mock_s3 decorating pytest fixture

Question

I wonder why mock_s3 decorator doesn't work when used as a decorator for pytest fixture. test_with_fixture fails while it provides the same code as the test_without fixture. Well, "the same" as it is decorated explicitly.

test_with_fixture raises AccessDenied error, but the type of S3 error it not relevant in this case. The problem is that, client.list_objects is not mocked in the test which uses fixture.

pytest - 3.1.2
moto - 1.0.1
boto3 - 1.0.4

import pytest
import boto3

from moto import mock_s3

BUCKET = 'Foo'


@pytest.fixture()
@mock_s3
def moto_boto():
    res = boto3.resource('s3')
    res.create_bucket(Bucket=BUCKET)


def test_with_fixture(moto_boto):
    client = boto3.client('s3')
    client.list_objects(Bucket=BUCKET)


@mock_s3
def test_without_fixture():     
    res = boto3.resource('s3')
    res.create_bucket(Bucket=BUCKET)

    client = boto3.client('s3')
    client.list_objects(Bucket=BUCKET)

Answer 1

An alternative is to use an 'autouse' test fixture in which you start and stop the moto server and create your test bucket.

This is based on mikegrima's comment on https://github.com/spulec/moto/issues/620.

import pytest
import boto3

from moto import mock_s3

BUCKET = 'Foo'


@pytest.fixture(autouse=True)
def moto_boto():
    # setup: start moto server and create the bucket
    mocks3 = mock_s3()
    mocks3.start()
    res = boto3.resource('s3')
    res.create_bucket(Bucket=BUCKET)
    yield
    # teardown: stop moto server
    mocks3.stop()


def test_with_fixture():
    client = boto3.client('s3')
    client.list_objects(Bucket=BUCKET)

Answer 2

Using a context manager:

import pytest
import boto3

from moto import mock_s3

BUCKET = 'Foo'


@pytest.fixture()
def moto_boto():
    with mock_s3():
        res = boto3.resource('s3')
        res.create_bucket(Bucket=BUCKET)
        yield


def test_with_fixture(moto_boto):
    client = boto3.client('s3')
    client.list_objects(Bucket=BUCKET)

Using the context manager, start and stop are invoked under the hood.

Answer 3

The problem of your fixture is that you are not using it later although it is in the signature of your test test_with_fixture(moto_boto). I suggest you to create a fixture that returns a function that can be instantiated within your test to create the mocked objects that your test requires (the s3 bucket). An example of such an implementation could be as follows:

import pytest
import boto3

from moto import mock_s3

BUCKET = 'Foo'

@pytest.fixture()
def moto_boto():
    @mock_s3
    def boto_resource():
        res = boto3.resource('s3')
        res.create_bucket(Bucket=BUCKET)
        return res
    return boto_resource

@mock_s3
def test_with_fixture(moto_boto):
        moto_boto()
        client = boto3.client('s3')
        client.list_objects(Bucket=BUCKET)

In this case I am using the moto library through a decorator in both the fixture and the test but the context manager could be similarly used as explained in the moto README