Make unregister_custom_function idempotent

[gtfierro]

Fixes #1492

Proposed Changes

• Replace the raised exception with a new warning; unregister_custom_function can now be called multiple times on the same function (idempotent)
• Adjust the issue 274 test by checking that the warning is raised, rather than the exception

[aucampia]

The change looks good, this changes public API, but I don't really think it is a massive problem in this case. I think the behaviour with this patch is more desirable.

[aucampia]

One thing that maybe bothers me a bit, what if you just want to unregister a custom function regardless of the action function identity. Maybe if func is none the check for function identity should not be performed.

[gtfierro]

We still have to check for membership inside the dictionary, else we will get a KeyError when deleting a non-existent function. Maybe something like this would work?

def unregister_custom_function(uri, func):
    if func is not None and _CUSTOM_FUNCTIONS.get(uri, (None, None))[0] != func:
        warnings.warn("This function is not registered as %s" % uri.n3())
    elif uri in _CUSTOM_FUNCTIONS:
        del _CUSTOM_FUNCTIONS[uri]

[aucampia]

yes, but lets wait for @nicholascar to give some input, I tagged him for review. I still the the change as is makes sense, but if we are going to change the function we can maybe go a bit further, even if not it is good to discuss some options.

[nicholascar]

Hmmm... I didn't realise func was in the function signature. I'm happy with this PR's update and will approve but I do think it would be better to be able to de-register a function based just on uri as a user shouldn't need to know the Python function to remove the SPARQL function.

We still have to check for membership inside the dictionary, else we will get a KeyError

Sure, but we can do this without knowing about func:

def unregister_custom_function(uri, func=None):
    if _CUSTOM_FUNCTIONS.get(uri):
            del _CUSTOM_FUNCTIONS[uri]
    else:
            warnings.warn("This function is not registered as %s" % uri.n3())

I think the test _CUSTOM_FUNCTIONS.get(uri, (None, None))[0] != func is a waste of time. Firstly it may as well just be _CUSTOM_FUNCTIONS.get(uri) != func as this yeilds the same test and secondly, who cares? Just deregister whatever uri is as that's the guarenteed unique custom function key.

If we just use func=None then we can implement the above and it people have legacy code parameterising the function with uri & func it will still work.

ASIDE: the function names should probably be deregister_custom_function and that seems to be more grammatically corect to me to describe the function's positive action of removing a registered function! Too late for this...

[gtfierro]

I agree, and thanks for the perspective. I've put your simplified implementation in my PR and adjusted the tests to match the new behavior

[nicholascar]

Thanks @gtfierro