When I first started writing unit tests in my early career, I thought it’s boring task and tried to avoid them. I think unit tests should never be ignored and it’s better to start writiing them before it’s too late :) Once you get used to them and use the right tools it will be really enjoyable task.
One of the struggles to write unit tests is mocking functions; changing the behavior of external calls, dependencies, database connections, etc. So, I decided to write a blog series about unit testing and start with mocking functions.
Let’s start with a simple example: A script that interacts with AWS S3 - It checks if there are dev buckets (dev buckets start with “dev-“)
Files structure
├── main.py
├── tests
│ ├── __init__.py
│ └── test_utils.py
└── utils
└── s3_utils.py
main.py
from utils.s3_utils import list_s3_buckets
def check_dev_buckets() -> bool:
"""
Checks if the dev buckets exist
"""
buckets = list_s3_buckets()
for bucket in buckets:
if bucket.startswith('dev-'):
return True
return False
if __name__ == '__main__':
if check_dev_buckets():
print('Dev buckets exist')
else:
print('No dev buckets found')
s3_utils.py
from boto3 import client
S3_CLIENT = client('s3')
def list_s3_buckets():
"""
Lists all S3 buckets
"""
response = S3_CLIENT.list_buckets()
buckets = [bucket['Name'] for bucket in response['Buckets']]
return buckets
test_utils.py
import mock
from main import check_dev_buckets
def test_check_dev_buckets():
with mock.patch('main.list_s3_buckets') as mock_list_s3_buckets:
mock_list_s3_buckets.return_value = ['dev-bucket-1', 'dev-bucket-2']
assert check_dev_buckets()
⚠️ Note
in mock.patch() you need to pass the function where it’s imported (used) not wehre it’s defined. it should be main.list_s3_buckets and not utils.s3_utils.list_s3_buckets