def _flattenTree(request, root, write):
    """
    Make C{root} into an iterable of L{bytes} and L{Deferred} by doing a depth
    first traversal of the tree.

    @param request: A request object which will be passed to
        L{IRenderable.render}.

    @param root: An object to be made flatter.  This may be of type C{unicode},
        L{bytes}, L{slot}, L{Tag <twisted.web.template.Tag>}, L{tuple},
        L{list}, L{types.GeneratorType}, L{Deferred}, or something providing
        L{IRenderable}.

    @param write: A callable which will be invoked with each L{bytes} produced
        by flattening C{root}.

    @return: An iterator which yields objects of type L{bytes} and L{Deferred}.
        A L{Deferred} is only yielded when one is encountered in the process of
        flattening C{root}.  The returned iterator must not be iterated again
        until the L{Deferred} is called back.
    """
    stack = [_flattenElement(request, root, write, [], None, escapeForContent)]
    while stack:
        try:
            frame = stack[-1].gi_frame
            element = next(stack[-1])
        except StopIteration:
            stack.pop()
        except Exception as e:
            stack.pop()
            roots = []
            for generator in stack:
                roots.append(generator.gi_frame.f_locals['root'])
            roots.append(frame.f_locals['root'])
            raise FlattenerError(e, roots, extract_tb(exc_info()[2]))
        else:
            if isinstance(element, Deferred):
                def cbx(originalAndToFlatten):
                    original, toFlatten = originalAndToFlatten
                    stack.append(toFlatten)
                    return original
                yield element.addCallback(cbx)
            else:
                stack.append(element)

    Make C{root} into an iterable of L{bytes} and L{Deferred} by doing a depth
    first traversal of the tree.

    @param request: A request object which will be passed to
        L{IRenderable.render}.

    @param root: An object to be made flatter.  This may be of type C{unicode},
        L{bytes}, L{slot}, L{Tag <twisted.web.template.Tag>}, L{tuple},
        L{list}, L{types.GeneratorType}, L{Deferred}, or something providing
        L{IRenderable}.

    @param write: A callable which will be invoked with each L{bytes} produced
        by flattening C{root}.

    @return: An iterator which yields objects of type L{bytes} and L{Deferred}.
        A L{Deferred} is only yielded when one is encountered in the process of
        flattening C{root}.  The returned iterator must not be iterated again
        until the L{Deferred} is called back.
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/_flatten.py