I have implemented a lambda project which has 4 modules/project inside it. Each module/project has python(s) files which implement module functionality.
I have to write the test cases for each module so that it goes through CircleCI and execute on themselves:

That the module is starting and stopping a stepfunction.
module is calling Rest service.
It is writing/reading files from S3 Bucket.

Everywhere, it is like a test driven development to write unit test, but now I have completed project implementation, how do I write automated test cases for my module ?
When it comes to unit testing, there's nothing special you have to do about AWS Lambda projects.
You Lambda handler is a Python function. Therefore, you can import it on your tests, call it with some input, and assert the output. Just like a normal Python function.
How do I write test cases for my AWS Python-based lambda project?