Specify typing for Django field in model (for Pylint)

I have created custom Django model-field subclasses based on CharField but which use to_python() to ensure that the model objects returned have more complex objects (some are lists, some are dicts with a specific format, etc.) -- I'm using MySQL so some of the PostGreSql field types are not available.

All is working great, but Pylint believes that all values in these fields will be strings and thus I get a lot of "unsupported-membership-test" and "unsubscriptable-object" warnings on code that uses these models. I can disable these individually, but I would prefer to let Pylint know that these models return certain object types. Type hints are not helping, e.g.:

class MealPrefs(models.Model):
    user = ...foreign key...
    prefs: dict[str, list[str]] = \
       custom_fields.DictOfListsExtendsCharField(
            default={'breakfast': ['cereal', 'toast'], 
                     'lunch': ['sandwich']},
       )

I know that certain built-in Django fields return correct types for Pylint (CharField, IntegerField) and certain other extensions have figured out ways of specifying their type so Pylint is happy (MultiSelectField) but digging into their code, I can't figure out where the "magic" specifying the type returned would be.

(note: this question is not related to the INPUT:type of Django form fields)

Thanks!

I had a look at this out of curiosity, and I think most of the "magic" actually comes for pytest-django.

In the Django source code, e.g. for CharField, there is nothing that could really give a type hinter the notion that this is a string. And since the class inherits only from Field, which is also the parent of other non-string fields, the knowledge needs to be encoded elsewhere.

On the other hand, digging through the source code for pylint-django, though, I found where this most likely happens:

in pylint_django.transforms.fields, several fields are hardcoded in a similar fashion:

_STR_FIELDS = ('CharField', 'SlugField', 'URLField', 'TextField', 'EmailField',
               'CommaSeparatedIntegerField', 'FilePathField', 'GenericIPAddressField',
               'IPAddressField', 'RegexField', 'SlugField')

Further below, a suspiciously named function apply_type_shim, adds information to the class based on the type of field it is (either 'str', 'int', 'dict', 'list', etc.)

This additional information is passed to inference_tip, which according to the astroid docs, is used to add inference info (emphasis mine):

astroid can be used as more than an AST library, it also offers some basic support of inference, it can infer what names might mean in a given context, it can be used to solve attributes in a highly complex class hierarchy, etc. We call this mechanism generally inference throughout the project.

astroid is the underlying library used by Pylint to represent Python code, so I'm pretty sure that's how the information gets passed to Pylint. If you follow what happens when you import the plugin, you'll find this interesting bit in pylint_django/.plugin, where it actually imports the transforms, effectively adding the inference tip to the AST node.

I think if you want to achieve the same with your own classes, you could either:

1. Directly derive from another Django model class that already has the associated type you're looking for.
2. Create, and register an equivalent pylint plugin, that would also use Astroid to add information to the class so that Pylint know what to do with it.