#31680: Preventing DeferredAttribute .__ get__ method unnecessary calls
-------------------------------------+-------------------------------------
Reporter: Sultan | Owner: nobody
Type: | Status: new
Cleanup/optimization |
Component: Database | Version: 3.1
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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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.
--
Ticket URL: <https://code.djangoproject.com/ticket/31680>
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