This article provides a template and a sample script in Python for parsing data that is sent to custom topics.

Script template

SELF_DEFINE_TOPIC_UPDATE_FLAG = '/user/update' # The custom topic /user/update.
SELF_DEFINE_TOPIC_ERROR_FLAG = '/user/update/error' # The custom topic /user/update/error.

# Call a function to convert the data that is sent by a device to a custom topic into JSON-formatted data when the device reports data to IoT Platform.
# topic: an input parameter that specifies the topic to which the device sends messages. The value is a string.   
# rawData: an input parameter. The value is a list of integers that cannot be empty.
# jsonObj: an output parameter. The value is a JSON object that indicates a dictionary.    
def transform_payload(topic, rawData):
   jsonObj = {}
   return jsonObj

Sample script

Note The following sample script can be used only to parse data that is sent to custom topics. If the Data Type parameter of the product is set to Custom, you also need to write a script for parsing Thing Specification Language (TSL) data. For information about how to write a script for parsing TSL data, see Example for parsing TSL data. 
# coding=UTF-8
SELF_DEFINE_TOPIC_UPDATE_FLAG = '/user/update' # The custom topic /user/update.
SELF_DEFINE_TOPIC_ERROR_FLAG = '/user/update/error' # The custom topic /user/update/error.

#  Sample data:
#  Custom topic: /user/update, to which data is sent.
#  Input parameters:
#     topic: /{productKey}/{deviceName}/user/update
#     bytes: 0x000000000100320100000000
#  Output parameters:
#  {
#     "prop_float": 0,
#     "prop_int16": 50,
#     "prop_bool": 1,
#     "topic": "/{productKey}/{deviceName}/user/update"
#   }
def transform_payload(topic, bytes):
    uint8Array = []
    for byteValue in bytes:
        uint8Array.append(byteValue & 0xff)

    jsonMap = {}
    if SELF_DEFINE_TOPIC_ERROR_FLAG in topic:
        jsonMap['topic'] = topic
        jsonMap['errorCode'] = bytes_to_int(uint8Array[0:1])

    elif SELF_DEFINE_TOPIC_UPDATE_FLAG in topic:
        jsonMap['topic'] = topic
        jsonMap['prop_int16'] = bytes_to_int(uint8Array[5:7])
        jsonMap['prop_bool'] = bytes_to_int(uint8Array[7: 8])
        jsonMap['prop_float'] = bytes_to_int(uint8Array[8:])

    return jsonMap

# Convert a byte array into data of the INT type.
def bytes_to_int(bytes):
    data = ['%02X' % i for i in bytes]
    return int(''.join(data), 16)