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The documentation for traceback.format_tb says "Return a list of "pre-processed" stack trace entries extracted from the traceback object tb. It is useful for alternate formatting of stack traces. The optional limit argument has the same meaning as for print_tb(). A "pre-processed" stack trace entry is a 4-tuple (filename, line number, function name, text) representing the information that is usually printed for a stack trace."
However, as of 3.5 it actually returns a StackSummary object, which is a subclass of list and contains FrameSummary objects. FrameSummary objects are not tuples. The documentation is accurate for 3.4 and earlier. I'm surprised this change was made because it seems like a compatibility break, but it's too late to do something about that now.
The documentation should be changed to correct the return value and add a "versionchanged: 3.5" note.

The documentation you've quoted is for traceback.extract_tb(). traceback.format_tb() documentation only says:
 A shorthand for ``format_list(extract_tb(tb, limit))``.
Issue 27910 is about updating the documentation of extract_tb(), format_list(), and StackSummary.from_list().
I'm closing this as a duplicate of issue 27910.