Python · testing

prefer-library-fake

python:prefer-library-fake

Tests should use maintained service fakes or emulators instead of hand-rolled third-party doubles.

Code: SARJ059
Default: error
Fix: none
Languages: python

Why

Hand-written doubles model only remembered protocol behavior and can let invalid requests pass.

Fix

Use the recognized library fake, emulator, or test container while keeping the production client.

Before / after

Executed by this rule’s unit tests.

Before

Test defines its own S3 client

tests/fakes/s3.py · focus

class FakeS3Client:
    def put_object(self, **kwargs):
        return kwargs

    def get_object(self, **kwargs):
        return kwargs

    def delete_object(self, **kwargs):
        return kwargs

After

Test uses the maintained AWS fake

tests/test_upload.py · focus

from moto import mock_aws

@mock_aws
def test_upload():
    assert upload_with_real_client()

Limits

Only test and shared-double paths are analyzed.
Only recognized external services and substantial hand-rolled doubles are reported.