Hi Volker,
On 2013-07-28, Volker Braun <vbraun.n...@gmail.com> wrote:
> The __classcall__ is used just as a shortcut (well arguably its not shorter
> but lets say it is for the sake of the argument) for the factory function
> pattern.
Sure. But the question is: SHOULD there be more than a factory function?
After all, we have a class, and we call the class as if to create an
instance---but what we get is in fact not an instance of this class, but
is instance of a *totally* different class.
OK, it is possible, Python let's you do it. But not all what can be done
should be done.
I have nothing against the following pattern:
class C(UniqueRepresentation):
@staticmethod
def __classcall_private__(cls, *args,**kwds):
if <condition 1>:
return A(*args,**kwds)
else:
return B(*args,**kwds)
# define some general methods of C
class B(C):
def __init__(...):
...
# define some methods of B that aren't in C
class A(C):
def __init__(...):
...
# define some methods of A that aren't in C
Then, calling C(...) would return an instance of either A or B, which
would be a unique representation, since C uses __classcall_private__,
not __classcall__, and hence the caching of UniqueRepresentation is
available for A and B.
And I think this pattern is totally fine, since C(...) returns instances
of sub-classes of C (namely of A resp. B)---hence, it returns instance
of C! I could imagine that a similar pattern would be available for
Partition versus PartitionTuple.
However, if C(...) is supposed to return objects that have nothing to do
with the class C, then C should really not be a class, but just a plain
factory function.
Best regards,
Simon
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