Is the idea here to support set_params(step_name=blah). Changing this line to

        new_estimators = list(getattr(self, attr))

should work for other implementations of _BaseComposition

@jnothman I don't follow what you mean. Please send me a PR or take over. thx

The problem with assigning the fitted transformer into the self.steps should be solved after #8350 (but assigning a new transformer in set_params will still raise an error)

The suggestion of @jnothman is also a nice way to fix it (and indeed fixes it for all subclass of BaseComposition), but it has the disadvantage of self.steps changing from tuple to list only after certain usage depending on what the user did.

But maybe in general, do we actually want to allow tuples? (if so, probably need to add a test for it)

I didn't realise this PR was fixing a regression, @agramfort, as #9587 shows this is.