Re: [Qemu-devel] [PATCH v6 02/12] qapi: Don't use info as witness of implicit object type

[Eric Blake]

On 10/02/2015 08:12 AM, Markus Armbruster wrote:

>> Actually, this only works for implicit objects.  Implicit enums instead
>> have self.name[-4:] == 'Kind'.  But qapi-types cares about implicit
>> objects only.  So if I hoist this, I may need something like:
>>
>> def is_implicit(self, type=None):
>>     if type and not isinstance(self, type):
>>         return Fals
>>     if isinstance(self, QAPISchemaObjectType):
>>         return self.name[0] == ':'
>>     if isinstance(self, QAPISchemaEnumType):
>>         return self.name[-4:] == 'Kind'
>>     return False
>>
>> where qapi-types would call entity.is_implicit(QAPISchemaObjectType).
> 
> Do it the OO-way: QAPISchemaEntity.is_implicit() returns False.  Any
> subclass that can have implicitly defined instances overrides it:
> QAPISchemaObjectType.is_implicit() tests for ':' prefix,
> QAPISchemaEnumType.is_implicit() tests for 'Kind' suffix (requires
> outlawing it for user enums, if we don't do it already), and so forth.

But there's still the issue of filtering by subclass.  For the
qapi-types visit_needed() filter, I either write:

if isinstance(entity, QAPISchemaObjectType) and entity.is_implicit()

or what I want to write:

if entity.is_implicit(QAPISchemaObjectType)

while still allowing the common case of is_implicit() when I don't care
which type is doing the testing.  Since python doesn't allow method
overloads, I'd have to repeat the:

    def is_implicit(self, type=None):
        if type and not isinstance(self, type):
            return False

prefix in each subclass that overrides the basic version.

>> where the indentation is okay.  I guess trying to avoid the \ is not
>> worth it, if the tools don't complain about it, and that this was a case
>> of me prematurely guessing (incorrectly) about what the tools don't like.
> 
> Quoting PEP8:

Thanks; that helps.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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