Merge pull request #7866 from luca-nardelli/improve-mandatory-variables-issues

Report which variable fails interpolation when they are mandatory

compose/config/interpolation.py

@@ -111,12 +111,14 @@ class TemplateWithDefaults(Template):
             var, _, err = braced.partition(':?')
             result = mapping.get(var)
             if not result:
+                err = err or var
                 raise UnsetRequiredSubstitution(err)
             return result
         elif '?' == sep:
             var, _, err = braced.partition('?')
             if var in mapping:
                 return mapping.get(var)
+            err = err or var
             raise UnsetRequiredSubstitution(err)
 
     # Modified from python2.7/string.py

tests/unit/config/interpolation_test.py

@@ -416,7 +416,7 @@ def test_interpolate_mandatory_no_err_msg(defaults_interpolator):
     with pytest.raises(UnsetRequiredSubstitution) as e:
         defaults_interpolator("not ok ${BAZ?}")
 
-    assert e.value.err == ''
+    assert e.value.err == 'BAZ'
 
 
 def test_interpolate_mixed_separators(defaults_interpolator):