A JSON Decoder that will evaluate strings as jinja2 expressions

json_jinja2_decoder.py

"""
TODO: Use an strict environment like SandboxedEnvironment
TODO: Create an "allowed list" of filters and functions to use in the expression
TODO: Make or check the context object is inmutable (Prevent thread-safe situations)

"""

import json
from datetime import datetime
from jinja2 import Environment, meta, sandbox


def now(format="%Y-%m-%d %H:%M:%S"):
    return datetime.now().strftime(format)

jinja_env = sandbox.SandboxedEnvironment()
jinja_env.globals['now'] = now


def evaluate_jinja_expression(value, context=None):
    if context is None:
        context = {}

    try:
        parsed_content = jinja_env.parse(value)

        # Check if it contains any variables or blocks
        if meta.find_undeclared_variables(parsed_content):
            # Render the template with the provided context
            template = jinja_env.from_string(value)
            return template.render(context)
        else:
            # DO nothing if no expression is found
            return value
    except Exception as e:
        print(f"Failed to evaluate expression: {e}")

    return value


class JinjaJSONDecoder(json.JSONDecoder):
    def __init__(self, *args, **kwargs):
        self.context = kwargs.pop("context", {})
        super().__init__(object_hook=self._deserialize, *args, **kwargs)

    def _deserialize(self, obj):
        if isinstance(obj, dict):
            return {
                k: evaluate_jinja_expression(v, self.context)
                if isinstance(v, str)
                else v
                for k, v in obj.items()
            }
        elif isinstance(obj, list):
            return [
                evaluate_jinja_expression(v, self.context) if isinstance(v, str) else v
                for v in obj
            ]
        return obj


json_string = """
{
    "name": "John Doe",
    "greeting": "Hello, {{ name | upper }}! today is {{ now() }}",
    "age": 30,
    "info": {
        "description": "User is {{ age }} years old."
    }
}
"""

context = {"name": "Jane Doe", "age": 25}

deserialized_data = json.loads(json_string, cls=JinjaJSONDecoder, context=context)

assert deserialized_data == {
    "name": "John Doe",
    "greeting": "Hello, JANE DOE! today is 2024-01-01 01:01:01",
    "age": 30,
    "info": {
        "description": "User is 25 years old."
    }
}