fields: use lazy evaluation to compute prefill value from template (#51301)

tests/form_pages/test_block.py

@@ -1325,3 +1325,45 @@ def test_block_with_dynamic_item_field(mock_urlopen, pub, blocks_feature):
     resp = resp.form.submit()
     # second field value is kept
     assert resp.form['f2'].value == '1'
+
+
+def test_block_used_in_later_prefill(pub, blocks_feature):
+    FormDef.wipe()
+    BlockDef.wipe()
+
+    block = BlockDef()
+    block.name = 'foobar'
+    block.fields = [
+        fields.StringField(id='123', required=True, label='Amount', type='string', varname='amount'),
+    ]
+    block.store()
+
+    formdef = FormDef()
+    formdef.name = 'form title'
+    formdef.fields = [
+        fields.PageField(id='0', label='1st page', type='page'),
+        fields.BlockField(
+            id='1', label='test', type='block:foobar', varname='data', max_items=3, hint='hintblock'
+        ),
+        fields.PageField(id='2', label='2nd page', type='page'),
+        fields.StringField(
+            id='3', label='sum', prefill={'type': 'string', 'value': '{{form_var_data|getlist:"amount"|sum}}'}
+        ),
+    ]
+    formdef.store()
+
+    app = get_app(pub)
+    resp = app.get(formdef.get_url())
+    resp.form['f1$element0$f123'] = '5'
+    resp = resp.form.submit('f1$add_element')
+    resp.form['f1$element1$f123'] = '3'
+    resp = resp.form.submit('f1$add_element')
+    resp.form['f1$element2$f123'] = '2'
+
+    resp = resp.form.submit('submit')  # -> 2nd page
+    assert resp.form['f3'].value == '10'
+
+    resp = resp.form.submit('previous')  # -> 1st page
+    resp.form['f1$element2$f123'] = '1'
+    resp = resp.form.submit('submit')  # -> 2nd page
+    assert resp.form['f3'].value == '9'

wcs/fields.py

@@ -421,7 +421,7 @@ class Field(object):
             if not Template.is_template_string(value):
                 return (value, explicit_lock)
 
-            context = get_publisher().substitutions.get_context_variables()
+            context = get_publisher().substitutions.get_context_variables(mode='lazy')
             try:
                 return (Template(value, autoescape=False, raises=True).render(context), explicit_lock)
             except TemplateError: