Join a list with a bitwise or operator in Python
22 August 2022 0 comments Python
The bitwise OR operator in Python is often convenient when you want to combine multiple things into one thing. For example, with the Django ORM you might do this:
from django.db.models import Q
filter_ = Q(first_name__icontains="peter") | Q(first_name__icontains="ashley")
for contact in Contact.objects.filter(filter_):
print((contact.first_name, contact.last_name))
See how it hardcodes the filtering on strings peter and ashley.
But what if that was a bit more complicated:
from django.db.models import Q
filter_ = Q(first_name__icontains="peter")
if include("ashley"):
filter_ | = Q(first_name__icontains="ashley")
for contact in Contact.objects.filter(filter_):
print((contact.first_name, contact.last_name))
So far, same functionality.
But what if the business logic is more complicated? You can't do this:
filter_ = None
if include("peter"):
filter_ | = Q(first_name__icontains="peter") # WILL NOT WORK
if include("ashley"):
filter_ | = Q(first_name__icontains="ashley")
for contact in Contact.objects.filter(filter_):
print((contact.first_name, contact.last_name))
What if the list of things you want to filter on depends on a list? You'd need to do the |= stuff "dynamically". One way to solve that is with functools.reduce. Suppose the list of things you want to bitwise-OR together is a list:
from django.db.models import Q
from operator import or_
from functools import reduce
def include(_):
import random
return random.random() > 0.5
filters = []
if include("peter"):
filters.append(Q(first_name__icontains="peter"))
if include("ashley"):
filters.append(Q(first_name__icontains="ashley"))
assert len(filters), "must have at least one filter"
filter_ = reduce(or_, filters) # THE MAGIC!
for contact in Contact.objects.filter(filter_):
print((contact.first_name, contact.last_name))
And finally, if it's a list already:
from django.db.models import Q
from operator import or_
from functools import reduce
names = ["peter", "ashley"]
qs = [Q(first_name__icontains=x) for x in names]
filter_ = reduce(or_, qs)
for contact in Contact.objects.filter(filter_):
print((contact.first_name, contact.last_name))
Side note
Django's django.db.models.Q is actually quite flexible with used with MyModel.objects.filter(...) because this actually works:
from django.db.models import Q
def include(_):
import random
return random.random() > 0.5
filter_ = Q() # MAGIC SAUCE
if include("peter"):
filter_ |= Q(first_name__icontains="peter")
if include("ashley"):
filter_ |= Q(first_name__icontains="ashley")
for contact in Contact.objects.filter(filter_):
print((contact.first_name, contact.last_name))