GitLocker: The Coding Marketplace

Description:

fallbackproperty 0.2.0

Requirements

Python 3.6+

What is it?
fallback_property transforms a function into a property and uses the
decorated function as fallback if no value was assigned to the property itself.
A special descriptor (fallback_property.FallbackDescriptor)
is used internally.

Django (or similar frameworks)
fallback_property is useful if you have a function that aggregates
values from related objects, which could already be fetched using an annotated
queryset.
The decorator will favor the precalculated value over calling the actual method.
It is especially helpful, if you optimize your application and want to
replace legacy or performance critical properties with precalulated values
using .annotate().

How to use it?
Simply define a function and use the decorator fallback_property
from fallback_property import fallback_property

class Foo:

@fallback_property()
def fallback_func(self):
return 7

Arguments
The fallback_property() has two optional arguments.

cached: bool = True
If the property is accessed multiple times, call the fallback function only once.

logging: bool = False
Log a warning if there was a fallback to the decorated, original method.

Usage Example (Django)
Suppose we have the following models
from django.db import models

class Pipeline(model.Model):
@property
def total_length(self):
return sum(self.parts.values_list('length', flat=True))

class Parts(models.Model):
length = models.PositiveIntegerField()
pipeline = models.ForeignKey(Pipeline, related_name='parts')
Calling pipline.total_length will always trigger another query and is
even more expensive when dealing with multiple objects. This can be
optimized by using .annotate() and fallback_property()
from django.db import models, QuerySet
from django.db.functions import Coalesce
from django.db.models import Sum
from fallback_property import fallback_property

class PipelineQuerySet(QuerySet):

def with_total_length(self):
return self.annotate(
total_length=Coalesce(
Sum('parts__length', output_field=models.IntegerField()),
0,
)
)

class Pipeline(model.Model):

@fallback_property(logging=True)
def total_length(self):
return sum(self.parts.values_list('length', flat=True))
You can now access the total_length without triggering another query or
get a warning, when the fallback function is used
pipeline = Pipeline.objects.with_total_length().first()
print(pipeline.total_length)
Important: The annotated value and the property must have the same name.

Related objects
When dealing with related objects in Django be aware that the ORM imposes certain limitations:
In the following example one might expect to get an instance of User, but instead the
value of the primary key is returned:
from django.db import models, QuerySet
from django.db.functions import Coalesce
from django.db.models import F
from fallback_property import fallback_property

class PartQuerySet(QuerySet):

def with_owner(self):
return self.annotate(
owner=Coalesce(
F('_owner'),
F('pipeline__owner'),
None,
)
)

class Pipeline(model.Model):
owner = models.ForeignKey(User)

class Parts(models.Model):
_owner = models.ForeignKey(User, blank=True, null=True, on_delete=models.SET_NULL)
length = models.PositiveIntegerField()
pipeline = models.ForeignKey(Pipeline, related_name='parts')

objects = PartQuerySet()

@fallback_property()
def owner(self):
return self._owner or self.pipline.owner

>>> print(Part.objects.with_owner().first().owner)
>>> 1

Development
This project is using poetry to manage all
dev dependencies.
Clone this repository and run
poetry install
poetry run pip install django
to create a virtual environment with all dependencies.
You can now run the test suite using
poetry run pytest
This repository follows the angular commit conventions.
You can register a pre-commit hook to validate your commit messages by using
husky. The configurations are already in place if
you have nodejs installed. Just run
npm install
and the pre-commit hook will be registered.