Inheritance in Python - TypeError

Question

I'm trying to learn python syntax and I don't understand why the example below doesn't work. I get this error:

TypeError: __init__() takes 1 positional argument but 2 were given

Code:

class Parent:
    def __init__(self):
        self.lastName = "Mustermann" 
        self.firstName = "Max"
    def get_name(self):
        return self.firstName+" "+self.lastName 
class Child(Parent):
    def __init__(self):
        self.firstName = "Moritz"
        self.lastName=Parent.lastName
p=Parent()
c = Child(p)
print(c.get_name())

I also don't understand why Parent.lastName should work (according to what I read). Parent is a class, so why would it access the lastName of the instance? What I really want is that Child inherits the lastName of Parent, but not the firstName.

c = Child(p) => c = Child(). You don't need an instance to inherit from a class — Jean-François Fabre
– Jean-François Fabre ♦, Commented Feb 14, 2019 at 22:01
Alternatively, a Child instance has a Parent, rather than being a Parent, in which case Child should not inherit from Parent, but instead Child.__init__ should take an instance of Parent as an argument. — chepner
– chepner, Commented Feb 14, 2019 at 22:13
Parent.lastName doesn't work - AttributeError: type object 'Parent' has no attribute 'lastName' but it is stopping at your first error. — AChampion
– AChampion, Commented Feb 14, 2019 at 22:23
@chepner That would make more sense, but I was trying to learn inheritance, thats why I constructed it this way. But I could invent better names for that example, because maybe thats what unconsciously tricked my brain into wrinting Child(p). — Azzinoth
– Azzinoth, Commented Feb 14, 2019 at 22:26

Jean-François Fabre · Accepted Answer · 2019-02-14 22:09:28Z

3

c = Child(p) is an error: the constructor doesn't take any argument except the self implicit one.

You don't need to copy lastName as Child inherits from Parent. But since you defined __init__ in Child you have to call Parent __init__ method as well for that (else parent constructor isn't called):

class Parent:
    def __init__(self):
        self.lastName = "Mustermann"
        self.firstName = "Max"
    def get_name(self):
        return self.firstName+" "+self.lastName
class Child(Parent):
    def __init__(self):
        Parent.__init__(self)
        self.firstName = "Moritz"

p=Parent()
c = Child()
print(c.get_name())

prints:

Moritz Mustermann

To call the parent __init__ method we used

Parent.__init__(self)

we could also have used super like this:

super(Child,self).__init__()

which amounts to the same thing here (single inheritance), so even like:

super().__init__()

edited Feb 14, 2019 at 22:09

answered Feb 14, 2019 at 22:04

Jean-François Fabre♦

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4 Comments

Patrick Artner Over a year ago

should'nt that be a super(Parent,self).__init__() call?

Jean-François Fabre Over a year ago

it doesn't work because it's super(Child,self).__init__() :)

AChampion Over a year ago

Just plain super().__init__() would work (in Py3).

Jean-François Fabre Over a year ago

it does. Adopting that syntax from now. Worked too much with old 2.x python versions so super wasn't my friend...

chepner · Accepted Answer · 2019-02-14 22:31:03Z

1

A child isn't (necessarily) a parent, it has a parent. Also, the names shouldn't be hard-coded in the __init__ method, but passed as arguments. Not every instance of Child or Person will have the same names (although in this simple example, a child will share its parent's last name).

(I changed Parent to Person since there's nothing particularly parent-like about the Parent class. A Child still has a Person as a parent, though.)

class Person:
    def __init__(self, firstName, lastName):
        self.lastName = firstName
        self.firstName = lastName

    def get_name(self):
        return self.firstName+" "+self.lastName 


class Child(Person):
    def __init__(self, firstName, parent):
        super().__init__(firstName, parent.lastName)
        self.parent = parent

p = Person("Max", "Mustermann")
c = Child("Moritz", p)
print(c.get_name())

edited Feb 14, 2019 at 22:31

answered Feb 14, 2019 at 22:16

chepner

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4 Comments

AChampion Over a year ago

c.get_name() will no longer work... perhaps Parent and Child should inherit from Person and define get_name() in Person. And it does usually take 2 to tango :)

chepner Over a year ago

Yeah, I never bothered looking at get_name :)

chepner Over a year ago

(I may also have taken the Child/Parent names too literally, but at least now the answer demonstrates both inheritance and composition.)

Azzinoth Over a year ago

@chepner You have taken them too literally, but your answer is still useful. I renamed Parent and Child into Person and Programmer. That makes my example much less confusing, which shows how important good variable names are. ;)

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