How common is the case of variable number of things to concatenate/union/merge?

From my experience, in most ceases this looks like:

    result = []
    for ...:
        # many complex statements
        # may include continue and break
        result.extend(items) # may be intermixed with result.append(item)

So concatenating purely lists from some sequence is very special case. And there are several ways to perform it.

    result = []
    for items in seq:
        result.extend(items)
        # nothing wrong with this simple code, really

    result = [x for items in seq for x in items]
    # may be less effective for really long sublists,
    # but looks simple

    result = list(itertools.chain.from_iterable(items))
    # if you are itertools addictive ;-)