Fix nullability of blank charfields in values and values_list (#1020)

Char fields with blank=True set should not be considered nullable in the
context of values() and values_list() querysets.

I'm also not a huge fan of the way these fields are made optional in the
constructur to the model classes, it feels like it would be better to
mark the arguments as having a default value, rather than allow sending
in None, but I'd rather keep this fix small and look at the overall
problem at a later point.

mypy_django_plugin/django/context.py

@@ -263,6 +263,9 @@ class DjangoContext:
         return attname
 
     def get_field_nullability(self, field: Union[Field, ForeignObjectRel], method: Optional[str]) -> bool:
+        if method in ("values", "values_list"):
+            return field.null
+
         nullable = field.null
         if not nullable and isinstance(field, CharField) and field.blank:
             return True

tests/typecheck/managers/querysets/test_values.yml

@@ -124,3 +124,21 @@
                     pass
                 class Book(models.Model):
                     authors = models.ManyToManyField(Author, related_name='books')
+
+-   case: queryset_values_blank_charfield
+    main: |
+        from myapp.models import Blog
+        values = Blog.objects.values('text').get()
+        reveal_type(values)  # N: Revealed type is "TypedDict({'text': builtins.str})"
+        reveal_type(values["text"])  # N: Revealed type is "builtins.str"
+    installed_apps:
+        - myapp
+    files:
+        -   path: myapp/__init__.py
+        -   path: myapp/models.py
+            content: |
+                from django.db import models
+
+                class Blog(models.Model):
+                    num_posts = models.IntegerField()
+                    text = models.CharField(max_length=100, blank=True)

tests/typecheck/managers/querysets/test_values_list.yml

@@ -265,3 +265,19 @@
                     pass
                 class Book(models.Model):
                     authors = models.ManyToManyField(Author, related_name='books')
+-   case: queryset_values_list_blank_charfield
+    main: |
+        from myapp.models import Blog
+        values = Blog.objects.values_list('text').get()
+        reveal_type(values)  # N: Revealed type is "Tuple[builtins.str]"
+    installed_apps:
+        - myapp
+    files:
+        -   path: myapp/__init__.py
+        -   path: myapp/models.py
+            content: |
+                from django.db import models
+
+                class Blog(models.Model):
+                    num_posts = models.IntegerField()
+                    text = models.CharField(max_length=100, blank=True)