[issue20371] datetime.datetime.replace bypasses a subclass's __new__

[Eugene Toder]

Eugene Toder added the comment:

Properly supporting subclasses in replace() is hard, at least without some cooperation from subclasses. For a proper replace()

x.replace(a=newval).b == x.b

should hold for every b not dependent on a, including ones added by subclasses. That is, it should replicate subclass state. Arguably, ideal replace() would also allow changing attributes defined by subclasses -- otherwise subclasses need to override it anyway, and all this effort was for nothing.

The best I can think of is to assume that subclasses are immutable and all "primary" properties are settable via constructor arguments with the same names. Then replace() can be implemented like this:

def replace(self, *args, **kwargs):
    sig = inspect.signature(self.__new__)
    bound = sig.bind_partial(type(self), *args, **kwargs)
    for arg in sig.parameters:
        if arg not in bound.arguments:
            bound.arguments[arg] = getattr(self, arg)
    return self.__new__(*bound.args, **bound.kwargs)

This will not work for subclasses defined in C, but at least we can show a nice error about that. This will also not work if __new__ uses *args or **kwargs instead of listing every property as its own argument.

(Another approach I can think of is patching properties on self, making a copy of self via __reduce__, and reverting values on self back. This doesn't rely on any signatures, but gets really dirty really quick.)

So I don't know if we want to implement this, or if returning base class from replace() is a better choice.

----------
nosy: +eltoder, serhiy.storchaka

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20371>
_______________________________________