arcpy.AddField_management(layerName, "BG10_INT", "LONG", 12, "", "", "", "NULLABLE", "")
expression = "toInt(!BG10!)"
codeblock = """def toInt(value):
    if value.isdigit():
        return int(value)
    else:
        return 0"""
arcpy.CalculateField_management (layerName, "BG10_INT", expression, "PYTHON", codeblock)

for row in rows:
    if row.BG10.isdigit():
        row.BG10_INT = int(row.BG10)
    else:
        row.BG10_INT = 0
    rows.updateRow(row)