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Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 def build_parameter_descriptions(obj, user_p=None, output="csv", show_none=True, ignore=None, plist=None):
 """ Creates a list of the decription of all the inputs of an object :param obj: object, that has parameters :param user_p: dict, user defined parameter descriptions :param show_none: if false, only shows descriptions of parameters that are not None :param ignore: list of parameters to not build :return: """

    if user_p is None:
        user_p = {}
    if ignore is None:
        ignore = []
    para = [[obj.__class__.__name__ + " inputs:", "", ""]]
    if plist is None:
        if not hasattr(obj, 'inputs'):
            raise exceptions.ModelError("Object must contain parameter 'inputs' or set plist as an input")
        plist = obj.inputs

    p_dict = {}
    if hasattr(obj, 'ancestor_types'):
        bt = obj.ancestor_types
        for otype in bt:  # cycles from lowest class, so descriptions get overridden
            if otype in vp:
                for item in vp[otype]:
                    p_dict[item] = vp[otype][item]
        for item in user_p:  # user defined definitions override base ones
            p_dict[item] = user_p[item]
    else:
        p_dict = user_p

    for item in plist:
        if show_none is False and getattr(obj, item) is None:
            continue
        if item in ignore:
            continue
        if item in p_dict:
            para.append([item, p_dict[item][1], p_dict[item][0]])
        else:
            para.append([item, "", ""])
    if output == "csv":
        out_obj = []
        for i in range(len(para)):
            out_obj.append(",".join(para[i]))
    elif output == "list":
        out_obj = para
    elif output == "dict":
        out_obj = OrderedDict()
        for i in range(len(para)):
            out_obj[para[i][0]] = {"description": para[i][2], "units": para[i][1]}
    else:
        raise ValueError("output must be either: 'csv', 'dict' or 'list'.")
    return out_obj