Programming for Engineers in Python. Inheritance

Transcription

1 Programming for Engineers in Python Inheritance 1

2 Plan Background and motivation Examples: TwoWheeler isinstance Binary tree Beverages 2

3 Inheritance Inheritance is commonly used to reflect is-a relationship between classes: B is a subtype of A. Class B inherits from class A Class A is said to be a base class Class B is said to be a derived class 3 Is a B A

4 Motivation Extract common attributes and methods to avoid code duplication and to better manage the code Class Car Number of wheels Current speed Color Engine Number of airbags and more Class Bus Number of wheels Current speed Color Engine Ride cost and more 4

5 Method Extract common attributes and methods to avoid code duplication and to better manage the code Implement convenient and suitable hierarchy: Is a Is a 5

6 Method 1. Determine a base/parent class 2. Extend it attributes and methods in child classes. Example: car, bus Common property: vehicle (has color, speed, engine, wheels ) Manage vehicle as a separate class 6 In OOD books: Child extends Parent

7 Method 1. Determine a base/parent class 2. Extend it attributes and methods in child classes. Is a Vehicle Car Bus Is a 7

8 Basic example 8 Class: Vehicle Number of wheels Speed Color Engine and more Example: Checking speed limit is done in one place -Vehicle class code. Class Car - has all Vehicle attributes - and additionally: Number of airbags and more Class Bus - has all Vehicle attributes - and additionally: Number of passengers and more

9 Example 1 - TwoWheeler class TwoWheeler: '''Vehicles that run on two wheels''' def init (self, d1, d2): # The wheel diameters, not necessarily equal self.diam1 = d1 self.diam2 = d2 def repr (self): # Default implementation return '('+str(self.diam1)+')('+str(self.diam2)+')' >>> t = TwoWheeler(30, 10) >>> print t (30)(10) 9

10 TwoWheeler Cont. class Bicycle(TwoWheeler): Is a Bicycle TwoWheeler def init (self, d1=0, d2=0, electric=false): self.diam1 TwoWheeler. init (self, = d1 d1, d2) self.diam2 = d2 self.electric = electric Same as parent >>> b = Bicycle(30, 10, False) >>> print b (30)(10) >>> print b.electrtic False Extend object s attributes 10

11 TwoWheeler Cont. class Bicycle(TwoWheeler): def init (self, d1=0, d2=0, electric=false): TwoWheeler. init (self, d1, d2) self.electric = electric def repr (self): if self.electric: prefix = 'Electric' else: prefix = 'Non-electric Invoke parent s constructor Extend parent s attributes prefix += ' bicycle ' str_parent_data = TwoWheeler. repr (self) return prefix + str_parent_data >>> b = Bicycle(30, 10, False) >>> print b Non-electric bicycle - (30)(10) Invoke parent s method 11

12 Manipulating inheritance class Bicycle(TwoWheeler): def init (self, d1, d2, electric = False): self.diam1 = d1 self.diam2 = d2 self.electric = electric >>> b = Bicycle(30, 10, False) >>> print b Traceback (most recent call last):... AttributeError: 'Bicycle' object has no attribute 'diam2' Question: Do we crash if we invoke parent s init in Bicycle s constructor? No, we don t. We don t influence attribute creation. 12

13 class Bicycle(TwoWheeler): pass def init (self, d1, d2, electric = False): self.diam1 = d1 self.diam2 = d2 self.electric = electric >>> b = Bicycle(30, 10, False) Traceback (most recent call last):... TypeError: init () takes at most 3 arguments (4 given) Why? 13 Python s flow: Does Bicycle class have init? - No. Go to parent class s init with 4 args: self, 30, 10, False

14 One parent class can be inherited several times TwoWheeler Bicycle Motorcycle Segway and so on 14

15 One class can inherit several parents. TwoWheeler Engine Motorcycle Not in syllabus 15

16 TwoWheeler Cont. class Motorcycle(TwoWheeler): def init (self, d1, d2, fuel_type): TwoWheeler. init (self, d1, d2) self.fuel_type = fuel_type self.curr_speed = 0 def repr (self): return 'Motorcycle - ' + TwoWheeler. repr (self) +\ '\nfuel: ' + self.fuel_type +\ \ncurrent speed: ' + str(self.curr_speed) 16 def get_speed(self): >>> m = Motorcycle(30, 10, 'Diesel') return >>> print self.curr_speed m Motorcycle - (30)(10) def set_speed(self, Fuel: Diesel new_speed): self.curr_speed Current speed: 0= new_speed Extend parent s methods

17 The class hierarchy - TwoWheeler Bicycle Motorcycle 17

18 isinstance & issubclass 18 >>> m = Motorcycle(20, 20, 'Diesel') >>> b = Bicycle(30, 10, False) >>> isinstance(m, Motorcycle) True >>> isinstance(m, TwoWheeler) True >>> isinstance(b, TwoWheeler) True >>> isinstance(b, Motorcycle) False >>> issubclass(motorcycle, TwoWheeler) True >>> issubclass(motorcycle, Bicycle) False >>> issubclass(twowheeler, Bicycle) False

19 Iterating through a list of objects b = Bicycle(30, 10, False) m = Motorcycle(20, 20, '95') list_of_objects = [b,m] for curr_object in list_of_objects: if isinstance(curr_object, TwoWheeler): print 'It is a TwoWheeler object' if isinstance(curr_object, Motorcycle): curr_object.set_speed(50) print curr_object We get two positive answers for m object 19 It is a TwoWheeler object Not-electric bicycle - (30, 10) It is a TwoWheeler object Fuel: 95, Current speed: 50, (20, 20)

20 Iterating through a list of objects >>> b = Bicycle(30, 10, False) >>> m = Motorcycle(20, 20, '95') >>> list_of_objects = [b,m] >>> \ for obj in list_of_objects: if issubclass(obj. class, TwoWheeler): print 'This is a TwoWheeler object' if isinstance(obj,bicycle): print 'Yes' if hasattr(obj, 'set_speed'): obj.set_speed(50) print obj 20 This is a TwoWheeler object Yes Non-electric bicycle - (30)(10) This is a TwoWheeler object Motorcycle - (20)(20) Fuel: 95 Current speed: 50

21 Binary tree example a Root b c d e f Every node has 0-2 children Leaf g 21 What is the depth? 4

22 Binary tree; v1 Cont. class BinTreeNode: def init (self, data, left = None, right = None): self.data = data self.left = left self.right = right def set_left(self, left): self.left = left def set_right(self, right): self.right = right 22

23 Binary tree; v1 Cont. >>> d = BinTreeNode('d') >>> e = BinTreeNode('e') >>> f = BinTreeNode('f') >>> g = BinTreeNode('g') >>> b = BinTreeNode('b') >>> c = BinTreeNode('c') >>> b.set_left(d) >>> c.set_left(e) >>> c.set_right(f) >>> f.set_right(g) >>> a = BinTreeNode('a', b, c) a b c d e f g 23

24 Computing depth; v1 class BinTreeNode: def get_depth(self): if not self.left and not self.right: # self is a leaf. Stop recursion. return 1 d_left = 0 d_right = 0 if self.left: d_left = self.left.get_depth() if self.right: d_right = self.right.get_depth() return 1 + max(d_left, d_right) 24

25 Computing depth; v1 Cont. 25 >>> d = BinTreeNode('d') >>> e = BinTreeNode('e') >>> f = BinTreeNode('f') >>> g = BinTreeNode('g') >>> b = BinTreeNode('b') >>> c = BinTreeNode('c') >>> b.set_left(d) >>> c.set_left(e) >>> c.set_right(f) >>> f.set_right(g) >>> a = BinTreeNode('a', b, c) >>> print a.get_depth() 4 a b c d e f g

26 Full binary tree. Revisited BinTreeNode Object with 1-2 children BinTreeLeaf Object without children 26

27 BinTreeLeaf class BinTreeLeaf(BinTreeNode): def init (self, data): BinTreeNode. init (self, data) def get_depth(self): # This is a leaf. No children to check return 1 27

28 Computing depth; v2 class BinTreeNode:... def get_depth(self): d_left = 0 d_right = 0 if self.left: d_left = self.left.get_depth() if self.right: d_right = self.right.get_depth() return 1 + max(d_left, d_right) 28

29 Computing depth; v2 Cont. >>> d = BinTreeLeaf('d') >>> e = BinTreeLeaf('e') >>> f = BinTreeLeaf('f') >>> b = BinTreeNode('b', left = d) >>> c = BinTreeNode('c', e, f) >>> a = BinTreeNode('a', b, c) >>> print a.get_depth() 3 b a c d e f 29

30 Binary tree; v2 Cont. >>> a = BinTreeLeaf('a') >>> b = BinTreeLeaf('b') >>> a.set_left(b) >>> We created a leaf WITH a child.. Bug! We shall guarantee data consistency, i.e., that a leaf doesn t have children and set_left, set_right methods can not be invoked! 30

31 BinTreeLeaf class BinTreeLeaf(BinTreeNode): def init (self, data): # Perform the initialization BinTreeNode. init (self, data) # Delete unnecessary attributes and methods delattr(self, 'left') delattr(self, 'right') self.set_left = None self.set_right = None... 31

32 Complete binary tree, Ver. 3. >>> a = BinTreeLeaf('a') >>> b = BinTreeLeaf('b') >>> print isinstance(a, BinTreeLeaf) True >>> print hasattr(a, 'left') False >>> a.set_left(b) Traceback (most recent call last):... a.set_left(b) TypeError: 'NoneType' object is not callable 32

33 Complete binary tree, Ver. 4. A more neat solution: Create a class with as much in common as possible, and then inherit from it, without deleting methods / attributes BinTreeObject Only data BinTreeNode Add right son Add left son BinTreeLeaf 33

34 Beverages Our goal is to create Beverages objects Requirements: Handle several kinds of beverages - soda drinks, dairy drinks, alcohol drinks. 34

35 Soda Dairy Beer name ml (volume) price calories Methods: repr raise_price add_water name ml (volume) price calories Organic / Not Methods: repr raise_price add_water add_cal name ml (volume) price calories alcohol Methods: repr raise_price add_water 35

36 Soda Dairy Beer name ml (volume) price calories name ml (volume) price calories Organic / Not name ml (volume) price calories alcohol These three classes have lots in common Methods: repr raise_price add_water Methods: repr raise_price add_water add_cal Methods: repr raise_price add_water We can (and should) avoid code duplication 36

37 Soda Dairy Beer name ml (volume) price calories name ml (volume) price calories Organic / Not name ml (volume) price calories alcohol But, there are also some differences Methods: repr raise_price add_water Methods: repr raise_price add_water add_cal Methods: repr raise_price add_water 37 Changes the calories per 100ml (dilute) Changes the alcohol percentage

38 >>> a = Beer('Tuineken',500,7,44,5.5) >>> a Tuineken 500ml >>> a 7$ Milk 44cal per 100ml 250ml 5.5% alcohol 2$ 39cal per 100ml Non-Organic >>> a =Dairy( Milk',250,2,39,False) Soda Dairy Beer repr >>> a =Soda('Cola',330,3.5,41) >>> a Cola 330ml 3.5$ 41cal per 100ml 38 name ml (volume) price calories name ml (volume) price calories Organic / Not name ml (volume) price calories alcohol

39 Let s define a general class, Beverage, with as much in common as possible And now, inherit! Beverage name ml (volume) price calories Methods: repr raise_price add_water Name, ml, price & calories Changes the calories per 100ml (dilute) 39 Soda Methods: Dairy isorganic Methods: update repr Add add_cal Beer alcohol Methods: update repr update add_water

40 class Beverage: def init (self, name, ml, price, cal): self.name = name self.ml = ml self.price = price self.cal = cal Beverage name ml (volume) price calories Methods: repr raise_price add_water def repr (self): res = self.name + '\n' + str(self.ml) + 'ml\n' + str(self.price) + '$\n' + str(self.cal) + 'cal per 100ml' return res def raise_price(self, dprice): self.price += dprice 40 def add_water(self, amount): new_ml = self.ml + amount self.cal *= self.ml/new_ml self.ml += amount #update calories per 100ml

41 Soda Methods: Beverage name ml (volume) price calories Methods: repr raise_price add_water class Soda(Beverage): pass 41

42 class Dairy(Beverage): isorganic Dairy Methods: update repr Add add_cal Beverage name ml (volume) price calories Methods: repr raise_price add_water def init (self, name, ml, price, cal, isorganic): Beverage. init (self, name, ml, price, cal) self.isorganic = 'Organic' if isorganic else 'Non-Organic def repr (self): res = Beverage. repr (self) res += '\n' + self.isorganic return res def add_cal(self,calories): self.cal += calories 42

43 class Beer(Beverage): alcohol Beer Methods: update repr update add_water Beverage name ml (volume) price calories Methods: repr raise_price add_water def init (self, name, ml, price, cal, alc): Beverage. init (self, name, ml, price, cal) self.alc = alc def repr (self): res = Beverage. repr (self) res += '\n' + str(self.alc) + '% alcohol' return res 43 def add_water(self,amount): new_ml = self.ml + amount self.cal *= self.ml/new_ml self.alc *= self.ml/new_ml self.ml += amount #update calories per 100ml #update alcohol level