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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_custom_value(self, key):
 """Return the first and only custom parameter matching the given name."""

        self._handled.add(key)
        values = self._lookup[key]
        if len(values) > 1:
            raise RuntimeError(
                "More than one value for this customParameter: {}".format(key)
            )
        if values:
            return values[0]
        return None