Build Dynamic REST API Mock Service

Peter Xie
3 min readMay 13, 2021


This is part of the REST API Testing in Python series.

This post will show you how to build a dynamic REST API mock service with magic using Flask before_request decorator.

Photo by Artur Tumasjan on Unsplash

The traditional approach of mock service is to build a mock service for each endpoint, and for the same endpoint you need to modify the mock service when you want to test a different scenario, e.g. return an error instead of a successful response, not to mention different mocks for different request methods, i.e. GET, POST, PUT, etc. For instance, you may create a mock service with Postman as follows:

Postman Mocks

Let’s re-think the mock services, what you want to mock is Response Code and Response Body, and you want a special response for each of your test cases. Why not set the expected Response Code and Body in your requests then? Yep, that’s what I am going to demonstrate here.


Use Flask before_request to mock any endpoints and any request methods. Note: This decorator function is called before handling a specific endpoint.
Send the response with the response_code and response_body set in the request headers.

Request Example:

Response-Code: 202
Response-Body: {"code": 0, "message": "all good"}

Response Example:

Status Code: 202
{"code": 0, "message": "all good"}


First, install the required packages as follows.

pip install flask requests

The mock service is just 15 lines of code. Let’s dive into it directly.

The code itself is quite self-explainable, the power of Python 😊. We set the response code to default 200 if it is not set in the request headers (line #8), and return a warning message to the client if the response body is not set (line #11). That’s it!

Start the mock service with python and you are ready to roll.
A quick test is to visit in your web browser and you will see this:

With requests, we can write a proper pytest test case as follows.

Just run pytest in the same folder or pytest for the test and you will see results as below. PASSED

Change the headers or assert to play around and you will see the power of the magic.

You can also use my test framework mentioned in REST API Testing in Python for a more comprehensive usage.


Alternatively, you can set a ‘response_file’ header in the request and read the expected response from a file. Or if it is inconvenient to set additional headers in your test or application code, you can build a ‘set_response’ endpoint in the mock service so that you can call this endpoint to set the expected response before calling the test endpoint.