Using while in list comprehension or generator expressions

Because the syntax of takewhile() and dropwhile() is not the clearest, here are the actual examples of your question:

>>> [i for i in itertools.takewhile(lambda x: x*x<30, range(10))]
[0, 1, 2, 3, 4, 5]
>>> [i for i in itertools.dropwhile(lambda x: x*x<30, range(10))]
[6, 7, 8, 9] 

Know that the author of itertools has questioned whether to deprecate these functions.

The various functions in itertools (takewhile() comes to mind) can help.