functools.partial

what is it?

functools.partial(func, /, *args, **keywords)

returns a new partial object which when called will behave like func called with the positional arguments args and keyword arguments keywords

in english (thanks danny):

• makes a new version of a function with one or more arguments already filled in
• new version of a function documents itself

example

from functools import partial
 
basetwo = partial(int, base=2)
 
basetwo('10010')
# returns 18

django specific uses

reference

reusable fields without subclassing

from functools import partial
 
from django.db import models
 
 
CreatedAtField = partial(
    models.DateTimeField,
    default=timezone.now,
    help_text="When this instance was created.",
)
 
 
class Book(models.Model):
    ...
    created_at = CreatedAtField()
 
 
class Author(models.Model):
    ...
    created_at = CreatedAtField()

upload_to callables for FileField

from functools import partial
 
from django.db import models
 
 
def user_upload_to(instance, filename, category):
    return f"users/{instance.id}/{category}/{filename}"
 
 
class User(models.Model):
    ...
    profile_picture = models.ImageField(
        upload_to=partial(user_upload_to, category="profile"),
    )
    background_picture = models.ImageField(
        upload_to=partial(user_upload_to, category="background"),
    )