Re: [PATCH v5 34/36] qapi/visit.py: assert tag_member contains a QAPISchemaEnumType

[John Snow <jsnow@redhat.com>]

On 10/7/20 8:43 AM, Markus Armbruster wrote:
> John Snow <jsnow@redhat.com> writes:
> 
>> This is true by design, but not presently able to be expressed in the
>> type system. An assertion helps mypy understand our constraints.
>>
>> Signed-off-by: John Snow <jsnow@redhat.com>
>> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
>> Reviewed-by: Cleber Rosa <crosa@redhat.com>
>> ---
>>   scripts/qapi/visit.py | 12 +++++++-----
>>   1 file changed, 7 insertions(+), 5 deletions(-)
>>
>> diff --git a/scripts/qapi/visit.py b/scripts/qapi/visit.py
>> index 14f30c228b7..4f11fd325b8 100644
>> --- a/scripts/qapi/visit.py
>> +++ b/scripts/qapi/visit.py
>> @@ -22,7 +22,7 @@
>>       mcgen,
>>   )
>>   from .gen import QAPISchemaModularCVisitor, ifcontext
>> -from .schema import QAPISchemaObjectType
>> +from .schema import QAPISchemaEnumType, QAPISchemaObjectType
>>   
>>   
>>   def gen_visit_decl(name, scalar=False):
>> @@ -84,15 +84,17 @@ def gen_visit_object_members(name, base, members, variants):
>>           ret += gen_endif(memb.ifcond)
>>   
>>       if variants:
>> +        tag_member = variants.tag_member
>> +        assert isinstance(tag_member.type, QAPISchemaEnumType)
> 
> I'm curious: do you need the local variable to make the assertion stick?
> 

No, but it only sticks to the binding and not the data. i.e. assertions 
to downcast work on the *name*.

(This comes up somewhere in the schema.py patches where I make a change 
that looks completely pointless, but it makes mypy happy.)

I could have left it alone. I just saw a lot of repeated multi-dots and 
habitually created a temporary local for the purpose.

>> +
>>           ret += mcgen('''
>>       switch (obj->%(c_name)s) {
>>   ''',
>> -                     c_name=c_name(variants.tag_member.name))
>> +                     c_name=c_name(tag_member.name))
>>   
>>           for var in variants.variants:
>> -            case_str = c_enum_const(variants.tag_member.type.name,
>> -                                    var.name,
>> -                                    variants.tag_member.type.prefix)
>> +            case_str = c_enum_const(tag_member.type.name, var.name,
>> +                                    tag_member.type.prefix)
>>               ret += gen_if(var.ifcond)
>>               if var.type.name == 'q_empty':
>>                   # valid variant and nothing to do