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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 _build_param_string(params):
 """Build query params string from a dictionary. Args: params (dict):
 A dictionary of params Returns: string: A valid url query params string. """

        pairs = []
        for key, value in params.iteritems():
            if value is None:
                value = ''
            pairs.append('{0}={1}'.format(key, value))
        if len(pairs) > 0:
            return '?{0}'.format('&'.join(pairs))
        return ''