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--- string return_value

The basic approach seems ok, but I'd add a new-line between the --- and your string return_value, like so:

---
string return_value

You write:

However, when I try to call it, I get an error that function accepted 0 arguments and got 1

Could you please add a snippet to your question that shows exactly what you're trying to do, and what the exact error message is?

--- string return_value

The basic approach seems ok, but I'd add a new-line between the --- and your string return_value, like so:

---
string return_value

You write:

However, when I try to call it, I get an error that function accepted 0 arguments and got 1

Could you please add a snippet to your question that shows exactly what you're trying to do, and what the exact error message is?

Edit:

EDIT: I noticed that one way to do it would be to add (some) argument to service_implementation so that it looks like
def service_implementation(request):
        return "answer"

Ah, yes.

When talking about 'services', we (I ?) always think of the abstracted ROS API level. So a 'no arguments service' is a ROS service that takes no arguments (like you defined), but at the C++/Python implementation level, you'll always need to accept (and handle) the request-reply pair that gets supplied by the ROS middleware to your client program.

For a 'no arguments service', that will just mean that the Request type has no (public) member fields. In Python, returning just a str probably then works because it's automatically transformed using the appropriate constructor of the Reply type.