Lately I'm trying to find ways to use Django in a more modern way. Being able to use type hints for validation, knowing what's going to be in form.cleaned_data. That sort of thing. I'm mostly failing.
The best I've managed lately is to reduce some boilerplate in class based validators. I'm not sure if it's really expected to make your own class based validators since I couldn't find any good documentation on it.
However, you can read through the source code to see how it's done there.
Typically you declare them like this:
from django.core.exceptions import ValidationError
from django.utils.deconstruct import deconstructible
@deconstructible
class FileSizeValidator:
def __init__(self, max_size):
self.max_size = max_size
def __call__(self, file):
if file.size > self.max_size:
raise ValidationError(_("Max file size is %s") % self.max_size)
def __eq__(self, other):
return isinstance(other, FileSizeValidator) and self.max_size == other.max_size
The reason __eq__ is needed is because Django needs to check if a validator has changed, so a new migration is needed. Quite annoying to need to write this, however. Another small issue is that my linter enjoys complaining about implementing __eq__ but not __hash__.
Dataclasses can save the day since they have default implementations of __eq__ and __hash__. So we can at least do this:
import dataclasses
from django.core.exceptions import ValidationError
from django.utils.deconstruct import deconstructible
@deconstructible
@dataclasses.dataclass
class FileSizeValidator:
max_size: int
def __call__(self, file: "FieldFile") -> None:
if file.size > self.max_size:
raise ValidationError(_("Max file size is %s") % self.max_size)
It's a small win.
There's a small difference. A dataclass's __eq__ checks the exact type, whereas before we were using isinstance, to allow for subclassing.
Still, in most cases, I would do this differently. Unless I have a lot of different file size requirements, I'd rather just use a partial:
from functools import partial
from django.core.exceptions import ValidationError
def validate_file_size(file: "FieldFile", max_size: int) -> None:
if file.size > max_size:
raise ValidationError(_("Max file size is %s") % max_size)
validate_small_file = partial(validate_file_size, max_size=524_288)
validate_medium_file = partial(validate_file_size, max_size=1_048_576)
validate_large_file = partial(validate_file_size, max_size=10_485_760)
Shorter, and if you ask me, easier to read.