Adding new value in new field using ArcPy

Question

I have a polyline feature class with this attribute table:

I need to get values without decimal numbers.

I created a new field called "newLength" and tried to insert the new values (whole numbers only) with this code:

import arcpy

# add new field
arcpy.env.workspace = "G:\desktop\Project\lyr\New File Geodatabase.gdb"
arcpy.AddField_management("dis", "newLength", "LONG")

fc = r"G:\desktop\Project\lyr\New File Geodatabase.gdb\dis"
with arcpy.da.UpdateCursor(fc, "Shape_Length") as cursor: # second argument = field name
    for row in cursor:
       newValue = '%.0f' % float(str(row).strip('[]')) # convert list to string

# add the new numbers to the new field
with arcpy.da.UpdateCursor(fc, "newLength") as cursor: # second argument = field name 
   for row in cursor:
      cursor.updateRow([newValue])

but the error is a new field with the same values in all rows:

whereas the second row is true and the first row is an error

Answer 1

You are only filling in the last value of "newValue" in all the rows. Try to modify it this way;

import arcpy

# add new field
arcpy.env.workspace = "G:\desktop\Project\lyr\New File Geodatabase.gdb"
arcpy.AddField_management("dis", "newLength", "LONG")

fc = r"G:\desktop\Project\lyr\New File Geodatabase.gdb\dis"


# add the new numbers to the new field
with arcpy.da.UpdateCursor(fc, ["Shape_Length", "newLength"]) as cursor: # second argument = field name 
   for row in cursor:
      newValue = '%.0f' % float(str(row[0]).strip('[]')) # convert list to string
      row[1] = newValue
      cursor.updateRow(row)

Answer 2

Your approach is overkill and the looping flawed. Add the new field as you have done and then simply run the calculate field tool and set the expression to be:

round(!Shape_Length!,0)

This will truncate the value, no need for any code. If you need to run this logic as a python script then simply right click on the tool result in the results panel and copy snippet, you'll have the code pre-formatted and ready to run.