Read YAML file and create Python objects

Question

I am newbie in Python and try to read a YAML-File. Based on its content, I want to create Python objects. I am using ruamel-yaml library. In my case, maybe I have Python-Classes Message, Signal and Signalgroup etc. (See the example file).

My approach would be to read the YAML file, check each line for a given key word and create the related object and fill it with data. I think it is the “old school” approach and maybe there is a much more effective approach to process the file.

Maybe using the function register_class/ rep. creating tags “from_yaml” but as the keys are indexed it would not work.

Message1:
Message2:
Message3:

Is there any much more professional approach?

# Yaml Testfile

- ModuleName: myTestModule
- Version: 1.0
- ModuleNumbers: [96,97,98,99]


- Message1:
   Name: AO3_
   DLC: 8
   Signal1:
     Name: Temperature
     Length: 16
   Signal2:
     Name: AnalogOut3
     Length: 16
     SignalGroup1:  #Comment
        Name: app_fcex
        Type: Bitfield
        Signal1:
            Name: drive_ready
            Length: 1
        Signal2:
            Name: error_active
            Length: 1
        Signal3:
            Name: warning_active            
            Length: 1
   Signal3:
     Name: Temperatur 2
     Length: 8
     ValueTable:
        Name: TempStates
        Item0:
           Name: INIT
           Value: 1
        Item1:
           Name: RUN
           Value: 2
        Item2:
           Name: DONE
           Value: 3
        Item3:
           Name: ERROR
           Value: 4
- Message2:
   name: AO2_
   object: RX2
   DLC: 8

Answer 1

I recommend you use tags in your YAML file, and drop the use of keys with names like Item1 , Item2 (replace with a list of tagged objects).

It is difficult to see the exact structure your data has, but an initial step could be to make the YAML document (assumed to be in a file input.yaml :

- ModuleName: myTestModule
- Version: 1.0
- ModuleNumbers: [96,97,98,99]


- !Message
  Name: AO3_
  DLC: 8
  Signal1:
    Name: Temperature
    Length: 16
  Signal2:
    Name: AnalogOut3
    Length: 16
    SignalGroup1:  #Comment
       Name: app_fcex
       Type: Bitfield
       Signal1:
           Name: drive_ready
           Length: 1
       Signal2:
           Name: error_active
           Length: 1
       Signal3:
           Name: warning_active
           Length: 1
  Signal3:
    Name: Temperatur 2
    Length: 8
    ValueTable:
       Name: TempStates
       items:
       - !Item
         Name: INIT
         Value: 1
       - !Item
         Name: RUN
         Value: 2
       - !Item
         Name: DONE
         Value: 3
       - !Item
         Name: ERROR
         Value: 4
- !Message
  name: AO2_
  object: RX2
  DLC: 8

and load that with:

import sys
import ruamel.yaml


class Item:
    def __init__(self, name=None, value=None):
        self.name = name
        self.value = value

    @classmethod
    def from_yaml(cls, constructor, node):
        for m in constructor.construct_yaml_map(node):
            pass
        return cls(m['Name'], m['Value'])

    def __repr__(self):
        return 'Item(name={.name}, value={.value})'.format(self, self)

class Message:
    def __init__(self, name=None, DLC=None, object=None, signals=None):
        self.name = name
        self.dlc = DLC
        self.object = object
        self.signals = [] if signals is None else signals

    @classmethod
    def from_yaml(cls, constructor, node):
        for m in constructor.construct_yaml_map(node):
            pass
        if 'Name' in m:
            name = m['Name']
        elif 'name' in m:
            name = m['name']
        else:
            name = None
        object = m['object'] if 'object' in m else None
        if 'DLC' in m:
            dlc = m['DLC']
        else:
            dlc = None
        if 'signals' in m:
            signals = m['signals']
        elif 'Signal1' in m:
            x = 1
            signals = []
            while True:
                name = "Signal{}".format(x)
                try:
                    signals.append(m[name])
                except KeyError:
                    break
                x += 1
        else:
            signals = None
        return cls(name, dlc, object, signals)

    def __repr__(self):
        return 'Message(name={}, DLC={}, object={}, signals{})'.format(
            self.name, self.dlc, self.object, '[...]' if self.signals else '[]',
        )

yaml = ruamel.yaml.YAML(typ='safe')
yaml.register_class(Item)
yaml.register_class(Message)
with open('input.yaml') as fp:
    data = yaml.load(fp)

The above has some, but limited checking on key availability (and eg normalizes Name and name for !Message .

With the above print('data') gives (wrapping done by hand):

[{'ModuleName': 'myTestModule'}, 
 {'Version': 1.0}, 
 {'ModuleNumbers': [96, 97, 98, 99]}, 
  Message(name=Signal4, DLC=8, object=None, signals[...]), 
  Message(name=AO2_, DLC=8, object=RX2, signals[])]

and print(data[3].signals[2]['ValueTable']['items'][2]) gives:

Item(name=DONE, value=3)

Further classes should of course be added as appropriate.