WSGI (Web Server Gateway Interface) is a standard interface between web servers and Python web applications or frameworks. To define a WSGI handler in a Python file, you need to create a Python function that follows the WSGI specification. This function should accept two parameters, the environment (environ) and a start_response callable.

Here's a simple example of a WSGI handler:

def wsgi_handler(environ, start_response):
    status = '200 OK'
    headers = [('Content-type', 'text/plain')]
    start_response(status, headers)

    response_body = 'Hello, WSGI!'
    return [response_body.encode('utf-8')]

In this example, wsgi_handler is a WSGI-compliant function that accepts the environment and start_response parameters, sets the status and headers, and returns a list containing a single byte string.

How to Return JSON from WSGI handler ?

To return JSON from a WSGI handler, you'll need to modify the response content type and convert the response body into a JSON formatted string. You can use the json module from Python's standard library to achieve this. Here's an example:

import json

def wsgi_handler(environ, start_response):
    status = '200 OK'
    headers = [('Content-type', 'application/json')]
    start_response(status, headers)

    response_data = {
        'message': 'Hello, WSGI!',
        'data': {
            'key': 'value',
        }
    }
    response_body = json.dumps(response_data)
    return [response_body.encode('utf-8')]

In this example, the Content-type header is set to 'application/json' to indicate that the response is a JSON object. The response_data dictionary is then converted to a JSON-formatted string using json.dumps() and returned as a byte string.

How to get query parameters from wsgi?

To get the article_id parameter from the query string, you can use the same approach as before, but you'll specifically look for the stop_id key in the query_params dictionary. Here's an example:

from urllib.parse import parse_qs

def wsgi_handler(environ, start_response):
    query_string = environ.get('QUERY_STRING', '')
    query_params = parse_qs(query_string)
    article_id = query_params.get('article_id', [None])[0]

    status = '200 OK'
    headers = [('Content-type', 'text/plain')]
    start_response(status, headers)

    if article_id is not None:
        response_body = f"article_id: {article_id}"
    else:
        response_body = "article_id not found in query parameters"

    return [response_body.encode('utf-8')]

In this example, the article_id value is retrieved from the query_params dictionary using the get method. If the stop_id key is not found in the dictionary, the get method returns a default value of [None]. Since parse_qs returns a dictionary with lists as values, we need to get the first element of the list by using [0].