def requestedFormat(request, acceptedFormat):
    """Return the response format requested by client

        Client could specify requested format using:
        (options are processed in this order)
            - `format` field in http request
            - `Accept` header in http request
        Example:
            chooseFormat(request, ['text/html','application/json'])
        Args:
            acceptedFormat: list containing all the accepted format
        Returns:
            string: the user requested mime-type (if supported)
        Raises:
            ValueError: if user request a mime-type not supported
        """
    if 'format' in request.args:
        fieldFormat = request.args.get('format')
        if fieldFormat not in acceptedFormat:
            raise ValueError('requested format not supported: ' + fieldFormat)
        return fieldFormat
    else:
        return request.accept_mimetypes.best_match(acceptedFormat)
