#31680: Preventing DeferredAttribute .__ get__ method unnecessary calls
-------------------------------------+-------------------------------------
Reporter: Sultan | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database layer | Version: 3.1
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 1 | UI/UX: 0
-------------------------------------+-------------------------------------
Description changed by Sultan:
Old description:
> To retrieve a deferred model attributes, the __get__ method is called
> twice. This is because it uses the getattr() function, which in turn
> causes the __get__ method to be called again.
>
> {{{
> def __get__(self, instance, cls=None):
> """
> Retrieve and caches the value from the datastore on the first
> lookup.
> Return the cached value.
> """
> if instance is None:
> return self
> data = instance.__dict__
> field_name = self.field.attname
> if field_name not in data:
> # Let's see if the field is part of the parent chain. If so
> we
> # might be able to reuse the already loaded value. Refs
> #18343.
> val = self._check_parent_chain(instance)
> if val is None:
> instance.refresh_from_db(fields=[field_name])
> **val = getattr(instance, field_name)**
> data[field_name] = val
> return data[field_name]
> }}}
>
> To prevent this unnecessary call, we can simply extract the value from
> the instance dict (at that moment it already contains the searched
> value):
>
> {{{
> def __get__(self, instance, cls=None):
> """
> Retrieve and caches the value from the datastore on the first
> lookup.
> Return the cached value.
> """
> if instance is None:
> return self
> data = instance.__dict__
> field_name = self.field.attname
> if field_name not in data:
> # Let's see if the field is part of the parent chain. If so
> we
> # might be able to reuse the already loaded value. Refs
> #18343.
> val = self._check_parent_chain(instance)
> if val is None:
> instance.refresh_from_db(fields=[field_name])
> # Now the instance dict contains the reloaded data.
> # Using getattr() will do extra call to __get__ method.
> **val = data[field_name]**
> data[field_name] = val
> return data[field_name]
> }}}
>
> This reduces the number of method calls.
New description:
To retrieve a deferred model attributes, the __get__ method is called
twice. This is because it uses the getattr() function, which in turn
causes the __get__ method to be called again.
{{{
def __get__(self, instance, cls=None):
"""
Retrieve and caches the value from the datastore on the first
lookup.
Return the cached value.
"""
if instance is None:
return self
data = instance.__dict__
field_name = self.field.attname
if field_name not in data:
# Let's see if the field is part of the parent chain. If so we
# might be able to reuse the already loaded value. Refs
#18343.
val = self._check_parent_chain(instance)
if val is None:
instance.refresh_from_db(fields=[field_name])
**val = getattr(instance, field_name)**
data[field_name] = val
return data[field_name]
}}}
To prevent this unnecessary call, we can simply extract the value from the
instance dict (at that moment it already contains the reloaded value):
{{{
def __get__(self, instance, cls=None):
"""
Retrieve and caches the value from the datastore on the first
lookup.
Return the cached value.
"""
if instance is None:
return self
data = instance.__dict__
field_name = self.field.attname
if field_name not in data:
# Let's see if the field is part of the parent chain. If so we
# might be able to reuse the already loaded value. Refs
#18343.
val = self._check_parent_chain(instance)
if val is None:
instance.refresh_from_db(fields=[field_name])
# Now the instance dict contains the reloaded data.
# Using getattr() will do extra call to __get__ method.
**val = data[field_name]**
data[field_name] = val
return data[field_name]
}}}
This reduces the number of method calls.
--
--
Ticket URL: <https://code.djangoproject.com/ticket/31680#comment:2>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
--
You received this message because you are subscribed to the Google Groups
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/django-updates/067.ffbe40c548b47fa374a89f0ccd4ef39f%40djangoproject.com.
