12-C02P02

C02P02 - gson

Implement a Python script called gson.py that reads 2 arguments:

• Path to a JSON file.
• Path inside the JSON file, which can describe properties & array indices.

The program should output:

• To stdout, the value given from the path. In case the program fails, it should exit with a non-zero status.
• To stderr, if the file is not a proper JSON or the path cannot be found in the JSON.

Examples

Lets say we have the following example.json file:

$ cat example.json

which outputs (taken from here)

{
  "squadName": "Super hero squad",
  "homeTown": "Metro City",
  "formed": 2016,
  "secretBase": "Super tower",
  "active": true,
  "properties": {
      "foo": "bar"
  },
  "members": [
    {
      "name": "Molecule Man",
      "age": 29,
      "secretIdentity": "Dan Jukes",
      "powers": [
        "Radiation resistance",
        "Turning tiny",
        "Radiation blast"
      ]
    },
    {
      "name": "Madame Uppercut",
      "age": 39,
      "secretIdentity": "Jane Wilson",
      "powers": [
        "Million tonne punch",
        "Damage resistance",
        "Superhuman reflexes"
      ]
    },
    {
      "name": "Eternal Flame",
      "age": 1000000,
      "secretIdentity": "Unknown",
      "powers": [
        "Immortality",
        "Heat Immunity",
        "Inferno",
        "Teleportation",
        "Interdimensional travel"
      ]
    }
  ]
}

Now, if we have our gson.py in the same directory, we should be able to run the following:

$ python gson.py example.json "members[0].name"

which outputs:

Molecule Man

Basically, the second argument describes a path in the JSON file.

Here are some more examples:

$ python gson.py example.json "members[0].powers[1]"
Turning tiny

$ python gson.py example.json "squadName"
Super hero squad

$ python gson.py example.json "properties"
{
    "foo": "bar"
}

$ python gson.py example.json "properties.foo"
bar

$ python gson.py example.json "shano.property"
Error: Property not found

$ echo $? 
1

We can even have more weird JSON formats:

{
  "items": [
    [1],
    [2]
  ]
}

$ python gson.py weird.json "items[0]"
[1]

$ python gson.py weird.json "items[0][0]"
1

Hints

• We are effectively trying to mimick https://lodash.com/docs/4.17.15#get or https://pypi.org/project/python-benedict/.
• Use https://docs.python.org/3/library/json.html to read & parse the JSON file.
• Use sys.argv to read the arguments.