Functions review
• Object Oriented Programming
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is and ==
is will return True if two variables point to the same
object, == if the objects referred to by the variables are equal
>>> a
>>> b
>>> b
True
>>> b
True
>>> b
>>> b
False
>>> b
True
= [1, 2, 3]
= a
is a
== a
= a[:]
is a
== a
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Object-Oriented Programming
(OOP)
• Represent problem-domain entities using a
computer language
• When building a software in a specific domain,
describe the different components of the domain
as types and variables
• Thus we can take another step up in abstraction
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Class as a Blueprint
A class is a blueprint of objects
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Car Example
• Members: 4 wheels, steering wheel, horn, color,…
• Every car instance has its own
• Methods: drive, turn left, honk, repaint,…
• Constructors: by color (only), by 4 wheels,
engine,…
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Shapes – 2D Point, Circle
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__init__
self
Attributes
Instances and memory
Copy (shallow / deep)
Methods
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Code – Define Classes
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Code – Using Classes
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Today
• Continue with 2D Shapes
• Rational numbers implementation
• It should feel like native language support
• Inspired by chapter 6 from the book Programming in Scala
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A Rectangle (design options)
• It is not always obvious what the attributes of an object
should be
• How would you represent a rectangle?
• (for simplicity ignore angle, assume the rectangle is
vertical or horizontal)
• There are several possibilities:
• One corner / center point + width and height
• Two opposing corners
• We shall select the width, height, lower-left corner
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A Rectangle - Implementation
In class Rectangle:
Shell:
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Rectangle in Memory
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Find Center
In class Rectangle:
Shell:
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Grow Rectangle
In class Rectangle:
Shell:
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Has Attributes?
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Print All Attributes
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Inheritance (briefly)
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The general idea
class Point(object) – what does object stands for?
Example: Animals
Polymorphism
• We have seen that already!
• Histogram example
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Histogram (polymorphism)
Source: Think Python
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Rational Numbers
• A rational number is a number that can be
expressed as a ratio n/d (n, d integers, d not 0)
• Examples: 1/2, 2/3, 112/239, 2/1
• Not an approximation!
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Specification
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print should work smoothly
Add, subtract, multiply, divide
Immutable
It should feel like native language support
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Constructing a Rational
• What are the attributes?
• How a client programmer will create a new Rational
object?
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Constructing a Rational
Shell:
?
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Reimplementing __str__
• __str__ method return a string representation of an object
• A more useful implementation of __str__ would print out
the values of the Rational’s numerator and denominator
• override the default implementation
In class Rational:
Shell:
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__repr__
• __repr__ method returns the “official” string
representation of an object
In class Rational:
Shell:
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Checking Preconditions
• Ensure the data is valid when the object is
constructed
In class Rational:
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Checking Preconditions
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Defining Operators
• Why not use natural arithmetic operators?
• Operator precedence will be kept
• All operations are method calls
From the book Programming in Scala
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Operator Overloading
• By defining other special methods, you can
specify the behavior of operators on user
defined types
• +, -, *, /, <, >,…
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Adding Rational Numbers
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Define __add__ Method
• Immutable
In class Rational:
Shell:
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Other Arithmetic Operations
In class Rational:
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Other Arithmetic Operations
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<, >, max
INCORRECT!
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<, >, max
In class Rational:
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<, >, max
How come max works?
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Default Arguments to Constructor
• Constructors other then the primary?
• Example: a rational number with a denominator
of 1 (e.g., 5/1  5)
• We would like to do: Rational(5)
• Default arguments
• Useful not solely for constructors
• Remember sorted (reverse, key are default arguments)?
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Revised Rational
In class Rational:
Shell:
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Greatest Common Divisor (gcd)
• 66/42 = 11/7
• To normalize divide the numerator and
denominator by their greatest common divisor (gcd)
• gcd(66,42) = 6  (66/6)/(42/6) = 11/7
• No need for Rational clients to be aware of this
• Encapsulation
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Off Topic: Calculate gcd
(Only if time allows)
• gcd(a,b) = g
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a = n * g
b = m * g
gcd(n,m)=1(otherwise g is not the gcd)
a = t * b + r = t * m * g + r  g is a
divisor of r
• gcd(a,b) = gcd(b,a%b)
• The Euclidean algorithm: repeat iteratively:
if (b == 0) return a
else repeat using a  b, b  a%b
• http://en.wikipedia.org/wiki/Euclidean_algorithm
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Correctness
• Example:
gcd(40,24)  gcd(24,16)  gcd(16,8) 
gcd(8,0)  8
• Prove: g
= gcd(a,b) = gcd(b,a%b)= g1
• g1 is a divisor of a ( g1 ≤ g)
• There is no larger divisor of a (
g1 ≥ g)
• ≤ : a = t * b + r  a = t * h * g1 + v * g1
 g1 is a divisor of a
• ≥ : assume g > g1  a = t * b + r  g is a
divisor of b and r  contradiction
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gcd Implementation
• Let’s leave it for next lesson (Recursion)
• Actually, we can use the implementation in the
module fractions.gcd
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Revised Rational
In class Rational:
Shell:
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Mixed Arithmetic's
• Now we can add and multiply rational numbers!
• What about mixed arithmetic?
• r + 2 won’t work 
• r + Rational(2) is not nice 
• Add new methods for mixed addition and
multiplication
• Will work thanks to polymorphism
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Usage
• The + method invoked is determined in each case by the
type of the right operand
• In our code it is implemented only for the operator + on
integers (in “real” life it should have been implemented
for every operator and every data type that is supported)
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Revised __add__
• Isinstance takes a value and a class object, and returns
True if the vlaue is an instance of the class
• Handles addition of integers correctly
• Type based dispatch – dispatches the computation to
different executions based on the types of the arguments
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Implicit Conversions
• 2 + r  2.+(r)  method call on 2 (int)  int class
contains no __add__ method that takes a Rational
argument 
• The problem: Python is asking an integer to add a
Rational object, and it doesn’t know how to do that
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__radd__ - Right Side Add
• __radd__ invoked when a Rational object
appears on the right side of the + operator
In class Rational:
Shell:
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Summary
• Customize classes so that they are natural
to use
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Attributes, methods, constructor
Method overriding
Encapsulation
Define operators as method
Method overloading
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Rational Numbers in Python
• Actually, there is a Python implementation
of Rational numbers
• It is called fractions
http://docs.python.org/library/fractions.html
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Next Week
• No class (tirgulim as usual)
• Next topic: Recursion
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