Using while in list comprehension or generator exp

Question

This question already has an answer here:

• Stopping list selection? 2 answers

I can use if and for in list comprehensions/generator expressions as

list(i for i in range(100) if i*i < 30)

I know this is not the most efficient but bear with me as the condition could be much more complicated and this is just an example. However, this still goes through hundred iterations and only yields a value in the first 6. Is there a way to tell the generator expression where to stop with something like this:

list(i for i in range(100) while i*i < 30)

However, while is not understood in generator expressions. So, my question is, how do I write a generator expression with a stopping condition so it does not continue computation, even if it doesn't yield new values.

Answer 1

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

Answer 2

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.