Fundamentals of Programming (Python) Object-Oriented Programming. Ali Taheri Sharif University of Technology Spring 2018

Transcription

1 Fundamentals of Programming (Python) Object-Oriented Programming Ali Taheri Sharif University of Technology

2 Outline 1. Python Data Types 2. Classes and Objects 3. Defining Classes 4. Working with Objects 5. Customizing Initializer 6. Adding Methods 7. Special Methods 8. Composition 9. Mutability 10.Sameness 11.Copying 2

3 Python Data Types So far we have seen several data types in Python Some of these data types are primitive data types Integer, Float, String, Boolean Some of them are more complicated List, Dictionary, Set, File In Python, we can build our own data types Using the other available types This is the subject of Object-Oriented programming 3

4 Classes and Objects An object is simply a variable A Class is simply a data type of an object We have been using objects! Just didn t call them objects For example: str is a data type (or class) We created objects of type (class) string animal = "cow" fruit = "apple" 4

5 Classes and Objects Object vs Variable Objects may contain variables, called attributes Objects may contain methods Class vs Type Programmer can define his own classes Attributes and methods are defined within class 5

6 Defining Classes Class header Keyword class begins definition Followed by name of class and colon ( : ) Initializer method init Executes each time an object is created Initializes attributes of class Object reference All methods must at least specify this one parameter Represents object of class from which a method is called Called self by convention 6

7 Defining Classes 7

8 Working with Objects Instantiating objects: p = Point() q = Point() Accessing objects attributes: p.x = 3 q.x = p.y + 5 q.y = p.x * q.x print(p.x, p.y, q.x, q.y) 8

9 Customizing Initializer 9

10 Adding Methods 10

11 Adding Methods class Point: def init (self, x=0, y=0): self.x = x self.y = y def distance_from_origin(self): return ((self.x**2) + (self.y**2)) ** 0.5 def distance(self, other): dx = self.x - other.x dy = self.y - other.y return (dx**2 + dx**2) ** 0.5 p = Point(2, 6) q = Point(-1, 4) d = p.distance(q) print(d) 11

12 Special Methods Operator Method Description + add (self, other) Addition - sub (self, other) Subtraction * mul (self, other) Multiplication / truediv (self, other) Division < lt (self, other) Less than <= le (self, other) Less that or equal > gt (self, other) Greater than >= ge (self, other) Greater than or equal == eq (self, other) Equal to!= ne (self, other) Not equal to 12

13 Special Methods Operator Method Description [index] getitem (self, index) Index operator in contains (self, value) Check membership len len (self) The number of elements str str (self) The string representation 13

14 Special Methods class Point: def init (self, x=0, y=0): self.x = x self.y = y def str (self): return '(%d,%d)' % (self.x, self.y) def getitem (self, index): if index == 0: return self.x elif index == 1: return self.y else: raise IndexError('Point index out of range.') >>> p = Point(2,3) >>> print(p) (2,3) >>> print(p[1]) 3 14

15 Composition 15

16 Mutability 16

17 Sameness When dealing with objects, the concept of sameness can be ambiguous Do the two objects contain the same data? Do they represent the same object? Shallow Equality Only compares the references, not the contents of the objects 17

18 Sameness The equality operator (==) performs shallow equality check by default on user-defined classes Unless the eq () method is implemented >>> p = Point(2,3) >>> q = Point(2,3) >>> p == q False Deep Equality Compares the contents of the objects Must be implemented manually through eq () or another custom method 18

19 Sameness Deep Equality class Point: def init (self, x=0, y=0): self.x = x self.y = y def eq (self, other): return (self.x == other.x and self.y == other.y) >>> p = Point(2,3) >>> q = Point(2,3) >>> p == q True 19

20 Copying The assignment operator make aliases Changing one results in changing the other >>> p = Point(2,3) >>> q = p >>> p is q True What if we want separate duplicates? We can copy objects using the copy module Shallow copy Deep copy 20

21 Copying The assignment operator make aliases Changing one results in changing the other >>> p = Point(2,3) >>> q = p >>> q.x = 5 >>> print(p) (5,3) What if we want separate duplicates? We can copy objects using the copy module Shallow copy Deep copy 21

22 Copying Shallow Copy Copies the object, but doesn t copy the embedded objects unless they are immutable 22

23 Copying Deep Copy Copies the object, and recursively copies the embedded objects class Rectangle: """ A class to manufacture rectangle objects """ def init (self, corner, w, h): self.corner = corner self.width = w self.height = h def eq (self, other): return (self.corner == other.corner and self.width == other.width and self.height == other.height) 23

24 Copying Deep Copy Copies the object, and recursively copies the embedded objects >>> import copy >>> r = Rectangle(Point(1,4), 7, 2) >>> s = copy.copy(r) >>> r == s True >>> s.corner.x = 5 >>> r == s True >>> t = copy.deepcopy(r) >>> r == t True >>> t.corner.x = 6 >>> r == t False 24