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from django.core import checks
from django.utils.functional import cached_property
NOT_PROVIDED = object()
class FieldCacheMixin:
"""
An API for working with the model's fields value cache.
Subclasses must set self.cache_name to a unique entry for the cache -
typically the field’s name.
"""
@cached_property
def cache_name(self):
raise NotImplementedError
def get_cached_value(self, instance, default=NOT_PROVIDED):
try:
return instance._state.fields_cache[self.cache_name]
except KeyError:
if default is NOT_PROVIDED:
raise
return default
def is_cached(self, instance):
return self.cache_name in instance._state.fields_cache
def set_cached_value(self, instance, value):
instance._state.fields_cache[self.cache_name] = value
def delete_cached_value(self, instance):
del instance._state.fields_cache[self.cache_name]
class CheckFieldDefaultMixin:
_default_hint = ("<valid default>", "<invalid default>")
def _check_default(self):
if (
self.has_default()
and self.default is not None
and not callable(self.default)
):
return [
checks.Warning(
"%s default should be a callable instead of an instance "
"so that it's not shared between all field instances."
% (self.__class__.__name__,),
hint=(
"Use a callable instead, e.g., use `%s` instead of "
"`%s`." % self._default_hint
),
obj=self,
id="fields.E010",
)
]
else:
return []
def check(self, **kwargs):
errors = super().check(**kwargs)
errors.extend(self._check_default())
return errors
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