Fixed #28164 -- Improved float conversions in DecimalField.to_python

Thanks Tim Graham and Adam Johnson for the reviews.

django/db/models/fields/__init__.py

@@ -1508,6 +1508,10 @@ class DecimalField(Field):
             validators.DecimalValidator(self.max_digits, self.decimal_places)
         ]
 
+    @cached_property
+    def context(self):
+        return decimal.Context(prec=self.max_digits)
+
     def deconstruct(self):
         name, path, args, kwargs = super().deconstruct()
         if self.max_digits is not None:
@@ -1522,6 +1526,8 @@ class DecimalField(Field):
     def to_python(self, value):
         if value is None:
             return value
+        if isinstance(value, float):
+            return self.context.create_decimal_from_float(value)
         try:
             return decimal.Decimal(value)
         except decimal.InvalidOperation:

tests/model_fields/test_decimalfield.py

@@ -15,6 +15,12 @@ class DecimalFieldTests(TestCase):
         f = models.DecimalField(max_digits=4, decimal_places=2)
         self.assertEqual(f.to_python(3), Decimal('3'))
         self.assertEqual(f.to_python('3.14'), Decimal('3.14'))
+        # to_python() converts floats and honors max_digits.
+        self.assertEqual(f.to_python(3.1415926535897), Decimal('3.142'))
+        self.assertEqual(f.to_python(2.4), Decimal('2.400'))
+        # Uses default rounding of ROUND_HALF_EVEN.
+        self.assertEqual(f.to_python(2.0625), Decimal('2.062'))
+        self.assertEqual(f.to_python(2.1875), Decimal('2.188'))
         with self.assertRaises(ValidationError):
             f.to_python('abc')