Testing Python AWS Lambda boto3 initialization

I had same issue with boto3 s3 client in my client class and moto in my pytest. I resolved it by wrapping boto3 client into a singleton:

This is my client code hello_world/app.py

class Singleton(type):
_instances = {}
def __call__(cls, *args, **kwargs):
    if cls not in cls._instances:
        cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
    return cls._instances[cls]

class S3BotoClient(metaclass=Singleton):
    def __init__(self, *args):
        print(f"creating s3 cient with args {args}")
        self.client = boto3.client("s3", *args)

def lambda_handler(event, context):
    s3 = S3BotoClient().client
    s3.list_buckets()

And this is a unit test:

from moto import mock_s3
from hello_world import app

@pytest.fixture()
def apigw_event():
    # return some sample event here

@mock_s3
def test_lambda_handler(apigw_event):
    ret = app.lambda_handler(apigw_event, "")
    ret = app.lambda_handler(apigw_event, "")
    # do assertions here

So S3 client is instantiated only once and after moto virtual env is initialized

While I'm not sure what the issue is with the above code, I'd advise you to use the moto library when you are trying to mock AWS services in Python (https://github.com/spulec/moto):

import boto3
from moto import mock_s3
from lambda_function import handler

class TestHandler(unittest.TestCase):

    @mock_s3
    def test(self, boto3_mock):
       # setup your connection to s3 or ssm. Make sure this matches the setup in the lambda file.
       conn = boto3.client('s3')
       conn.create_bucket(Bucket='mybucket') # setup your fake resources
       # call your lambda function

In addition - and as a somewhat personal preference - I would advise against putting too much logic in your actual lambda function. Just take the incoming event send it to other functions/classes as much as possible. This should help simplify testing.

If you really want to keep using @patch instead of the moto library, I got the code to work with the following:

from mock import patch
from example import lambda_function

class TestStringMethods(unittest.TestCase):

    @patch('example.lambda_function.boto3.client')
    def test_my_model(self, some_mock):
        # further setup of test
        lambda_function.my_handler(None, None)

Here, lambda_function is the file containing your handler, and it is located in the directory/package example. You could also mock boto3 itself with 'example.lambda_function.boto3' and return a client yourself.