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Development Methods

Last updated: 2023-09-27 16:20:05

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Function Form

The general form of a Python function is as follows:
import json

def main_handler(event, context):
    print("Received event: " + json.dumps(event, indent = 2)) 
    print("Received context: " + str(context))
  return("Hello World")

Execution Method

An execution method should be specified when each SCF function is created. The execution method of the Python programming language is similar to index.main_handler, where index refers to the index.py entry file, and main_handler refers to the main_handler entry function. When submitting the zip code package by uploading a local zip package or uploading it through COS, the root directory of the package should contain the specified entry file and the file should contain the specified entry function. The filename and function name should match those entered in the execution method to ensure successful execution.

Input Parameters

In the Python environment, the input parameters include 'event' and 'context', both of which are Python dict types.
event: this parameter is used to pass the trigger event data.
context: this parameter is used to pass runtime information to your handler.
The event parameter varies with trigger or event source. For more information on its data structure, see Trigger Overview.

Response

Your handler can use return to return a value. The return value will be handled differently depending on the function invocation type.
In the Python environment, a serializable object such as dict object can be directly returned:
def main_handler(event, context):
    resp = {
        "isBase64Encoded": False,
        "statusCode": 200,
        "headers": {"Content-Type":"text/html","Key":["value1","value2","value3"]},
        "body": "<html><body><h1>Heading</h1><p>Paragraph.</p></body></html>"
}
    return(resp)
Two methods are available to return values:
Sync invocation: the return value of a sync invocation will be serialized in JSON and returned to the caller for subsequent processing. The function testing in the console is sync invocation, which can capture the function’s return value and display it after the invocation is completed.
Async invocation: the return value of an async invocation will be discarded, since the invocation method responds right after the function is triggered, without waiting for function execution to complete.
Note
The return value of both sync and async invocations will be recorded in the function logs.

Exception Handling

You can throw an exception using raise Exception inside the function.
If you capture and handle exceptions before returning, and do not continue to throw them, the function will be considered to have executed successfully and will return the information specified in return in the entry function. In the following example code, if the function executes successfully, the return value is Hello World.
# -*- coding: utf8 -*-
def main_handler(event, context):
  try:
  print("try exception")
  raise Exception("err msg")
  except Exception as e:
      print(e)
  return("Hello World")
If exceptions are not captured before returning, they will be thrown outside the entry function and captured by the function platform. At this point, the function is considered to have failed, and the function return information is replaced with failure information. In the following example code, the function fails.
# -*- coding: utf8 -*-
def main_handler(event, context):
  print("try exception")
  raise Exception("err msg")
  return("Hello World")
The function returns information similar to the following:
{
  "errorCode":-1,
  "errorMessage":"user code exception caught",
  "requestId":"a325b967-ef5b-4aa3-a329-c6bb0df72948",
  "stackTrace":"Traceback (most recent call last):\n  File \"/var/user/index.py\", line 4, in main_handler\n    raise Exception(\"err msg\")\nException: err msg",
  "statusCode":430
}
The errorCode field indicates a code error, and errorMessage provides error details. The stackTrace field indicates an error stack, and statusCode provides error details. For more information about statusCode, see Function Status Code.
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