How to dynamically compose an OR query filter in Django?

| |

👻 See our latest reviews to choose the best laptop for Machine Learning and Deep learning tasks!

From an example you can see a multiple OR query filter:

Article.objects.filter(Q(pk=1) | Q(pk=2) | Q(pk=3))

For example, this results in:

[<Article: Hello>, <Article: Goodbye>, <Article: Hello and goodbye>]

However, I want to create this query filter from a list. How to do that?

e.g. [1, 2, 3] -> Article.objects.filter(Q(pk=1) | Q(pk=2) | Q(pk=3))

👻 Read also: what is the best laptop for engineering students in 2022?

How to dynamically compose an OR query filter in Django? filter: Questions

List comprehension vs. lambda + filter

5 answers

I happened to find myself having a basic filtering need: I have a list and I have to filter it by an attribute of the items.

My code looked like this:

my_list = [x for x in my_list if x.attribute == value]

But then I thought, wouldn"t it be better to write it like this?

my_list = filter(lambda x: x.attribute == value, my_list)

It"s more readable, and if needed for performance the lambda could be taken out to gain something.

Question is: are there any caveats in using the second way? Any performance difference? Am I missing the Pythonic Way‚Ñ¢ entirely and should do it in yet another way (such as using itemgetter instead of the lambda)?

957

Answer #1

It is strange how much beauty varies for different people. I find the list comprehension much clearer than filter+lambda, but use whichever you find easier.

There are two things that may slow down your use of filter.

The first is the function call overhead: as soon as you use a Python function (whether created by def or lambda) it is likely that filter will be slower than the list comprehension. It almost certainly is not enough to matter, and you shouldn"t think much about performance until you"ve timed your code and found it to be a bottleneck, but the difference will be there.

The other overhead that might apply is that the lambda is being forced to access a scoped variable (value). That is slower than accessing a local variable and in Python 2.x the list comprehension only accesses local variables. If you are using Python 3.x the list comprehension runs in a separate function so it will also be accessing value through a closure and this difference won"t apply.

The other option to consider is to use a generator instead of a list comprehension:

def filterbyvalue(seq, value):
   for el in seq:
       if el.attribute==value: yield el

Then in your main code (which is where readability really matters) you"ve replaced both list comprehension and filter with a hopefully meaningful function name.

957

Answer #2

This is a somewhat religious issue in Python. Even though Guido considered removing map, filter and reduce from Python 3, there was enough of a backlash that in the end only reduce was moved from built-ins to functools.reduce.

Personally I find list comprehensions easier to read. It is more explicit what is happening from the expression [i for i in list if i.attribute == value] as all the behaviour is on the surface not inside the filter function.

I would not worry too much about the performance difference between the two approaches as it is marginal. I would really only optimise this if it proved to be the bottleneck in your application which is unlikely.

Also since the BDFL wanted filter gone from the language then surely that automatically makes list comprehensions more Pythonic ;-)

How to dynamically compose an OR query filter in Django? filter: Questions

How do I do a not equal in Django queryset filtering?

5 answers

MikeN By MikeN

In Django model QuerySets, I see that there is a __gt and __lt for comparative values, but is there a __ne or != (not equals)? I want to filter out using a not equals. For example, for

Model:
    bool a;
    int x;

I want to do

results = Model.objects.exclude(a=True, x!=5)

The != is not correct syntax. I also tried __ne.

I ended up using:

results = Model.objects.exclude(a=True, x__lt=5).exclude(a=True, x__gt=5)
784

Answer #1

You can use Q objects for this. They can be negated with the ~ operator and combined much like normal Python expressions:

from myapp.models import Entry
from django.db.models import Q

Entry.objects.filter(~Q(id=3))

will return all entries except the one(s) with 3 as their ID:

[<Entry: Entry object>, <Entry: Entry object>, <Entry: Entry object>, ...]

How to do a less than or equal to filter in Django queryset?

1 answers

I am attempting to filter users by a custom field in each users profile called profile. This field is called level and is an integer between 0-3.

If I filter using equals, I get a list of users with the chosen level as expected:

user_list = User.objects.filter(userprofile__level = 0)

When I try to filter using less than:

user_list = User.objects.filter(userprofile__level < 3)

I get the error:

global name "userprofile__level" is not defined

Is there a way to filter by < or >, or am I barking up the wrong tree.

209

Answer #1

Less than or equal:

User.objects.filter(userprofile__level__lte=0)

Greater than or equal:

User.objects.filter(userprofile__level__gte=0)

Likewise, lt for less than and gt for greater than. You can find them all in the documentation.

Shop

Learn programming in R: courses

$

Best Python online courses for 2022

$

Best laptop for Fortnite

$

Best laptop for Excel

$

Best laptop for Solidworks

$

Best laptop for Roblox

$

Best computer for crypto mining

$

Best laptop for Sims 4

$

Latest questions

NUMPYNUMPY

psycopg2: insert multiple rows with one query

12 answers

NUMPYNUMPY

How to convert Nonetype to int or string?

12 answers

NUMPYNUMPY

How to specify multiple return types using type-hints

12 answers

NUMPYNUMPY

Javascript Error: IPython is not defined in JupyterLab

12 answers

News


Wiki

Python OpenCV | cv2.putText () method

numpy.arctan2 () in Python

Python | os.path.realpath () method

Python OpenCV | cv2.circle () method

Python OpenCV cv2.cvtColor () method

Python - Move item to the end of the list

time.perf_counter () function in Python

Check if one list is a subset of another in Python

Python os.path.join () method