Fixing Aiohttp JSON Mimetype Error in Python

Introduction

When working with asynchronous HTTP requests in Python using aiohttp, developers may encounter the error “Attempt to decode JSON with unexpected mimetype”. This error suggests that the aiohttp library expects a MIME type of ‘application/json’ but has encountered a different type when attempting to parse a JSON response.

Understanding the Error

The error occurs in the context where an HTTP response is expected to be in JSON format, but the server has responded with a different mimetype in its Content-Type header. The aiohttp checks this header to verify the response type before parsing it as JSON, which leads to the error if an unexpected value is found.

Error Causes

• Inappropriate server response: The server might be sending a response with an incorrect Content-Type header.
• Client-side expectations: The client expects JSON but the server’s response is in a different format.
• Endpoints mishandling: The endpoint used doesn’t correspond to a JSON response or is not designed to serve JSON.

Solution 1: Content-Type Assertions

Solution description: Check and assert the Content-Type header of the API response is indeed ‘application/json’.

Steps to Implement:

1. Perform the API call using aiohttp.
2. Get response headers using response.headers.
3. Assert the Content-Type header to be ‘application/json’.
4. Handle the response accordingly if the assertion fails.

The complete code example:

import aiohttp
import asyncio

async def fetch_json(url):
    async with aiohttp.ClientSession() as session:
        async with session.get(url) as response:
            content_type = response.headers.get('Content-Type')
            if 'application/json' in content_type:
                return await response.json()
            else:
                raise ValueError('Unexpected Content-Type for JSON response')

loop = asyncio.get_event_loop()
try:
    response = loop.run_until_complete(fetch_json('https://example.com/api'))
    print(response)
except Exception as e:
    print(e)
finally:
    loop.close()

The advantages of this solution include providing clear error handling and keeping your code intentional about the expected response type. The limitations could be that assert statements may not be suitable for production code and there may be a need for more dynamic handling of ‘Content-Type’.

Solution 2: Flexible JSON Parsing

Solution description: Loosen the JSON parsing constraints to handle different but acceptable content types.

Steps to Implement:

1. Perform the API call using aiohttp.
2. Before parsing the response, check if the content type is acceptable.
3. Parse the response content as JSON if allowed content type is present.

The complete code example:

import aiohttp
import asyncio

async def fetch_json(url):
    acceptable_types = ['application/json', 'text/javascript', 'text/plain']
    async with aiohttp.ClientSession() as session:
        async with session.get(url) as response:
            content_type = response.headers.get('Content-Type')
            if any(atype in content_type for atype in acceptable_types):
                return await response.json(content_type=None)
            else:
                raise ValueError('Content-Type for JSON response not acceptable')

loop = asyncio.get_event_loop()
try:
    response = loop.run_until_complete(fetch_json('https://example.com/api'))
    print(response)
except Exception as e:
    print(e)
finally:
    loop.close()

Advantages of this solution include flexibility in handling different content types which can be considered as JSON. The limitation is that it could potentially allow unexpected response formats and could bypass meaningful errors regarding the API’s contract.