fix lint

Author: derrickaw

sdks/python/apache_beam/yaml/yaml_mapping.py

@@ -239,8 +239,8 @@ def _expand_javascript_mapping_func(
 
   if expression:
     unpacking_code = '\n'.join([
-        f'  var {name} = __row__.{name};'
-        for name in original_fields if name in expression
+        f'  var {name} = __row__.{name};' for name in original_fields
+        if name in expression
     ])
     source_code = f"""
     const udf = function(__row__) {{