[1.8.x] Fixed #24391 -- Made BoundField.value() cache callable values.

Backport of 65441bbdb0 from master

django/forms/forms.py

@@ -31,6 +31,8 @@ def pretty_name(name):
         return ''
     return name.replace('_', ' ').capitalize()
 
+UNSET = object()
+
 
 def get_declared_fields(bases, attrs, with_base_fields=True):
     """
@@ -134,6 +136,7 @@ class BaseForm(object):
         # Instances should always modify self.fields; they should not modify
         # self.base_fields.
         self.fields = copy.deepcopy(self.base_fields)
+        self._bound_fields_cache = {}
 
     def __str__(self):
         return self.as_table()
@@ -161,7 +164,9 @@ class BaseForm(object):
         except KeyError:
             raise KeyError(
                 "Key %r not found in '%s'" % (name, self.__class__.__name__))
-        return BoundField(self, field, name)
+        if name not in self._bound_fields_cache:
+            self._bound_fields_cache[name] = BoundField(self, field, name)
+        return self._bound_fields_cache[name]
 
     @property
     def errors(self):
@@ -527,6 +532,7 @@ class BoundField(object):
         else:
             self.label = self.field.label
         self.help_text = field.help_text or ''
+        self._initial_value = UNSET
 
     def __str__(self):
         """Renders this field as an HTML widget."""
@@ -621,12 +627,16 @@ class BoundField(object):
         if not self.form.is_bound:
             data = self.form.initial.get(self.name, self.field.initial)
             if callable(data):
-                data = data()
-                # If this is an auto-generated default date, nix the
-                # microseconds for standardized handling. See #22502.
-                if (isinstance(data, (datetime.datetime, datetime.time)) and
-                        not getattr(self.field.widget, 'supports_microseconds', True)):
-                    data = data.replace(microsecond=0)
+                if self._initial_value is not UNSET:
+                    data = self._initial_value
+                else:
+                    data = data()
+                    # If this is an auto-generated default date, nix the
+                    # microseconds for standardized handling. See #22502.
+                    if (isinstance(data, (datetime.datetime, datetime.time)) and
+                            not getattr(self.field.widget, 'supports_microseconds', True)):
+                        data = data.replace(microsecond=0)
+                    self._initial_value = data
         else:
             data = self.field.bound_data(
                 self.data, self.form.initial.get(self.name, self.field.initial)

tests/forms_tests/tests/test_forms.py

@@ -4,6 +4,7 @@ from __future__ import unicode_literals
 import copy
 import datetime
 import json
+import uuid
 
 from django.core.exceptions import NON_FIELD_ERRORS
 from django.core.files.uploadedfile import SimpleUploadedFile
@@ -1376,6 +1377,20 @@ class FormsTestCase(TestCase):
         self.assertEqual(bound['password'].value(), 'foo')
         self.assertEqual(unbound['password'].value(), None)
 
+    def test_boundfield_initial_called_once(self):
+        """
+        Multiple calls to BoundField().value() in an unbound form should return
+        the same result each time (#24391).
+        """
+        class MyForm(Form):
+            name = CharField(max_length=10, initial=uuid.uuid4)
+
+        form = MyForm()
+        name = form['name']
+        self.assertEqual(name.value(), name.value())
+        # BoundField is also cached
+        self.assertIs(form['name'], name)
+
     def test_boundfield_rendering(self):
         """
         Python 2 issue: Test that rendering a BoundField with bytestring content