ui.clipboard should raise if the clipboard API isn't available

[falkoschindler]

As discussed in #4236, ui.clipboard warns and returns an empty string if the clipboard API isn't available. I think it should raise instead so that the user code can decide how to handle the error case. As this is technically a breaking change, this issue is scheduled for 3.0.

[evnchn]

I have thought about it for a bit.

I don't think it should raise an exception, but it should return None.

None and an empty string is distinguishable by is None, and I can pretty much say for 99% use cases, since people (1) don't type check and (2) this is not TypeScript, that it will be fine and work with situations such as if clipboard_result: statements.

Also, there is no concern that the clipboard API, when working, returns something like undefined or null, because according to MDN Web Docs, it returns an empty string if the clipboard is empty, does not contain text, or does not include a textual representation among the objects representing the clipboard's contents

Compared to raising an exception, which breaks all code if no action is taken when upgrading, and empty string, which breaks only if you do something like assert isinstance(a, str)

We can also push out the feature earlier.

Also, exceptions are just generally hard to deal with in my opinion. There is no way to deal with an exception without indenting in, but you can absolutely use the ternary operator in python for basically one-line if-else.

[evnchn]

But then, in the Zen of Python by Tim Peters:

Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.

Question: Is not having access to the Clipboard API under HTTP (non-localhost) an "error"? Is raising a warning via log.warning considered "silent" or not?

And the most important question of all which we wont get the answer to (not easily): Can Tim Peters please come explain what he means?