Sometimes you need to build a list in a loop, but break from the loop if some condition is met, keeping the list up to that point. This is so common it doesn't really need an example.

Trying to shoehorn the break keyword in to the generator expression syntax doesn't look pretty, unless you add `and break if expr` to the end, but that has its own issues. Besides, overloading `while` is much cuter...

    ls = [ expr for name in iterable while expr ]

    ls = [ expr for name in iterable if expr while expr ]

    ls = [ expr for name in iterable if expr else expr while expr ]

With regular generator expressions, it gives you a kind of base case, which may assist with showing off.

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