Using Python's inspect module to load test data

August 07, 2023
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The problem

In one of my Django project, a pattern started to emerge in my tests, where I needed mock data coming out a 3rd party API. The 3rd party API docs often shows an example, which is great to get a basic payload to use in tests. However, I couldn’t paste it directly on my test code:

  • It is in JSON format, which isn’t necessarily valid Python. For example if the example contains some null or boolean values (false in JSON vs False in Python).
  • It might be a few 100 lines long, distracting from the test code
  • May contain a lot of data irrelevant to the test.

Still, I like to have a snapshot of the example, in my code: some fields might be useful in the future, and having the full payload in git enables me see if some fields are being added or removed by the 3rd party.

Initial solution

My solution is to paste the example in a JSON file, and write a small utility function to load the json from the file. To separate it from my test, while leaving it close, I started to put it in a data directory directly adjacent to my test file:

├── data/
│   └── example.json

Then, I wrote a small utility function to load the data, in the test module:

import json
from pathlib import Path

def load_data(filename):
    fixture_path = Path(__file__).parent / "data" / filename
    return json.loads(fixture_path.read_text())

I used it once, and moved on. Then I needed something similar, but also wanted the ability to override some fields, so I wrote another one, adding a overrides parameter:

def load_data(filename, **overrides):
    fixture_path = Path(__file__).parent / "data" / filename
    base_data = json.loads(fixture_path.read_text())
    return {**base_data, **overrides}

After a while, as I added more integrations, a few versions popped up in my codebase. They were all similar, but because of the __file__ magic, I couldn’t easily reuse my function: the fixture would be loaded from directory adjacent to my utility function, not from the test module. However, I knew that Python introspection would probably let me do what I wanted.

The reusable solution

A quick search brought up a Stack Overflow answer describing exactly what I needed, using the inspect module. Here is how my final solution looks like:

import inspect
import json
from pathlib import Path
from typing import Any

def load_json(
    fixture_name: str,
    **overrides: Any,
) -> dict[str, Any]:
    Load JSON from a fixture located a `data` directory.

    The directory is adjacent to the caller.
    caller_file = inspect.stack()[1].filename
    data_path = Path(caller_file).parent / "data"
    fixture_path = data_path / fixture_name
    loaded_data = json.loads(fixture_path.read_text())
    return {

This solution has some limitations (the caller must be the Python module adjacent to the test data, doesn’t work with a top level array, …), but it works fine for my use case. I can now reuse it across my codebase and improve it as my need evolve.

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