An object that enables chaining multiple iterables to serve them lazily as a queryset-compatible object.
Project description
This module provides a way to chain multiple finite iterables for consumption as a queryset-compatible object.
Quickstart
Let’s start with an example. Say we have a couple of abstract database models which enables us to reuse fields later on:
class Titled(db.Model):
title = db.CharField(max_length=100)
class Meta:
abstract = True
class Dynamic(db.Model):
duration = db.PositiveIntegerField()
class Meta:
abstract = True
We also have concrete database models that share some of those fields:
class Video(Titled, Dynamic):
RESOLUTION = (
(1, '240p'), (2, '320p'), (3, '480p'),
(4, '720p'), (5, '1080p')
)
author = db.CharField(max_length=100)
resolution = db.IntegerField(choices=RESOLUTION)
class Song(Titled, Dynamic):
GENRE = (
(1, 'Country'), (2, 'Folk'), (3, 'Polka'),
(4, 'Western'), (5, 'World')
)
artist = db.CharField(max_length=100)
genre = db.IntegerField(choices=GENRE)
Our database already contains some data:
>>> Video.objects.all() [<Video: Psy - Gangnam Style (253 s at 1080p)>, <Video: Justin Bieber - Baby (225 s at 720p)>, <Video: Lady Gaga - Bad Romance (308 s at 320p)>, <Video: Shakira - Waka Waka (211 s at 480p)>] >>> Song.objects.all() [<Song: Gotye feat. Kimbra - Somebody That I Used to Know (244 s; Folk)>, <Song: Coldplay - Clocks (307 s; Polka)>, <Song: Muse - Madness (279 s; Country)>, <Song: Florence + The Machine - Spectrum (218 s; Folk)>]
A basic chain
Let’s create a simple chain:
>>> from dj.chain import chain >>> media = chain(Video.objects.all(), Song.objects.all())
We can collectively call queryset-related methods on it:
>>> media.count() 8
We can also filter it further:
>>> list(media.filter(duration__gt=250)) [<Video: Psy - Gangnam Style (253 s at 1080p)>, <Video: Lady Gaga - Bad Romance (308 s at 320p)>, <Song: Coldplay - Clocks (307 s; Polka)>, <Song: Muse - Madness (279 s; Country)>]
Check the cumulative length:
>>> media.filter(duration__gt=250).count() 4
Use indices and slices:
>>> media.filter(duration__gt=250)[1] <Video: Lady Gaga - Bad Romance (308 s at 320p)> >>> list(media[3:6]) [<Video: Shakira - Waka Waka (211 s at 480p)>, <Song: Gotye feat. Kimbra - Somebody That I Used to Know (244 s; Folk)>, <Song: Coldplay - Clocks (307 s; Polka)>] >>> list(media[1::3]) [<Video: Justin Bieber - Baby (225 s at 720p)>, <Song: Gotye feat. Kimbra - Somebody That I Used to Know (244 s; Folk)>, <Song: Florence + The Machine - Spectrum (218 s; Folk)>]
Use cumulative sorting and filtering:
>>> list(media.order_by('title'))
[<Video: Justin Bieber - Baby (225 s at 720p)>,
<Video: Lady Gaga - Bad Romance (308 s at 320p)>,
<Song: Coldplay - Clocks (307 s; Polka)>,
<Video: Psy - Gangnam Style (253 s at 1080p)>,
<Song: Muse - Madness (279 s; Country)>,
<Song: Gotye feat. Kimbra - Somebody That I Used to Know (244 s; Folk)>,
<Song: Florence + The Machine - Spectrum (218 s; Folk)>,
<Video: Shakira - Waka Waka (211 s at 480p)>]
>>> list(media.order_by('-duration').filter(duration__lt=300))
[<Song: Muse - Madness (279 s; Country)>,
<Video: Psy - Gangnam Style (253 s at 1080p)>,
<Song: Gotye feat. Kimbra - Somebody That I Used to Know (244 s; Folk)>,
<Video: Justin Bieber - Baby (225 s at 720p)>,
<Song: Florence + The Machine - Spectrum (218 s; Folk)>,
<Video: Shakira - Waka Waka (211 s at 480p)>]
Etc.
Chaining heterogenic iterables
We can add iterables that aren’t querysets to the bunch:
>>> from collections import namedtuple
>>> Book = namedtuple('Book', "author title page_count")
>>> books=(
... Book(author='Charles Dickens', title='A Tale of Two Cities', page_count=869),
... Book(author='Miguel de Cervantes', title='Don Quixote', page_count=1212),
... )
>>> media=chain(Video.objects.all(), books)
>>> media.count()
6
>>> list(media)
[<Video: Psy - Gangnam Style (253 s at 1080p)>,
<Video: Justin Bieber - Baby (225 s at 720p)>,
<Video: Lady Gaga - Bad Romance (308 s at 320p)>,
<Video: Shakira - Waka Waka (211 s at 480p)>,
Book(author='Charles Dickens', title='A Tale of Two Cities', page_count=869),
Book(author='Miguel de Cervantes', title='Don Quixote', page_count=1212)]
You can also use cumulative ordering in this case. The only thing you need to keep in mind is that iterables which are not querysets should be presorted for the cumulative result to be ordered correctly. An example:
>>> list(media.order_by('title'))
[Book(author='Charles Dickens', title='A Tale of Two Cities', page_count=869),
<Video: Justin Bieber - Baby (225 s at 720p)>,
<Video: Lady Gaga - Bad Romance (308 s at 320p)>,
Book(author='Miguel de Cervantes', title='Don Quixote', page_count=1212),
<Video: Psy - Gangnam Style (253 s at 1080p)>,
<Video: Shakira - Waka Waka (211 s at 480p)>]
Custom filtering, sorting and transformations
Chains provide special overridable static methods used while yielding values:
xfilter(value) - yield a value only if xfilter(value) returns True. See known issues below.
xform(value) - transforms the value JIT before yielding it back. It is only called for values within the specified slice and those which passed xfilter.
xkey(value) - returns a value to be used in comparison between elements if sorting should be used. Individual iterables should be presorted for the complete result to be sorted properly. Any cumulative order_by clauses are executed before the xkey method is used.
Known issues
If slicing or xfilter is used, reported len() is computed by iterating over all iterables so performance is weak. Note that len() is used by list() when you convert your chain to a list or when iterating over the chain in Django templates. If this is not expected, you can convert to a list using a workaround like this:
list(e for e in some_chain)
Indexing on chains uses iteration underneath so performance is weak. This feature is only available as a last resort. Slicing on the other hand is also lazy.
Collective filter and exclude silently skip filtering on incompatible iterables. Use xfilter(value) as a workaround.
How do I run the tests?
The easiest way would be to run:
$ DJANGO_SETTINGS_MODULE="dj._chaintestproject.settings" django-admin.py test
Change Log
0.9.0
code separated from lck.django
support for collective sort using queryset-like order_by on a chain
fix for slices with custom steps
PEP8-fied all sources
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