boB Stepp wrote:
> On Wed, Feb 22, 2017 at 10:25 PM, boB Stepp <robertvst...@gmail.com>
> wrote:
>> I am trying to wrap my head around the mechanics of inheritance in
>> Python 3. I thought that all attributes of a superclass were
>> accessible to an instance of a subclass. But when I try the
>> following:
>>
>> py3: class A:
>> ... def __init__(self):
>> ... self.aa = 'class A'
>> ...
>> py3: class B(A):
>> ... def __init__(self):
>> ... self.bb = 'class B'
>> ...
>> py3: a = A()
>> py3: b = B()
>> py3: b.aa
>> Traceback (most recent call last):
>> File "<stdin>", line 1, in <module>
>> AttributeError: 'B' object has no attribute 'aa'
>>
>> I am unsuccessful...
>
> I have partially answered my own question, but I am not sure I
> understand the mechanics yet. Apparently I must explicitly assign
> class A's attribute, aa, to an instance of class B as follows:
>
> py3: class B(A):
> ... def __init__(self):
> ... self.aa = super().aa
> ... self.bb = 'class B'
> ...
> py3: b = B()
> py3: b.aa
> 'class A'
>
> I was expecting that all of A's methods and attributes would
> seamlessly be available to any instance of class B, but apparently
> this is wrong-headed on my part. Instead, it looks like I need to
> explicitly attach A's methods and attributes to B's instances via B's
> self.
>
> I will continue to mull this over.
You are making this harder than need be. If you have one class
class A:
def __init__(self):
# complex calculation --> a
self.a = a
self.b = "whatever"
you could split the code into two methods, to keep it manageable, say:
class A:
def __init__(self):
self.init_a()
self.b = "whatever"
def init_a(self):
# complex calculation
self.a = a
This will obviously continue to work with two classes:
class A:
def init_a(self):
# complex calculation
self.a = a
class B(A):
def __init__(self):
self.init_a()
self.b = "whatever"
Now what if init_a() is actually called __init__? Just renaming will not
work
class A:
def __init__(self):
# complex ...
self.a = a
class B(A)
def __init__(self):
self.__init__() # this will invoke B.__init__ again
# --> infinite recursion
self.b = "whatever"
so you either have to be explicit
class B(A)
def __init__(self):
A.__init__(self) # this will invoke A.__init__
# because we told it to
self.b = "whatever"
or use "black magic"
class B(A):
def __init__(self):
super().__init__() # have Python figure out
# what to call
self.b = "whatever"
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