Simple Boto3 mock in Python with Pytest monkeypatch

2021-01-20 (Wednesday) | 200 words (~1 minutes reading)

If you’re anything like me, then you might find mocking dependencies in Python tests quite fiddly and unintuitive (it’s certainly less straightforward than Jest in TypeScript).

I needed to mock one part of the Python boto3 library for AWS, and I got it working using Pytest’s monkeypatch.

This shows a test for fetching a value from a JSON-encoded secret string in AWS Secrets Manager, but you can extrapolate from this roughly how to mock out other boto3 APIs in a similar way.

import json

import pytest
import boto3

from src.foobar import foobar

TEST_AWS_REGION = "test-region-1"
TEST_SECRET_NAME = "foobar-secret-name"
TEST_SECRET_VALUE = "foobar-secret-value"



class MockBoto3Client:
  # noinspection PyPep8Naming
  @staticmethod
  def get_secret_value(SecretId: str):
    assert SecretId == TEST_SECRET_NAME
    return {
      "SecretString": json_util.dumps(
        {
          "key": TEST_SECRET_VALUE,
        }
      )
    }


class MockBoto3Session:
  @staticmethod
  def client(service_name: str, region_name: str):
    assert service_name == "secretsmanager"
    assert region_name == TEST_AWS_REGION
    return MockBoto3Client


@pytest.fixture
def mock_boto3(monkeypatch: pytest.MonkeyPatch):
  monkeypatch.setattr("src.foobar.SECRET_NAME", TEST_SECRET_NAME)
  monkeypatch.setattr(boto3.session, "Session", MockBoto3Session)


def test_docdb_host(mock_boto3):
    assert foobar() == TEST_SECRET_VALUE

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