如何使用arcpy计算TEXT字段？

有一个DEM影像，先创建了不同网眼数目的渔网，之后想要将每张渔网的各个孔提出来成为单独的shp，这个操作需要渔网有TEXT的字段，所以想要添加一个TEXT字段，之后将其赋值上FID的字符串。 运行报错说不是PYTHON的语句，请问应该怎么写呢？

# -*- coding: cp936 -*-

import arcpy

import os

from arcpy import env

env.workspace = "D:\\dasanshang\\Scrip"

# Set coordinate system of the output fishnet

env.outputCoordinateSystem = arcpy.SpatialReference("WGS 1984 UTM Zone 46N")

originCoordinate = '305165.223137 3208374.91521'

yAxisCoordinate = '305165.223137 3208375.91521'

#创建渔网

for i in range(10,100,10):

    outFeatureClass = "fishnet"+str(i)+".shp"

    # Set the origin of the fishnet

    cellSizeWidth = 0

    cellSizeHeight = 0

    numRows = str(i)

    numColumns = str(i)

    oppositeCoorner = '403565.223137 3320484.91521'

    NoLables = 'NO_LABLES'

    templateExtent = '#'

    geometryType = 'POLYGON'

    arcpy.CreateFishnet_management(outFeatureClass, originCoordinate, yAxisCoordinate, cellSizeWidth, cellSizeHeight,numRows, numColumns, oppositeCoorner, "NO_LABELS", "#", geometryType)



#Spilt成不同的shp 用于分割栅格

for i in range(10,100,10):

    in_features = "fishnet" + str(i) + ".shp"

    split_features = "fishnet" + str(i) + ".shp"

    

    out_workspace = str(env.workspace) + "\\temp\\fishnet"+ str(i) 

    os.mkdir(os.path.join(str(env.workspace)+"\\temp" , "fishnet"+str(i)))

    #给每个渔网新建字段

    fieldname = "ID" + str(i)

    arcpy.AddField_management (in_features, fieldname, "TEXT")

    expression = 'str(!FID!)'

    #给新建字段赋值

    arcpy.CalculateField_management (in_features, fieldname,expression,"PYTHON3")



    split_field = fieldname

    arcpy.Split_analysis(in_features, split_features,split_field, out_workspace, "#")