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itertools.takewhile

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itertools.takewhile enables you to take items from a sequence until a condition first becomes False. 

def is_even(x):
    return x % 2 == 0

lst = [0, 2, 4, 12, 18, 13, 14, 22, 23, 44]
result = list(itertools.takewhile(is_even, lst))

print(result)

This outputs [0, 2, 4, 12, 18].

Note that, the first number that violates the predicate (i.e.: the function returning a Boolean value) is_even is, 13. Once takewhile encounters a value that produces False for the given predicate, it breaks out.

The output produced by takewhile is similar to the output generated from the code below. 

def takewhile(predicate, iterable):
	for x in iterable:
		if predicate(x):
			yield x
		else:
			break

Note: The concatenation of results produced by takewhile and dropwhile produces the original iterable.

result = list(itertools.takewhile(is_even, lst)) + list(itertools.dropwhile(is_even, lst))

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Itertools Module:
* Itertools Module
* itertools.dropwhile
* Combinations method in Itertools Module
* Take a slice of a generator
* Zipping two iterators until they are both exhausted
* Grouping items from an iterable object using a function
* itertools.takewhile
* itertools.count
* itertools.product
* itertools.repeat
* Get an accumulated sum of numbers in an iterable
* Cycle through elements in an iterator
* itertools.permutations
* Chaining multiple iterators together
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