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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 _is_valid_function(module_name, function):
    '''
    Determine if a function is valid for a module
    '''
    try:
        functions = __salt__['sys.list_functions'](module_name)
    except salt.exceptions.SaltException:
        functions = ["unable to look up functions"]
    return "{0}.{1}".format(module_name, function) in functions