Python assert_dict_matches recursive test helper function

The Jest library for Node.js has a helpful expect.objectContaining function that makes it easy to assert only on the keys of interest in a particular test.

Python’s unittest library does have a similar assertDictEqual method, but it asserts on all keys.

I wanted to do be able to easily assert on only the keys of interest in Python tests, so I put together this quick test helper function to do so. It makes use of assertDictEqual, but only after extracting the keys of interest in a recursive way.

from typing import Dict
from unittest import TestCase

def assert_dict_matches(expected: Dict, actual: Dict):
	Assert that `actual` matches `expected` for the keys and indexes that are
	present in `expected`, recursively.
	tc = TestCase()
	tc.maxDiff = None
	matched = match_keys(expected, actual)
	tc.assertDictEqual(expected, matched)

def match_keys(expected, actual):
	if isinstance(expected, dict):
		assert isinstance(actual, dict)
		return {
			k: match_keys(expected[k], v)
			for k, v in actual.items()
			if k in expected.keys()
	if isinstance(expected, list):
		assert isinstance(actual, list)
		return [
			match_keys(expected[i], v) for i, v
			in enumerate(actual[:len(expected)])
	return actual

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