Strategies for Mocking AWS Services for Testing

Sat, 19 Sep 2026 02:10:03 +0000

Stubber

Boto3 comes with the Stubber class, but it doesn’t work well for all calls. In those cases

Moto

Moto is a Python package that allows for easy mocking of AWS services. The canonical way to do this is to use the @mock_<service> decorator outside of your test function and call boto3 functions there. Moto will detect and intercept the boto3 calls. Code example from the Moto documentation:

import boto3
from moto import mock_s3
from mymodule import MyModel

@mock_s3
def test_my_model_save:
 conn = boto3.resource('s3', region_name='us-east-1')
 # We need to create the bucket since this is all in Moto's 'virtual' AWS account
 conn.create_bucket(Bucket='mybucket')

 model_instance = MyModel('steve', 'is awesome')
 model_instance.save

 body = conn.Object('mybucket', 'steve').get[
 'Body'].read.decode("utf-8")

 assert body == 'is awesome'

Patching Clients and Resources

This is kind of useless for most testing, where there is an external module or package under test. The Moto documentation advises users to set up their boto3 clients and/or resources within the functions that use them, but this isn’t always workable. Instead, since you’re already importing the module or package under test, you can use Moto’s patch_client or patch_resource to patch the instantiated client or resource:

Testing Python Code with Pytest

Mon, 14 Mar 2022 19:20:53 +0000

Mocking External Code

To mock outside function calls with Pytest, install the pytest-mock package and use mocker as a parameter to your test function. Then you set the patch with the following syntax: mocker.patch.object(Class, "function_name", replacement_function). Note how there aren’t any parantheses for the functions - either in the string representation or the actual function. This is because if they were used, the function would be evaluated at the call, rather than passed in to the patch.object function. These are best done in setup functions rather than the test function itself.

Testing on The Edge Cases

Strategies for Mocking AWS Services for Testing

Stubber

Moto

Patching Clients and Resources

Testing Python Code with Pytest

Mocking External Code