How do I inherit a class in python?

Question

I am trying to associate a competitor with the race that they have competed in, how would I do the show() function through inheriting the event class? I'm still wrapping my head around inheritance so I apologise if this question is obvious..

import sqlite3

print "hello"

class competitor(object):
    def __init__(self, name, dob, number, events):
        self.name = name
        self.dob = dob
        self.number = number ##this needs to check db for existing
        self.events = events

    def listEvents(self):
        for all in self.events:
            self.show()

class event(competitor):
    def __init__(self, name, distance, date, time):
        self.name = name
        self.distance = distance #meters
        self.date = date # [dd,mm,yyyy]
        self.time = time # [d,h,m,s]

    def printDate(self):
        date = str(self.date[0]) + "/" + str(self.date[1]) + "/" + str(self.date[2])
        ##print date
        return date

    def printTime(self):
        if (self.time[0] > 0):
            time = str(self.time[0]) + "." + str(self.time[1]) + ":" + str(self.time[2]) + "." + str(self.time[3])
            return time
        else:
            time = str(self.time[1]) + ":" + str(self.time[2]) + "." + str(self.time[3])
            return time

    def getKmPace(self):
        time_s = self.time[0]*3600*24 + self.time[1]*3600 + self.time[2]*60 + self.time[3]
        time_m = time_s/60.0
        pace = time_m/(self.distance/1000.0)
        return pace

    def show(self):
        print "Event: ", self.name, " Date: ", self.printDate()
        print "Distance: ",self.distance/1000.0,"KM, Time: ", self.printTime()
        print "Pace per 1 KM: ", self.getKmPace(), " minutes."


kdl = event("20KM",20000,[26,4,2014],[0,1,27,36])
kdl_bad = event("20KM",20000,[26,4,2013],[0,2,35,37])
kdl.show()

richard = competitor("Richard", 1993, 1, [kdl,kdl_bad])
richard.listEvents()

Hmm, I'm not exactly sure what you're asking. I can see that you have an event class, which inherits from the competitor class. (This seems like on odd inheritance relationship to me, since saying "an event is a competitor" doesn't really make logical sense). You also have a function called show, but it's not a part of either of those classes, at least according to the indentation in your pasted code. Is that just a copy/paste error? Should show and getkmPace actually be part of the event class? — dano
– dano, Commented May 1, 2014 at 17:32
Well, think about your class design more closely. Is an "event" a more specific "competitor"? Or does an "event" have "competitors? Inheritance is usually used when you're describing an "is-a" relationship, not a "has-a". — Santa
– Santa, Commented May 1, 2014 at 17:32
getKmPace(self) and show()functions are outside of the event class? — Sergio Martinez
– Sergio Martinez, Commented May 1, 2014 at 17:33
@Santa how would I describe has-a relationship in this case? — user1902692
– user1902692, Commented May 1, 2014 at 17:40

Santa · Accepted Answer · 2014-05-01 17:40:20Z

3

Well, think about your class design more closely. Is an "event" a more specific "competitor"? Or does an "event" have "competitors? Inheritance is usually used when you're describing an "is-a" relationship, not a "has-a".

In your case, a competitor has a reference to multiple event objects. Your class design for both are already on the right track. Your use of inheritance, however, is not.

A simple fix:

class competitor(object):
    def __init__(self, name, dob, number, events):
        self.name = name
        self.dob = dob
        self.number = number ##this needs to check db for existing
        self.events = events

    def listEvents(self):
        for event in self.events:
            event.show()

class event(object):
    def __init__(self, name, distance, date, time):
        self.name = name
        self.distance = distance #meters
        self.date = date # [dd,mm,yyyy]
        self.time = time # [d,h,m,s]

    def printDate(self):
        date = str(self.date[0]) + "/" + str(self.date[1]) + "/" + str(self.date[2])
        return date

    def printTime(self):
        if (self.time[0] > 0):
            time = str(self.time[0]) + "." + str(self.time[1]) + ":" + str(self.time[2]) + "." + str(self.time[3])
        else:
            time = str(self.time[1]) + ":" + str(self.time[2]) + "." + str(self.time[3])
        return time

    def getKmPace(self):
        time_s = self.time[0]*3600*24 + self.time[1]*3600 + self.time[2]*60 + self.time[3]
        time_m = time_s/60.0
        pace = time_m/(self.distance/1000.0)
        return pace

    def show(self):
        print "Event: ", self.name, " Date: ", self.printDate()
        print "Distance: ",self.distance/1000.0,"KM, Time: ", self.printTime()
        print "Pace per 1 KM: ", self.getKmPace(), " minutes."

kdl = event("20KM",20000,[26,4,2014],[0,1,27,36])
kdl_bad = event("20KM",20000,[26,4,2013],[0,2,35,37])

print 'First event:'
kdl.show()

print 'Richard has two events:'
richard = competitor("Richard", 1993, 1, [kdl,kdl_bad])
richard.listEvents()

answered May 1, 2014 at 17:40

Santa

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2 Comments

user1902692 Over a year ago

thank you very much, so what would you call this if it's not inheritance?

Santa Over a year ago

@user1902692 It's called composition. When you describe Shape -> Rectangle -> Square, that is inheritance, as a Square is a Rectangle is a Shape. And you want a Square to inherit the fields and methods of a Rectangle (e.g. the area calculation algorithm). But when you describe a Car -> Doors -> Handles, that is composition, as a Door is not a Car, but a Car has Doors. You don't want a Door to inherit Car.RevvEngine(), for example. So you don't lump them in one class hierarchy and compose them together.

Adam Smith · Accepted Answer · 2014-05-01 18:07:00Z

1

I don't think this is a case for inheritance. To apply inheritance in this example would be something like:

class event(object): # not inheriting from competitor
    # your code for the event
    #     ...

class 20KM_event(event):
    def __init__(self, date, time):
        super(20KM_event,self).__init__("20KM",20000, date, time)
    # if any specific methods are required for JUST the
    # 20KM event, put them here. Otherwise you're done

kdl = 20KM_event([26,4,2014],[0,1,27,36])

for your competitor, often this is something that should be handled by the event in question. They are members of the event, after all, so maybe something like:

class event(object):
    def __init__(self,name,distance,date,time):
        self.name = name
        self.distance = distance
        self.date = date
        self.time = time
        self.competitors = []
    def addCompetitor(self,*args):
        """Add a competitor to this event
USAGE: self.addCompetitor(competitor) OR
       self.addCompetitor(name, dob, number, events)"""
        if len(args) == 1 and isinstance(args[0],competitor):
            target = args[0]
        else:
            target = competitor(*args)
        target.events.append(self)
        self.competitors.append(target)

edited May 1, 2014 at 18:07

answered May 1, 2014 at 17:42

Adam Smith

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4 Comments

Padraic Cunningham Over a year ago

@Adam Smith, in your first example, what does using super(self,event) do as opposed to super(20KM_event,self).__init__("20KM",20000, date, time)?

Adam Smith Over a year ago

@PadraicCunningham did I use the wrong notation? Crud I'm rusty on my supers. One sec...

Padraic Cunningham Over a year ago

@AdamSmith, I rarely use super and any time I have it has been like super(20KM_event,self).__init__("20KM",20000, date, time), I was just wondering if there were some significance. I could be wrong.

Adam Smith Over a year ago

@PadraicCunningham no I just did it from memory and did it wrong. The correct notation is super(type, obj) not super(obj, parent) lol. I rarely need to worry about the parameters because I code in Python3 and have yet to need multiple inheritance. super().__init__ works fine for every use case I've had personally, but no reason not to be explicit when I'm not sure of OP's use case.

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