因为在使用的时候没找到可参考资料，在师傅指导下才完成的数据筛选，记录一下过程。因为是新新新手，请大家谨慎食用。

一、在函数表达式界面，点加号按钮新添py脚本，起个名字

二、qgis他给出的默认代码，保留import部分，去掉函数的部分自己写，比如我的是这样。

from qgis.core import *
from qgis.gui import *

@qgsfunction(group='Custom', referenced_columns=[])
def my_filter(value1):
    """把字符串变成数字"""

    if "X" in value1:
        # print(value1, "/n============")
        value3 = [str.replace(value1, "X", ",")]
        # print(value3, "/n============")
        if "+" in value3[0]:
            value2 = [str.replace(value3[0], "+", ",")]
            # print("+++++", value2)

        else:
            value2 = value3
            # print("$$$$$$", value2)
    elif "+" in value1:
        value2 = [str.replace(value1, "+", ",")]

    elif "Y2" in value1:
        value2 = [str.replace(value1, "Y2", "")]
    elif "Z2" in value1:
        value2 = [str.replace(value1, "Z2", "")]
    elif "Y3" in value1:
        value2 = [str.replace(value1, "Y3", "")]
    else:
        value2 = value1

    if type(value2) == list:
        list1 = value2[0].split(",")
        for i in range(len(list1)):
            list1[i] = int(list1[i])
        list1.sort(reverse=1)
        value2 = list1[0]

    return value2

在写的过程中最难的是调试这些数据类型处理过程中的报错。。。

还有就是要理解qgis的处理对象，因为我要按照字段处理每个单元格的值，我处理的是单元格值对象，他是该字段定义的数据类型如STR，而不是列表或者array等其他数据类型。

三、在vscode调试时，需要输入单个数据进行调试，在vscode中添加这样一段进行调试使用。

if __name__ == "__main__":
    for i in range(10):
        num = input("输入你的数据")
        val = my_filter(num)
        print(val)

10.20更新

把上述问题用gdal解决了一下！遇到的问题有数据创建和保存时各个层级的数据的关系，中文编码如中文字段的创建存在乱码问题

from osgeo import ogr, gdal


def pip_filter(inshp):
    # 1.打开原数据读取信息
    datasource = ogr.Open(inshp)
    print("数据信息", datasource)
    inlayer = datasource.GetLayer(0)
    print(inlayer)
    print("图层中要素数:", inlayer.GetFeatureCount())
    # 获取文件空间参考
    spatial = inlayer.GetSpatialRef()
    # 2.创建新文件，要素另存
    gdal.SetConfigOption("GDAL_FILENAME_IS_UTF8", "YES")
    gdal.SetConfigOption("SHAPE_ENCODING", "GBK")

    driver = ogr.GetDriverByName("ESRI Shapefile")
    new_file = driver.CreateDataSource("filtered_pipe.shp")
    # 设置图层对象
    new_layer = new_file.CreateLayer("pipe", spatial, ogr.wkbLineString)
    inlayer_defn = inlayer.GetLayerDefn()

    # 复制全部的字段
    field_names = []
    for i in range(0, inlayer_defn.GetFieldCount()):
        field_defn = inlayer_defn.GetFieldDefn(i)
        field_name = field_defn.name

        field_names.append(field_name)
        # 输出layer创建字段
        new_layer.CreateField(field_defn)

    # 2.遍历要素，将数据清洗为有效值
    inlayer.ResetReading()
    for i in range(inlayer.GetFeatureCount()):
        feature1 = inlayer.GetNextFeature()
        # while feature1:
        # print("-------------------开始判断---------------")
        #     feature1=layer.GetNextFeature()

        # print('要素:',feature1.keys())
        value1 = feature1.GetField("管径")

        # print('初始值:',value1,)
        # print(type(value1))
        if "X" in value1:
            # print(value1, "/n============")
            value3 = [str.replace(value1, "X", ",")]
            # print(value3, "/n============")
            if "+" in value3[0]:
                value2 = [str.replace(value3[0], "+", ",")]
                # print("+++++", value2)

            else:
                value2 = value3
                # print("$$$$$$", value2)
        elif "+" in value1:
            value2 = [str.replace(value1, "+", ",")]

        elif "Y2" in value1:
            value2 = [str.replace(value1, "Y2", "")]
        elif "Z2" in value1:
            value2 = [str.replace(value1, "Z2", "")]
        elif "Y3" in value1:
            value2 = [str.replace(value1, "Y3", "")]
        else:
            value2 = value1

        if type(value2) == list:
            list1 = value2[0].split(",")
            for i in range(len(list1)):
                list1[i] = int(list1[i])
            list1.sort(reverse=1)
            value2 = list1[0]
        feature1.SetField("管径", value2)
        if int(feature1.GetField("管径")) < 600:
            pass
        elif int(feature1.GetField("管径")) >= 600:
            # print(feature1.keys())
            # print(feature1.GetField('管径'))
            new_layer.CreateFeature(feature1)
    new_file.Destroy()
    # new_file.destroy()
    # inlayer.SetAttributeFilter("管径 < 600")

    # for feature in new_layer:
    #     print (feature.GetField("管径"))


# 3.筛选数据
# for feature in new_layer:
#     print( '看看结果',feature.GetField("管径"))


if __name__ == "__main__":
    filepath = r"D:\BaiduSyncdisk\code\python\mygdal\pipe_data\rain_pipe.shp"
    pip_filter(filepath)

3.qgis打开数据，乱码问题的解决：