Re: [PATCH v2 04/19] qapi/schema: declare type for QAPISchemaObjectTypeMember.type

[Markus Armbruster <armbru@redhat.com>]

John Snow <jsnow@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".

But "be set" can also be read as "will have a value".

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.

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.

> 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 <jsnow@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 []