[Tutor] super constructor usage

[Mats Wichmann]

On 03/30/2017 05:39 AM, Rafael Knuth wrote:
>>>> I am trying to wrap my head around the super constructor.
> 
> Is it possible to embed a super constructor into an if / elif
> statement within the child class?
> 
> if message == "string A": return X
> elif: return Y
> 
> How should I modify my code below?
> (I couldn't solve that by myself)
> 
> class A:
>     def __init__(self, message):
>         self.message = message
>         print(message)
> 
> class B(A):
>     def __init__(self, message):
>         print("This is the message from your parent class A:")
>         super(B, self).__init__(message)
> 
> B("BlaBla")

For grins, try this (decorated with prints).  There's an additional
argument allowed to the class B initializer, but set as a default
argument so that if you omit it it defaults to True.

===
class A(object):
    def __init__(self, msg):
        print('A: Initializing instance of', self.__class__.__name__)
        self.message = msg

class B(A):
    def __init__(self, msg, doinit=True):
        print('B: Initializing instance of', self.__class__.__name__)
        if doinit:
            super().__init__(msg)

print("Instantiating an A:")
a = A("some message")
print(a.message)

print("Instantiating a B:")
b = B("some message")
print(b.message)

print("Instantiating a B without calling superclass __init__:")
c = B("some message", False)
print(c.message)
===
When you run it:

Instantiating an A:
A: Initializing instance of A
some message
Instantiating a B:
B: Initializing instance of B
A: Initializing instance of B
some message
Instantiating a B without calling superclass __init__:
B: Initializing instance of B
Traceback (most recent call last):
  File "constr-tutor.py", line 22, in <module>
    print(c.message)
AttributeError: 'B' object has no attribute 'message'
===

So note that: the instance attribute "message" is set in the class A
initializer, as in all your postings in this thread.  Just like Alan
pointed out, there are implications if you don't call up to the
superclass, and it shows up pretty clearly here: in the third case,
where we decide not to call the parent's  __init__, this initialization
doesn't happen and the attribute is missing, so accessing it blows up
with an AttributeError exception.