Alan Gauld via Tutor wrote:
> On 30/11/17 15:37, Shall, Sydney wrote:
>
>> My problem is with constructing a sub-class.
>>
>> My sub-class is constructed as follows:
>>
>> import Population_ProductivityV24 as POCWP
>
> Note that POCWP is an alias for the *module* Population_ProductivityV24.
> It is not a class.
>
>> line 27 : class SimulateCycleZero(POCWP):
>
> Here you define a class that subclasses your imported module.
> Frankly I'm surprised that you don't get an error there
> but hey...
>
>> line 28 : def __init__(self, dirname_p):
>
> But this is now an init for a subclass of module.
>
>> The error message is as follows:
>>
>> File
>>
"/Users/sydney/AnacondaProjects/Capital/Capital_with_productivity/Current_Versions/Simulate_Cycle_Zero_V3.py",
>> line 27, in <module>
>> class SimulateCycleZero(POCWP):
>>
>> TypeError: module.__init__() takes at most 2 arguments (3 given)
>
> So I'm guessing the use of a module to subclass
> has confused things and I confess I'm not clear on
> exactly what is going on and why you get this message.
A class is an instance of its metaclass.
class A:
pass
is roughly equivalent to
A = type("A", (), {}) # classname, base classes, class attributes
and
class B(A):>>> class A:
... def __init__(self, *args):
... print("__init__{}".format(args))
...
>>> class B(A()): pass
...
__init__()
__init__('B', (<__main__.A object at 0x7f3db8a1c048>,), {'__module__':
'__main__', '__qualname__': 'B'})
>>> assert isinstance(B, A)
>>> isinstance(B, A)
>>>
>>>
True
>>> B()
>>>
>>>
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'A' object is not callable
foo = 42
is roughly equivalent to
B = type(A)("B", (A,), {"foo": 42})
When you subclass from an instance of A instead of A itself this becomes
a = A()
B = type(a)("B", (a,), {"foo": 42})
which can be simplified to
B = A("B", (a,), {"foo": 42})
If this succeeds B is bound to an instance of A, but usually you'll see a
TypeError, either immediately as the OP,
>>> class A: pass
...
>>> class B(A()): pass
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object() takes no parameters
or later when you try to instantiate B:
>>> class A:
... def __init__(self, *args):
... print("__init__{}".format(args))
...
>>> class B(A()): pass
...
__init__()
__init__('B', (<__main__.A object at 0x7f3db8a1c048>,), {'__module__':
'__main__', '__qualname__': 'B'})
>>> isinstance(B, A)
>>>
>>>
True
>>> B()
>>>
>>>
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'A' object is not callable
> But I'm pretty sure you don;t want to subclass your
> imported module and thats the mistake.
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