How to mock the boto3 client session requests for secretsmanager to either return a specific response or raise an exception

Given code like:

def do_stuff():

    session = boto3.session.Session()
    client = session.client(service_name="secretsmanager", region_name=region_name)
    client.get_secret_value(SecretId=sendgrid_api_key_arn)

how do I mock client.get_secret_value("some-value") to return some value

and how do I mock it to throw an exception

@patch("boto3.session")
def test_get_sendgrid_api_key_secret_when_client_error(mock_session):
        session = mock_session.Session();
        client = session.client()
        client.get_secret_value().return_value = 
                 {"SecretString": "my-secret"} <- this is wrapped in a MagicMock which is useless.

Solution

You need to set a tree of return values:

boto3.session.Session needs to return a mock object
That mock object needs a client method that returns another mock object
And that mock object needs a get_secret_value method that returns a fake value

If I assume that target.py exists and contains:

import boto3.session


def do_stuff():
    session = boto3.session.Session()
    client = session.client(service_name="secretsmanager", region_name='myregion')
    return client.get_secret_value(SecretId='some-secret-id')

Then I can test it like this:

from unittest import mock

import target


@mock.patch("boto3.session.Session")
def test_do_stuff(mock_session_class):
    mock_session_object = mock.Mock()
    mock_client = mock.Mock()
    mock_client.get_secret_value.return_value = {'SecretString': 'my-secret'}
    mock_session_object.client.return_value = mock_client
    mock_session_class.return_value = mock_session_object

    res = target.do_stuff()
    assert res['SecretString'] == 'my-secret'

This works, although I suspect there's a more graceful way to set it up.