How to insert a attribute with a dynamic value in many object or add an element dynamically in many array?

Quick Answer (TL;DR)

• Usually it is easy to painlessly transform JSON with JMESPath
  • One pain-free key is to utilize JSON structures that are specifically normalized for optimal use with JMESPath
  • One pain-free key is knowing when to use dictionary (aka objects // associative-arrays // mappings) name-value pairs in your JSON to make all parts of the JSON capable of unambiguous reference
• Unfortunately, the goal in this specific question is not doable with standard JMESPath, because JMESPath lacks a token to refer to the JSON data root in the current-node context

Detailed Answer

Context

• JMESPath query language
• python 3.x using JMESPath 0.9.4 [but any JMESPath engine will do]

Problem

• Scenario:
  • DeveloperSObosskay972 wishes to transform JSON data from one representation to another
  • DeveloperSObosskay972 wants to rely solely on JMESPath expressions to complete the transformation
  • DeveloperSObosskay972 wants to refer to one part of the JSON structure in another part, to allow for dynamic cross-referencing of the datastructure

Attempt 01 (not what we really want)

• the "almost" solution that is not quite what we want
• this code ...

import jmespath
vdata001aa = """<<json.load(JSON Format Example 1)>>"""
vresult = jmespath.compile('@|[*].{"Nature":@.Nature,"EV":@.EV,"Moves":@.Moves,"MovesCount":@.MovesCount,"MaxMouvCount":`3`}').search(vdata001aa)
pprint.pprint(vresult)

• produces this result ...

[{'EV': 'lal',
  'MaxMouvCount': 3,
  'Moves': ['caca1', 'caca2'],
  'MovesCount': 2,
  'Nature': 'loi'},
 {'EV': 'lal',
  'MaxMouvCount': 3,
  'Moves': ['caca1', 'caca2', 'caca3'],
  'MovesCount': 3,
  'Nature': 'loi2'},
 {'EV': 'lil',
  'MaxMouvCount': 3,
  'Moves': ['caca1', 'caca2', 'caca3'],
  'MovesCount': 3,
  'Nature': 'loi3'},
 {'EV': None,
  'MaxMouvCount': 3,
  'Moves': None,
  'MovesCount': None,
  'Nature': None}]

• This is not what we want, because:
  • we had to hardwire the value 3 for MaxMouvCount which is technically "cheating"
    • it's cheating because we want a dynamic value, not a hard-wired value
  • this produces a superfluous element where every value except MaxMouvCount is null (python happens to call this None)
  • we only wanted three elements, not four

Attempt 02 (not what we really want)

• The reason why Attempt 01 does not work well is because the original JSON stucture is not well-normalized for JMESPath
• In order to address this, we add dictionary name-value pairs to the original data
• With this approach, we make every element of the JSON data a value attached to a dictionary key (aka javascript object name-value pairs)
  • Here we use the term dictionary which is known in other contexts as object or hash or associative array or mapping
  • We don't care about the terminology, so much as the ability to refer to all parts of the top-level JSON as name-value pairs

• reformat your original JSON JSON Format Example 1 so it looks like this instead
• this reformatted JSON will make all parts of your data unambiguously addressible

{
"jsontop": {
    "settings_info": {
      "MaxMouvCount": 3
    },
    "nature_table": [
     {
       "Nature": "loi",
       "EV": "lal",
       "Moves": [
         "caca1",
         "caca2"
       ],
       "MovesCount": 2
     },
     {
       "Nature": "loi2",
       "EV": "lal",
       "Moves": [
         "caca1",
         "caca2",
         "caca3"
       ],
       "MovesCount": 3
     },
     {
       "Nature": "loi3",
       "EV": "lil",
       "Moves": [
         "caca1",
         "caca2",
         "caca3"
       ],
       "MovesCount": 3
     }
    ]
}

• this code ...

import jmespath
vdata001aa  = """<<json.load(**RE-NORMALIZED** JSON Format Example 1)>>"""
vresult     = jmespath.compile('@|jsontop.nature_table[*].{"Nature":@.Nature,"EV":@.EV,"Moves":@.Moves,"MovesCount":@.MovesCount,"MaxMouvCount":jsontop.settings_info.MaxMouvCount}').search(vdata001aa)
pprint.pprint(vresult)
pass

• produces this result ...

[{'EV': 'lal',
  'MaxMouvCount': None,
  'Moves': ['caca1', 'caca2'],
  'MovesCount': 2,
  'Nature': 'loi'},
 {'EV': 'lal',
  'MaxMouvCount': None,
  'Moves': ['caca1', 'caca2', 'caca3'],
  'MovesCount': 3,
  'Nature': 'loi2'},
 {'EV': 'lil',
  'MaxMouvCount': None,
  'Moves': ['caca1', 'caca2', 'caca3'],
  'MovesCount': 3,
  'Nature': 'loi3'}]

• this is not what we want, because we get None (aka null) where we expected 3

Explanation

• Attempt 02 would have worked, if JMESPath supported a token to refer to the JSON root object
• An alternate query which would work is (if for example, the dollar-sign character worked as a reference to the JSON data root)

import jmespath
vdata001aa  = """<<json.load(**RE-NORMALIZED** JSON Format Example 1)>>"""
vresult     = jmespath.compile('@|jsontop.nature_table[*].{"Nature":@.Nature,"EV":@.EV,"Moves":@.Moves,"MovesCount":@.MovesCount,"MaxMouvCount":$.jsontop.settings_info.MaxMouvCount}').search(vdata001aa)
pprint.pprint(vresult)
pass

• There is actually a project on GitHub that addresses this limitation of JMESPath
  • https://github.com/grofers/go-codon/wiki/Jmespath-extensions#3-added--to-refer-to-root-node

Conclusion

• Attempt 01 and Attempt 02 show that the current (stable) version of JMESPath does not quite have the ability to meet the requirement
• You will have to break out of JMESPath to get the desired dynamic value from your JSON and populate the reformatted data
• Alternatively, you will have to add an extension to JMESPath itself, which is arguably less desirable than using the functionality of the hosting language