How to pass variable to JSON, for python?

Question

I am new in work with JSON, so sorry in advance for the stupid question.

I want to write JSON with the variable in the value field. It looks like this:

def print_json(user_name):
    opened_json = open('way/to/json/file')
    tmp = json.load(opened_json)

    res = tmp(['path_to_folder'](user_name))
    print(res)

def main(user_name):
    print_json(user_name)


main('user')

It is JSON:

{"path_to_folder": "/Users/" + user_name + "/my_folder/"}

Awaiting for that output:

/Users/user/my_folder/

Please, tell me if any solution here exists.

Thanks in advance!

EDIT: My problem, that I can't add variable to JSON correctly. It marked red. Wrong syntax, when I try to concat.

tmp will be a regular python dictionary containing the json object(dictionary) from your file. — Tom Dalton
– Tom Dalton, Commented Jun 4, 2020 at 13:00
{"path_to_folder": "/Users/" + user_name + "/my_folder/"} isn't valid JSON to start with. — chepner
– chepner, Commented Jun 4, 2020 at 13:12
Sorry, it's really unclear what you are asking. You have an existing json file, or you want to create a totally new json file? — Tom Dalton
– Tom Dalton, Commented Jun 4, 2020 at 13:12

L3viathan · Accepted Answer · 2020-06-04 14:40:33Z

6

What you want isn't directly possible in JSON, because it doesn't support "templating".

One solution would be to use a templating language such as Jinja to write a JSON template, then load this file without the json library and fill in the values using Jinja, and finally use json.loads to load a dictionary from your rendered string.

Your json-like file could look something like this:

{"path_to_folder": "/Users/{{ user_name }}/my_folder/"}

Your Python code:

import json
from jinja2 import Environment, FileSystemLoader

env = Environment(
    FileSystemLoader("path/to/template")
)
template = env.get_template("template_filename.json")

def print_json(username):
    return json.loads(
        template.render(user_name=username)
    )
...

In fact, if this is a simple one-time thing, it might even be better to use Python's built-in templating. I would recommend old-style formatting in the case of JSON, because otherwise you'll have to escape a lot of braces:

JSON file:

{"path_to_folder": "/Users/%(user_name)s/my_folder/"}

"Rendering":

with open("path/to/json") as f:
    rendered = json.loads(f.read() % {"user_name": username})

edited Jun 4, 2020 at 14:40

answered Jun 4, 2020 at 13:13

L3viathan

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4 Comments

Vas23Vewe Over a year ago

Thank you! I though that it works like usual concat for string...

Vas23Vewe Over a year ago

TypeError: no loader for this environment specified. Not sure, but maybe I made a mistake in code.

L3viathan Over a year ago

You'll have to give the Environment object some arguments, see this. I'll change it in the answer.

Vas23Vewe Over a year ago

The final editing works fine. Thank you! Last question: my JSON consists of many rows and now it prints me all of them. How can I call specific value? Where I should add path to it?

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