[Telemetry] Validate args in js_template rendering

telemetry/telemetry/util/js_template.py

Index: telemetry/telemetry/util/js_template.py
diff --git a/telemetry/telemetry/util/js_template.py b/telemetry/telemetry/util/js_template.py
index 61f5d5f9b1d5d56e013d753e78db13fc78a407c0..c167ebc3afa1922baed36966e49d940cd65a4f4b 100644
--- a/telemetry/telemetry/util/js_template.py
+++ b/telemetry/telemetry/util/js_template.py
@@ -38,12 +38,16 @@ def Render(template, **kwargs):
       to interpolate with values.
     **kwargs: Values to be interpolated in the template.
   """
+  unused = set(kwargs)
+
   def interpolate(m):
     field_spec = m.group('field_spec').strip()
     field = RE_FIELD_IDENTIFIER.match(field_spec)
     if not field:
       raise KeyError(field_spec)
-    value = kwargs[field.group('name')]
+    key = field.group('name')
+    value = kwargs[key]
+    unused.discard(key)
     if field.group('modifier') == '@':
       if not isinstance(value, str):
         raise ValueError('Literal value for %s must be a string' % field_spec)
@@ -51,4 +55,8 @@ def Render(template, **kwargs):
     else:
       return RenderValue(value)
 
-  return RE_REPLACEMENT_FIELD.sub(interpolate, template)
+  result = RE_REPLACEMENT_FIELD.sub(interpolate, template)
+  if unused:
+    raise TypeError('Unexpected arguments not used in template: %s.' % (
+      ', '.join(repr(str(k)) for k in sorted(unused))))

[nednguyen, 2017/02/02 14:33:50]

On another hand, ca  we also output error if there are template variables
without matching argument?

[perezju, 2017/02/02 14:42:44]

Those will raise a KeyError when trying to read the arg in line 49. See
testRenderRaisesWithUnknownIdentifier.

+  return result