How do I mock an external API call in Jest or pytest?

Question

My unit tests call a real external API and fail when the network is unavailable. How do I replace that call while still checking my application's behavior?

IT-QA Assistant · Accepted Answer

*AI-drafted answer — reviewed for correctness. Please verify against your own environment and versions; corrections and better answers are welcome below.*

Replace the network dependency at a boundary your code controls, then assert the result and the outgoing arguments. Dependency injection avoids module-loading complications in Jest:

```js
async function getName(fetchUser) {
  const user = await fetchUser(7);
  return user.name;
}

test('returns the API user name', async () => {
  const fetchUser = jest.fn().mockResolvedValue({ name: 'Ada' });
  await expect(getName(fetchUser)).resolves.toBe('Ada');
  expect(fetchUser).toHaveBeenCalledWith(7);
});
```

`mockResolvedValue` produces a resolved promise; use `mockRejectedValue` for an asynchronous failure. See Jest mock functions (https://jestjs.io/docs/mock-function-api).

In pytest, patch the name where your application looks it up. If `service.py` imports `fetch_user` and `get_name(7)` calls it:

```python
import service

def test_get_name(monkeypatch):
    def fake_fetch_user(user_id):
        assert user_id == 7
        return {"name": "Ada"}

monkeypatch.setattr(service, "fetch_user", fake_fetch_user)
    assert service.get_name(7) == "Ada"
```

The fixture restores the attribute afterward. See pytest monkeypatch (https://docs.pytest.org/en/stable/how-to/monkeypatch.html).

Also test timeouts, malformed responses, and error handling. Match the real dependency's synchronous or asynchronous contract. These tests verify application logic, not the actual HTTP adapter or provider compatibility; keep separate integration or contract tests for those.

How do I mock an external API call in Jest or pytest?

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