[berry] How to access nested json objects.

[chromoxdor]

Hi there,

I have written a small script to publish Zigbee device events for devices running ESPEasy.
My script works fine. But I couldn't get the first field and therefore couldn't get to the nested object for further processing when handling this as a map.
The workaround is to convert the json map to a string and extract the field name to use with find() to get the nested object.

I wonder if there is a way to handle this without converting the map to a string.
I would also appreciate constructive criticism on my code as this is my first time using berry.

Here is a demo json:
{"TH1":{"Device":"0x96E4","Name":"TH1","Temperature":20.2,"Humidity":47.1,"Endpoint":1,"LinkQuality":51}}
And here is the script:

#
# SetOption111 1
# 1 = use frequency output for buzzer pin instead of on/off signal, for piezo buzzers
#
# SetOption83 1
# Uses Zigbee device friendly name instead of 16 bits short addresses
#
# Set friendly names:
# ZbName <device>,<name>
#
#######################################################################################

# change broadcast ip and udp port here

var broadcast_ip = "192.168.1.255"
var udp_port = 8266


#######################################################################################
import string
u = udp()

def send_to_espeasy(payload_json)
  var p0 = str(payload_json)
  var i=string.find(p0,":")
  var p1=string.split(p0,i-1)
  var p2 = p1[0]
  i=string.find(p2,"'")
  p1 = string.split(p2,i+1)
  var object_name = p1[1]
  var nested_object = payload_json.find(object_name)
  
  if nested_object.find("Name")
   var name = str(nested_object.find("Name"))
   var sendvalue = ""
   print(name)
# temp/hum sensor:
    if nested_object.find("Humidity") != nil && nested_object.find("Temperature") != nil # does the payload contain 'both' fields
     var temp = str(nested_object.find("Temperature"))
     var hum = str(nested_object.find("Humidity"))
     sendvalue = temp + "," + hum
     print("temp/hum")

# windowopen: 
    elif nested_object.find("ZoneStatusChange") != nil   
     sendvalue = str(nested_object.find("ZoneStatusChange"))
     print("window")
  
# buttons:  
    elif nested_object.find("LidlPower") != nil           
     sendvalue = str(nested_object.find("LidlPower"))
     print("button")
 
    else
     print("not in list")
     sendvalue = "" 
    end
    
    if sendvalue != ""
     var event= "event,zb_" + name + "=" + sendvalue
     print(event)
     u.begin("", 0)
     u.send(broadcast_ip, udp_port, bytes().fromstring(event)) 
    end

 end
end

tasmota.add_rule("ZbReceived",send_to_espeasy)

[sfromis]

I'd recommend against doing any JSON parsing at the string level, to instead do all the processing at the map level, much less code than trying to take apart strings.

"Discovering" what elements are in a map, including nested maps, is based on the keys() function on a map.
If you simply want the first key: object_name = payload_json.keys()()
What happens here is that keys() returns an iterator function, and calling that function once will return the first available value.

You can also iterate through a plain map with a construct like:

  for key : payload_json.keys()
    var value = payload_json[key]
    print(key, value)
  end

And here's an example of a more generalized construct for traversing multiple nested map levels recursively, in this case calling a function on each leaf value:

  def traverse(f, from)
    for key : from.keys()
      var entry = from[key]
      if classname(entry) == 'map'
        self.traverse(f, entry)
      else
        f(key, entry)
      end
    end
  end

An example of things you can do shorter would be:

    if nested_object.find("Humidity") != nil && nested_object.find("Temperature") != nil # does the payload contain 'both' fields
     var temp = str(nested_object.find("Temperature"))
     var hum = str(nested_object.find("Humidity"))
     sendvalue = temp + "," + hum

Same results with:

    var temp = nested_object.find("Temperature")
    var hum = nested_object.find("Humidity")
    if hum != nil && temp != nil # does the payload contain 'both' fields
     sendvalue = f"{temp},{hum}"

Using f-strings can be nice for building strings, without explicit string conversions, and you could also apply a formatting for more control over details like decimals.

[chromoxdor]

Thanks a lot for this detailed answer!

What happens here is that keys() returns an iterator function, and calling that function once will return the first available value.

Where can i read about this. I couldn't find anything regarding this?

[sfromis]

The available function on a map is in the docs: https://berry.readthedocs.io/en/latest/source/en/Chapter-7.html#keys-method

And the for statement docs explains how an iterator works.
https://berry.readthedocs.io/en/latest/source/en/Chapter-4.html#for-statement

The range construct like for i : 1..6 also works like an iterator, producing values until the end.

Hence, each call of an iterator will produce a value, or raise an exception if no more. Unsurprisingly, first call will give first available value.

For context, it is also a normal construct to be able to have a function in a variable, or simply as a return value, and then call that function with an argument list. And just () is the empty argument list.

[chromoxdor]

Thanks again. That is very helpful!