Awesome you have come back to this! This went in as part of https://github.com/matplotlib/matplotlib/pull/5599/files

I did that out of an abundance of caution to not change a different API. It looks like there are a handful of setters that do in fact return something.

I do not see why they are incompatible:

def set_foo(self, value):
    self.foo = value
    return self.foo

but agree they should go long term.

@tacaswell by "incompatible" I mean that if returning setters where an actual feature supported by matplotlib, there are certain setters that could not be replicated using traitlets.

This kind of setter could be replicated with traitlets:

def set_foo(self, value):
    new_value = change_it(value)
    self.foo = new_value
    return new_value

However something like this could not:

def set_foo(self, value):
    new_value, byproduct = change_it(value)
    self.foo = new_value
    return byproduct

From a quick skim it looks like all of the returning setters return the same thing as their paired getter (ex set_title returns the Text object that is the title).

If there are any in the second class I am nominally 👍 on deprecating / addressing on a case-by-case basis.

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