SynchronousOnlyOperation error in custom converter when running on ASGI

Using Django Deployment
1

Hello, I have a module in my application that implements a model with a hierarchical structure. I am using a custom url converter that parses a series of slugs and returns a list of those objects:

from module.models import Object

class slugGraph:
    regex = r"([-a-zA-Z0-9_]/?)+"

    def to_python(self, value):
        ret = []
        for slug in value.split("/"):
            try:
                ret.append(object.objects.get(slug=slug))
            except Object.DoesNotExist:
                raise ValueError
        return ret

    def to_url(self, value):
        ret = ""
        for x in value:
            ret += x.slug
            ret += "/"
        ret = ret[:-1] # removes last slash
        return ret

This works perfectly well on the development web server, but when I try to deploy the code via ASGI, I get the following error:

SynchronousOnlyOperation - You cannot call this from an async context - use a thread or sync_to_async.

The application errors out on the line:
ret.append(object.objects.get(slug=slug))
Is using converters to return an object like this impossible when deploying using ASGI?
Thank you for your time and I you can help me :slight_smile:

2

You either need to convert the query to use aget, or wrap the query in a sync_to_async call, or since you’re possibly going to execute this query multiple times, you could make that entire to_python method synchronous.

Side note: If you’re deploying this in an async environment, you really should be doing your development using the Daphne-version of runserver to have it running async as well.

3

Thank you for a quick reply, that has worked. If i understand correctly I have to now change every call to the custom converter (for example when retrieving a captured variable from the url) asynchronously, correct?

Wonderful, I did not know that, that makes testing a lot easier.

4

Correct. Your alternatives are to either create two different flavors of the method (sync and async), or to change the callers such that they’re all calling your function from a sync context.

1 Like
5

Great, thank you for the quick replies again :slight_smile:

6

I am so sorry, but I am a new to concurrent code in python. How would I implement two flavors of the same method that can be called with the same method name?

7

You can’t. They need to have two different names. That’s why we have the situations like get and aget.

1 Like