如何在单元测试中正确修补boto3呼叫

Question

I'm new to Python unit testing, and I want to mock calls to the boto3 3rd party library. Here's my stripped down code:

real_code.py:

import boto3

def temp_get_variable(var_name):
  return boto3.client('ssm').get_parameter(Name=var_name)['Parameter']['Value']

test_real_code.py:

import unittest
from datetime import datetime
from unittest.mock import patch

import real_code

class TestRealCode(unittest.TestCase):

    @patch('patching_config.boto3.client')
    def test_get_variable(self, mock_boto_client):

        response = {
            'Parameter': {
                'Name': 'MyTestParameterName',
                'Type': 'String',
                'Value': 'myValue',
                'Version': 123,
                'Selector': 'asdf',
                'SourceResult': 'asdf',
                'LastModifiedDate': datetime(2019, 7, 16),
                'ARN': 'asdf'
            }
        }

        mock_boto_client.get_variable.return_value = response

        result_value = real_code.get_variable("MyTestParameterName")

        self.assertEqual("myValue", result_value)

When I run it the test fails with

Expected :myValue
Actual   :<MagicMock name='client().get_parameter().__getitem__().__getitem__()' id='2040071816528'>

What am I doing wrong? I thought by setting mock_boto_client.get_variable.return_value = response it would mock out the call and return my canned response instead. I don't understand why I am getting a MagicMock object instead of the return value I tried to set. I'd like to set up my test so that when the call to get_parameter is made with specific parameters, the mock returns the canned response I specified in the test.

Answer 1

There are two issues with your test code. The first is that when your mock object mock_boto_client called, it returns a new mock object. This means that the object that get_parameter() is being called on is different than the one you are attempting to set a return value on. You can have it return itself with the following:

mock_boto_client.return_value = mock_boto_client

You can also use a different mock object:

foo = MagicMock()
mock_boto_client.return_value = foo

The second issue that you have is that you are mocking the wrong method call. mock_boto_client.get_variable.return_value should be mock_boto_client.get_parameter.return_value . Here is the test updated and working:

import unittest
from datetime import datetime
from unittest.mock import patch

import real_code

class TestRealCode(unittest.TestCase):

    @patch('boto3.client')
    def test_get_variable(self, mock_boto_client):

        response = {
            'Parameter': {
                'Name': 'MyTestParameterName',
                'Type': 'String',
                'Value': 'myValue',
                'Version': 123,
                'Selector': 'asdf',
                'SourceResult': 'asdf',
                'LastModifiedDate': datetime(2019, 7, 16),
                'ARN': 'asdf'
            }
        }

        mock_boto_client.return_value = mock_boto_client
        mock_boto_client.get_parameter.return_value = response

        result_value = real_code.get_variable("MyTestParameterName")

        self.assertEqual("myValue", result_value)