Calculate Field - Why does the field populate with the same number, but prints correclty?

Question

Why does the field populate with the same number, but prints correclty?

import arcpy
import rivers
arcpy.env.overwriteOutput = True
arcpy.env.workspace = r"C:\Data"

import math
def sinuosity(shape):
    channel = shape.length
    deltaX = shape.firstPoint.X -shape.lastPoint.X
    deltaY = shape.firstPoint.Y -shape.lastPoint.Y
    valley = math.sqrt(pow(deltaX, 2) + pow(deltaY, 2))
    return channel/valley

fc = "streams.shp"
new_field = "Sindex"
with arcpy.da.SearchCursor(fc, ["OID@", "SHAPE@"]) as cursor:
        for row in cursor:
            oid = row[0]
            shape =row[1]
            si = rivers.sinuosity(shape)
            print (f"Stream ID {oid} has an sinuosity index of {si:.3f}")

arcpy.AddField_management (fc, new_field, "DOUBLE" )
arcpy.CalculateField_management(fc,new_field, si)

Answer 1

Your print() is inside a SearchCursor. In your SearchCursor you also define the variable si for your CalculateField. So you write in CalculateField_management for all features the last si from the SearchCursor.

SearchCursor can not modify rows. Try it with an arcpy.da.UpdateCursor():

# not tested
fc = "streams.shp"
new_field = "Sindex"
arcpy.AddField_management(fc, new_field, "DOUBLE" )
with arcpy.da.UpdateCursor(fc, ["OID@", "SHAPE@", new_field]) as cursor:
        for row in cursor:
            oid = row[0]
            shape =row[1]
            si = rivers.sinuosity(shape)
            # set si for new_field
            row[2]=si
            print (f"Stream ID {oid} has an sinuosity index of {si:.3f}")
            # save / update the actual row
            cursor.updateRow(row)