Add simple query projection

[jg-rp]

This PR adds a select method to the query iterator interface that returns a projection of each JSONPath match by selecting a subset of its values.

Given example data from Stefan Goessner's original JSONPath article, this example selects just the title and price fields for each book with an ISBN.

import json
import jsonpath

# data = ...

it = jsonpath.query("$..book[?(@.isbn)]", data).select("title", "price")
print(json.dumps(list(it), indent=4))

Output

[
    {
        "title": "Moby Dick",
        "price": 8.99
    },
    {
        "title": "The Lord of the Rings",
        "price": 22.99
    }
]

Normally, without the call to select, we'd get the entire book object.

# ...
it = jsonpath.query("$..book[?(@.isbn)]", data).values()
print(json.dumps(list(it), indent=4))

Output

[
    {
        "category": "fiction",
        "author": "Herman Melville",
        "title": "Moby Dick",
        "isbn": "0-553-21311-3",
        "price": 8.99
    },
    {
        "category": "fiction",
        "author": "J. R. R. Tolkien",
        "title": "The Lord of the Rings",
        "isbn": "0-395-19395-8",
        "price": 22.99
    }
]

We can select nested values too.

import json
import jsonpath

data = {
    "store": {
        "book": [
            {
                "category": "reference",
                "author": "Nigel Rees",
                "title": "Sayings of the Century",
                "price": 8.95,
            },
            {
                "category": "fiction",
                "author": "Evelyn Waugh",
                "title": "Sword of Honour",
                "price": 12.99,
            },
            {
                "category": "fiction",
                "author": "Herman Melville",
                "title": "Moby Dick",
                "isbn": "0-553-21311-3",
                "price": 8.99,
            },
            {
                "category": "fiction",
                "author": "J. R. R. Tolkien",
                "title": "The Lord of the Rings",
                "isbn": "0-395-19395-8",
                "price": 22.99,
                "foo": {"bar": 42},
            },
        ],
        "bicycle": {"color": "red", "price": 19.95},
    }
}


it = jsonpath.query("$..book[?(@.isbn)]", data).select("title", "price", "foo.bar")
print(json.dumps(list(it), indent=4))

Output

[
    {
        "title": "Moby Dick",
        "price": 8.99
    },
    {
        "title": "The Lord of the Rings",
        "price": 22.99,
        "foo": {
            "bar": 42
        }
    }
]