#set document(title: "13.5 Multiple inheritance and mixin classes", author: "OpenStax / XYZ Homework") #set page(width: 8.5in, height: auto, margin: 1in) #import "@preview/cetz:0.5.2" #set text(font: ("STIX Two Text", "Libertinus Serif", "New Computer Modern"), size: 10.5pt, lang: "en") #show math.equation: set text(font: ("STIX Two Math", "New Computer Modern Math")) #set par(justify: true, leading: 0.62em, spacing: 0.9em) #set enum(spacing: 1.1em) // room between list items so tall inline fractions don't collide #set list(spacing: 1.1em) #set table(stroke: 0.5pt + rgb("#c7ccd3")) #let BLUE = rgb("#183B6F") // brand navy — section bars + example/solution labels (white on navy 11.09:1) #let ORANGE = rgb("#A94509") // brand primary-700 — AA-safe deep orange for TEXT (5.93:1 on white; raw brand #F37021 is 2.94:1 and must never carry text) #let RED = rgb("#DC2626") // brand error-600 #let GREEN = rgb("#059669") // brand success-600 (decoration only; small green text uses green-text #007942) #show heading.where(level: 1): it => block(width: 100%, above: 0pt, below: 16pt, fill: gradient.linear(BLUE, rgb("#2C5AA0")), inset: (x: 14pt, y: 12pt), radius: 3pt, text(fill: white, weight: "bold", size: 19pt, it.body)) #show heading.where(level: 2): it => block(width: 100%, above: 18pt, below: 10pt, fill: BLUE, inset: (x: 10pt, y: 6pt), radius: 2pt, text(fill: white, weight: "bold", size: 12pt, it.body)) #show heading.where(level: 3): it => text(fill: ORANGE, weight: "bold", size: 12.5pt, it.body) #show heading.where(level: 4): it => text(fill: BLUE, weight: "bold", size: 10.5pt, it.body) #let examplebox(label, title, body) = block(width: 100%, breakable: true, fill: rgb("#EFF1F5"), stroke: 0.5pt + rgb("#CFDDF0"), radius: 4pt, inset: 10pt, above: 12pt, below: 12pt)[ #block(below: 6pt)[#box(fill: BLUE, inset: (x: 6pt, y: 2pt), radius: 2pt, text(fill: white, weight: "bold", size: 8.5pt, label)) #h(0.4em) #strong[#title]] #body] // rail = decorative left rule (raw brand token); labelcolor = AA-safe label text shade #let notebox(label, rail, labelcolor, tint, body) = block(width: 100%, breakable: true, fill: tint, stroke: (left: 3pt + rail), inset: (left: 10pt, rest: 8pt), radius: (right: 4pt), above: 11pt, below: 11pt)[ #text(fill: labelcolor, weight: "bold", size: 7.5pt, tracking: 0.5pt)[#upper(label)] #linebreak() #body] #let solutionbox(body) = block(above: 4pt, below: 8pt)[ #text(fill: BLUE, weight: "bold", size: 8.5pt)[Solution] #linebreak() #body] #let figph(msg) = block(width: 100%, height: 60pt, fill: rgb("#f6f7f9"), stroke: (paint: rgb("#c7ccd3"), dash: "dashed"), radius: 4pt, inset: 10pt)[ #align(center + horizon, text(fill: rgb("#889"), style: "italic", size: 9pt, msg))] // Standardize inlined figure sizes: measure the natural CeTZ canvas, then scale to a // consistent envelope (aspect-aware; see build_typst.py FIG_* constants). Unlike the // print preamble, dimensions are FLOORED: in an editor a user can trim a figure to a // degenerate 1-D shape (a bare line), and w/h or tw/w would then divide by zero. #let _STD_W = 3.5 #let _WIDE_W = 5.6 #let _MAX_H = 3.4 #let _ASPECT_WIDE = 2.2 #let _UPSCALE_MAX = 1.15 #let stdfig(body) = context { let m = measure(body) let w = calc.max(m.width / 1in, 0.01) let h = calc.max(m.height / 1in, 0.01) let tw = if w / h > _ASPECT_WIDE { _WIDE_W } else { _STD_W } let s = calc.min(tw / w, _MAX_H / h, _UPSCALE_MAX) align(center, box(scale(x: s * 100%, y: s * 100%, reflow: true, body))) } #show figure: set block(breakable: false) #set figure(gap: 8pt) #show figure.caption: set text(size: 8.5pt, fill: rgb("#555")) == 13.5#h(0.6em)Multiple inheritance and mixin classes === Learning objectives By the end of this section you should be able to - Construct a class that inherits from multiple superclasses. - Identify the diamond problem within multiple inheritance. - Construct a mixin class to add functionality to a subclass. === Multiple inheritance basics #strong[Multiple inheritance] is a type of inheritance in which one class inherits from multiple classes. A class inherited from multiple classes has all superclasses listed in the class definition inheritance list. Ex: class SubClass(SuperClass\_1, SuperClass\_2). #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #emph[Multiple inheritance organization] #link("https://www.openstax.org/r/multiple-inheritance-organization")[Multiple inheritance organization; ch 13, video 8] ] #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #emph[Implementing multiple inheritance] Consider the program: class A:   def \_\_init\_\_(self, a\_attr=2):     self.a\_attr = a\_attr class B:   def \_\_init\_\_(self, b\_attr=4):     self.b\_attr = b\_attr class C(A, B):   def \_\_init\_\_(self, a\_attr=5, b\_attr=10, c\_attr=20):     A.\_\_init\_\_(self, a\_attr)     B.\_\_init\_\_(self, b\_attr)     self.c\_attr = c\_attr b\_inst = B(2) c\_inst = C(1, 2) ] === The diamond problem and mixin classes Multiple inheritance should be implemented with care. The #strong[diamond problem] occurs when a class inherits from multiple classes that share a common superclass. Ex: Dessert and BakedGood both inherit from Food, and ApplePie inherits from Dessert and BakedGood. #figure(figph[A conceptual diagram illustrating a classification hierarchy. 'ApplePie' is shown as a subtype of both 'Dessert' and 'BakedGood', which in turn are both subtypes of 'Food'.], alt: "A conceptual diagram illustrating a classification hierarchy. 'ApplePie' is shown as a subtype of both 'Dessert' and 'BakedGood', which in turn are both subtypes of 'Food'.", caption: [#strong[The diamond problem.]If both Dessert and BakedGood override a Food method, the overridden Food method that ApplePie inherits is ambiguous. Thus, diamond shaped inheritance should be avoided.]) Mixin classes promote modularity and can remove the diamond problem. A mixin class: - Has no superclass - Has attributes and methods to be added to a subclass - Isn't instantiated (Ex: Given MyMixin class, my\_inst = MyMixin() should never be used.) #strong[Mixin classes] are used in multiple inheritance to add functionality to a subclass without adding inheritance concerns. #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #emph[Creating a mixin class] The following code isn't correct, as not all plants are carnivorous. Follow the programmer through the programmer's edits to improve the program. Note: Rose, Pitcher, and VenusFlyTrap represent plants that all photosynthesize. Pitcher and VenusFlyTrap represent plants that are also carnivorous and can eat. A pass statement is used in Python to indicate that code is to be written later and prevents certain errors that would result if no code was written or a comment was used instead. class Plant:   def photosynth(self):     print("Photosynthesizing")   def eat(self):     print("Eating") class Rose(Plant):   pass class Pitcher(Plant):   pass class VenusFlyTrap(Plant):   pass ] #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #emph[Fixing the diamond problem with a mixin class] The program below represents multiple inheritance and has the diamond problem. Edit the program to use a mixin class, OnlineMixin. OnlineMixin: - Replaces OnlineShop. - Is a superclass of FoodFast. - Has one method, process\_online(), which prints "Connecting to server" and is called by FoodFast's process\_order(). The output should match: Processing meal order Connecting to server Enjoy your FoodFast order class Business: def process\_order(self): print("Processing order") class Restaurant(Business): def process\_order(self): print("Processing meal order") class OnlineShop(Business): \# Change to mixin class def process\_order(self): print("Processing online order") class FoodFast(Restaurant, OnlineShop): def process\_order(self): super().process\_order() \# Call mixin method print("Enjoy your FoodFast order") franchise\_1 = FoodFast() franchise\_1.process\_order() ]