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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_ext_namespaces(config):
    '''
    Get namespaces from the existing configuration.

    :param config:
    :return:
    '''
    namespaces = {}
    if not config:
        return namespaces

    for ns in config:
        constraint_version = tuple(config[ns].get('py-version', []))
        if not constraint_version:
            raise salt.exceptions.SaltSystemExit("An alternative version is configured, but not defined "
                                                 "to what Python's major/minor version it should be constrained.")
        else:
            namespaces[ns] = constraint_version

    return namespaces