On 13.02.15 05:41, Ethan Furman wrote:
So there are basically two choices:
1) always use the type of the most-base class when creating new instances
pros:
- easy
- speedy code
- no possible tracebacks on new object instantiation
cons:
- a subclass that needs/wants to maintain itself must override all
methods that create new instances, even if the only change is to
the type of object returned
2) always use the type of self when creating new instances
pros:
- subclasses automatically maintain type
- much less code in the simple cases [1]
cons:
- if constructor signatures change, must override all methods which
create new objects
And switching to (2) would break existing code which uses subclasses
with constructors with different signature (e.g. defaultdict).
The third choice is to use different specially designed constructor.
class A(int):
class A(int):
... def __add__(self, other):
... return self.__make_me__(int(self) + int(other))
... def __repr__(self):
... return 'A(%d)' % self
...
A.__make_me__ = A
A(2) + 3
A(5)
class B(A):
... def __repr__(self):
... return 'B(%d)' % self
...
B.__make_me__ = B
B(2) + 3
B(5)
We can add special attribute used to creating results of operations to
all basic classes. By default it would be equal to the base class
constructor.
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