code: def _build_fields(self, defaults, fields):
          
          return dict(list(defaults.get('@fields', {}).items()) + list(fields.items()))
docs: Return provided fields including any in defaults
  
          >>> f = LogstashFormatter()
          # Verify that ``fields`` is used
          >>> f._build_fields({}, {'foo': 'one'}) == \
                  {'foo': 'one'}
          True
          # Verify that ``@fields`` in ``defaults`` is used
          >>> f._build_fields({'@fields': {'bar': 'two'}}, {'foo': 'one'}) == \
                  {'foo': 'one', 'bar': 'two'}
          True
          # Verify that ``fields`` takes precedence
          >>> f._build_fields({'@fields': {'foo': 'two'}}, {'foo': 'one'}) == \
                  {'foo': 'one'}
          True