test_mock

# start_func.py

async def start_func(testcls, first, **second):
    second['new_arg'] = 'wow'
    await testcls.send(first, **second)

if __name__ == '__main__':
    from end_func import TestCls
    import asyncio

    testargs = {
        'bar': 'bar',
        'baz': 'baz'
    }
    cls = TestCls()

    loop = asyncio.get_event_loop()
    loop.run_until_complete(start_func(cls, 'foo', **testargs))
    loop.close()

# end_func.py

class TestCls:
    async def send(self, first, **second):
        second['newer_arg'] = 'WOOOOOOOOOOOOOOOOO'
        await self._send_request(first, **second)

    async def _send_request(self, first, **second):
        print('nothing')

# test_mock.py

import pytest
from unittest import mock
from start_func import start_func
from end_func import TestCls
import json

class HTTP_mock:
    async def _send_request(self, arg, **kwargs):
        print(json.dumps([arg, kwargs]))


@pytest.mark.asyncio
async def test_send_mock(capsys):
    testargs = {
        'bar': 'testbar',
        'baz': 'testbaz'
    }
    cls = TestCls()
    with mock.patch('end_func.TestCls._send_request', new=HTTP_mock._send_request) as http_mock:
        first_arg = 'footest'
        await start_func(cls, 'footest', **testargs)
        captured = capsys.readouterr()
        res = json.loads(captured.out)
        print('captured', res)
        assert first_arg == res[0]
        assert all(item in res[1].items() for item in testargs.items())