IHMO a contextlib.contextmanager would be simpler. Something like:

@contextlib.contextmanager
def delim_if(self, condition, delimiter="()"):
    if condition:
        self.write(self.delimiter[0]) 
    try:
        yield
    finally:
        if condition:
            self.write(self.delimiter[-1])

# I'm not sure that this one works as expected :-)
def delim(self, delimiter):
    return self.delim_if(True, delimiter)

We started with this, but it introduced a regression in the import time and I reverted it.

#17376

You wouldn't have such issue if ast.unparse would be a submodule....

I dislike the submodule idea more:

#17377 (comment)

Maybe it would be more readable if "()" is always passed explicitly. @pablogsal: what do you think?

I agree, I don't think delimiter should not have a default.