How to Fix 422 Client Error: Unprocessable Entity when using the json parameter of Request Python Module

The aim of this page📝 is to explain how to debug and handle errors in Python code based on the particular example of a 422 Client Error: Unprocessable Entity error that occurred when making a request to an Iglu schema registry.

• A 422 Client Error: Unprocessable Entity error occurs when the server understands the content type of the request entity, but the server was unable to process the contained instructions.
• In this case, the error message indicated that the request body was invalid.
• The cause of this issue is that the data being sent in the request body was not in the expected format.
• The json parameter of the requests library expects a JSON-serializable object, not a string.
• If you pass a string to the json parameter, it will be sent as-is in the body of the request and will not be serialized to JSON.

Starting with Requests version 2.4.2, you can use the json= parameter (which takes a dictionary) instead of data= (which takes a string) in the call: — https://stackoverflow.com/a/26344315/11082684

• To fix this issue, you could deserialize the contents of a JSON file into a Python object before passing it to the json parameter.
• You can use the json.load() function to deserialize the contents of a file into a Python object.

BROKEN CODE

Here is a particular example I have experienced (I know make_iglu_request takes way too many params, will refactor)

def patch_schema(iglu: Iglu) -> None:
    ...
    for env in envs:
        ...
        with handle_patching(schema):
            ...
            with open("temp_schema.json", mode="rt", encoding="utf-8") as patched_schema:
                schema_to_patch = patched_schema.read() # <<< ERR!
            schema_patcher_http_requestor.make_iglu_request(requests.put,
                                                            env,
                                                            schema_endpoint,
                                                            iglu,
                                                            schema_to_patch)
    ...

def make_iglu_request(request_func: Callable, env: str, schema_endpoint: str, iglu: Iglu, schema_to_patch: json) -> None:
    ... # 🠉 ERR in the type hint!
    try:
        res = request_func(url,
                           headers=headers,
                           verify=os.environ['CACERT'],
                           json=schema_to_patch)
        ...
    except requests.exceptions.RequestException as err:
        ...

This code reads the contents of a file into a string and assigns it to the schema_to_patch variable. However, this string is then passed to the make_iglu_request function and ultimately to the json parameter of the request_func function. This causes an error because the json parameter expects a JSON-serializable object, not a string.

422 Client Error: Unprocessable Entity for url: https://com-snowplowanalytics-dev.iglu.snplow.net/api/schemas/com.example-company/onmouse_img/jsonschema/1-0-0
{"message":"The request body was invalid."}

RUBBERDUCK

FIX

To fix this issue, you could change your code to deserialize the contents of the temp_schema.json file into a Python object before passing it to the make_iglu_request function. For example, you could change your code to something like this:

with open("temp_schema.json", mode="rt", encoding="utf-8") as patched_schema:
    schema_to_patch = json.load(patched_schema)

This will deserialize the contents of the temp_schema.json file into a Python object, which can then be passed to the make_iglu_request function and ultimately to the json parameter of the request_func function.

LINKS

• https://stackoverflow.com/a/26344315/11082684