Dynamic function signature in help()

Question

I am building a library that automatically discovers remote API endpoints and populates an object with methods corresponding to the inferred endpoints. The dynamically created methods have the following structure:

def proxy(*args, **kwds):
    # 1. Validate arguments: Do arguments match remote API?
    # 2. Forward call to remote API
    # 3. Parse and return response
    pass

This works so far. However, I would like to set the method signature shown with help(proxy) dynamically. It would increase the user interface greatly if users can see the inferred signatures directly and also use tab-completion for keyword arguments. Is there a way to decorate, annotate, or otherwise manipulate the function to achieve this?

The difference to Dynamically define functions with varying signature is that I only what to change the signature from the caller perspective. The definition of the method should be with *args and **kwds.

Would it work to dynamically define a wrapper function with the desired signature? — cherrywoods, Oct 24 '22 at 08:18
Sure wrapping the function would be fine but how do you define the wrapper? — sauerburger, Oct 25 '22 at 08:00
I was thinking maybe one could define the wrapper as in the linked SO question using `eval`, but I haven't really thought it through. — cherrywoods, Oct 25 '22 at 08:06
Another idea would be to somehow use or mimic `@typing.overload` dynamically but neither can I tell you how this might work concretely. — cherrywoods, Oct 25 '22 at 08:14

score 1 · Accepted Answer · answered Oct 25 '22 at 08:29

Poking around the source of the inspect module, I realized that the information in __signature__ is honored. Therefore, one can create a new, more informative signature dynamically as follows

from inspect import Signature, Parameter

def proxy(*args, **kwds):
    pass

user_param = Parameter("user", kind=Parameter.POSITIONAL_OR_KEYWORD)
comment_param = Parameter("comment", kind=Parameter.POSITIONAL_OR_KEYWORD)
sig = Signature([user_param, comment_param])

proxy.__signature__ = sig 
proxy.__doc__ = "Post a new comment as given user"
proxy.__name__ = "post_comment"

Calling help(proxy) will show

post_comment(user, comment)
    Post a new comment as given user

and tab-completion suggests user= or comment=.

Please note, that the new signature is not enforced. Calling proxy(no_such_argument="hello") will not raise a TypeError, so the validation of the signature still needs to take place inside the proxy function.

Dynamic function signature in help()

1 Answers1