On 02/02/2021 23:08, Greg Ewing wrote:
On 3/02/21 4:52 am, Phil Thompson wrote:
Thanks - that's fairly definitive, although I don't really understand
why __new__ has this particular requirement.
The job of tp_new is to initialise the C struct. To do this,
it first has to initialise the fields of the struct it
inherits from, then initialise any fields of its own that
it adds, in that order.
Understood.
Initialising the inherited fields must be done by calling
the tp_new for the struct that it inherits from. You don't
want to call the tp_new of some other class that might have
got inserted into the MRO, because you have no idea what
kind of C struct it expects to get.
I had assumed that some other magic in typeobject.c (eg. conflicting
meta-classes) would have raised an exception before getting to this
stage if there was a conflict.
Cooperative calling is a nice idea, but it requires rather
special conditions to make it work. All the methods must
have exactly the same signature, and it mustn't matter what
order they're called in. Those conditions don't apply to
__new__, especially at the C level where everything is much
more strict type-wise.
Thanks for the explanation.
Phil
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