On Mon, Jan 15, 2024 at 8:53 AM Markus Armbruster <arm...@redhat.com> wrote: > > John Snow <js...@redhat.com> writes: > > > declare, but don't initialize the type of "type" to be QAPISchemaType - > > Declare > > > and allow the value to be initialized during check(). This creates a > > form of delayed initialization for QAPISchemaType objects where the > > static typing only represents the fully-realized object, which occurs > > after check() has been called. > > > > This avoids the need for several "assert type is not None" statements > > littered throughout the code by asserting it "will always be set." > > Uh, I find "will always be set" confusing. > > I think you mean "cannot be None".
Yuh > > But "be set" can also be read as "will have a value". ...yuh > > It actually doesn't exist until .check(), and once it exists, it cannot > be None. > > > Note that the static typing information for this object will be > > incorrect prior to check() being called. If this field is accessed > > before it is initialized in check(), you'll be treated to an > > AttributeError exception. > > We could now enter a philosophical debate on whether the static typing > is actually incorrect. Clearly v: T asserts that the value of @v is > always a T, and the type checker proves this. Does it also assert that > @v exists? The type checker doesn't care, and that's a feature. *clutches TAPL and cries* > > Maybe start with the problem, and then develop the solution: > > A QAPISchemaObjectTypeMember's type gets resolved only during .check(). > We have QAPISchemaObjectTypeMember.__init__() initialize self.type = > None, and .check() assign the actual type. Using .type before .check() > is wrong, and hopefully crashes due to the value being None. Works. > > However, it makes for awkward typing. With .type: > Optional[QAPISchemaType], mypy is of course unable to see that it's None > before .check(), and a QAPISchemaType after. To help it over the hump, > we'd have to assert self.type is not None before all the (valid) uses. > The assertion catches invalid uses, but only at run time; mypy can't > flag them. > > Instead, declare .type in .__init__() as QAPISchemaType *without* > initializing it. Using .type before .check() now certainly crashes, > which is an improvement. Mypy still can't flag invalid uses, but that's > okay. > OK. --js > > Fixes stuff like this: > > > > qapi/schema.py:657: error: "None" has no attribute "alternate_qtype" > > [attr-defined] > > qapi/schema.py:662: error: "None" has no attribute "describe" > > [attr-defined] > > > > Signed-off-by: John Snow <js...@redhat.com> > > --- > > scripts/qapi/schema.py | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/scripts/qapi/schema.py b/scripts/qapi/schema.py > > index e39ed972a80..48a51dcd188 100644 > > --- a/scripts/qapi/schema.py > > +++ b/scripts/qapi/schema.py > > @@ -794,7 +794,7 @@ def __init__(self, name, info, typ, optional, > > ifcond=None, features=None): > > assert isinstance(f, QAPISchemaFeature) > > f.set_defined_in(name) > > self._type_name = typ > > - self.type = None > > + self.type: QAPISchemaType # set during check() > > self.optional = optional > > self.features = features or [] >
