Provide more information in AnsibleUndefinedVariable (#71666)

* Provide more information in AnsibleUndefinedVariable

Fixes #55152

changelogs/fragments/55152-add-more-info-to-AnsibleUndefinedVariable.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - Provide more information in AnsibleUndefinedVariable (https://github.com/ansible/ansible/issues/55152)

lib/ansible/template/vars.py

@@ -104,8 +104,8 @@ class AnsibleJ2Vars(Mapping):
             value = None
             try:
                 value = self._templar.template(variable)
-            except AnsibleUndefinedVariable:
-                raise
+            except AnsibleUndefinedVariable as e:
+                raise AnsibleUndefinedVariable("%s: %s" % (to_native(variable), e.message))
             except Exception as e:
                 msg = getattr(e, 'message', None) or to_native(e)
                 raise AnsibleError("An unhandled exception occurred while templating '%s'. "

test/integration/targets/template/runme.sh

@@ -22,3 +22,6 @@ ansible-playbook filter_plugins.yml -v "$@"
 
 # https://github.com/ansible/ansible/issues/68699
 ansible-playbook unused_vars_include.yml -v "$@"
+
+# https://github.com/ansible/ansible/issues/55152
+ansible-playbook undefined_var_info.yml -v "$@"

test/integration/targets/template/undefined_var_info.yml

@@ -0,0 +1,15 @@
+- hosts: localhost
+  gather_facts: no
+  vars:
+    foo: []
+    bar: "{{ foo[0] }}"
+  tasks:
+    - debug:
+        msg: "{{ bar }}"
+      register: result
+      ignore_errors: yes
+
+    - assert:
+        that:
+          - '"foo[0]" in result.msg'
+          - '"object has no element 0" in result.msg'