[FIX] fields: implement _description_digits

Otherwise, fields_get returns the string, not the digits
Add test

odoo/addons/test_new_api/demo_data.xml

@@ -45,5 +45,11 @@
         <record id="bool_4" model="domain.bool">
             <field name="bool_undefined" eval="False"/>
         </record>
+
+        <record id="decimal_new_api_number" model="decimal.precision">
+            <field name="name">New API Precision</field>
+            <field name="digits" eval="3"/>
+        </record>
+
     </data>
 </odoo>

odoo/addons/test_new_api/models.py

@@ -284,6 +284,7 @@ class MixedModel(models.Model):
     _description = 'Test New API Mixed'
 
     number = fields.Float(digits=(10, 2), default=3.14)
+    number2 = fields.Float(digits='New API Precision')
     date = fields.Date()
     now = fields.Datetime(compute='_compute_now')
     lang = fields.Selection(string='Language', selection='_get_lang')

odoo/addons/test_new_api/tests/test_new_fields.py

@@ -443,6 +443,12 @@ class TestFields(common.TransactionCase):
         record.number = 2.4999999999999996
         self.assertEqual(record.number, 2.50)
 
+    def test_21_float_digits(self):
+        """ test field description """
+        precision = self.env.ref('test_new_api.decimal_new_api_number')
+        description = self.env['test_new_api.mixed'].fields_get()['number2']
+        self.assertEqual(description['digits'], (16, precision.digits))
+
     def check_monetary(self, record, amount, currency, msg=None):
         # determine the possible roundings of amount
         if currency:

odoo/fields.py

@@ -1290,7 +1290,9 @@ class Float(Field):
             return self._digits
 
     _related__digits = property(attrgetter('_digits'))
-    _description_digits = property(attrgetter('_digits'))
+
+    def _description_digits(self, env):
+        return self.get_digits(env)
 
     def convert_to_column(self, value, record, values=None, validate=True):
         result = float(value or 0.0)