takewhile and dropwhile

takewhile() returns elements while a condition is True, while dropwhile() skips elements while a condition is True, then yields all remaining elements. These functions operate on the leading portion of a sequence based on a predicate, making them especially useful for sorted or ordered data and for lazy evaluation over potentially infinite iterators.

takewhile() - Take While Condition is True

takewhile() yields elements from the start of the iterable as long as the predicate returns True. It stops permanently at the first element that fails the predicate, even if later elements would pass.

from itertools import takewhile

numbers = [1, 2, 3, 4, 5, 3, 2, 1]

# Take while less than 4
result = list(takewhile(lambda x: x < 4, numbers))
print(result)

[1, 2, 3]

dropwhile() - Drop While Condition is True

dropwhile() discards elements from the start of the iterable as long as the predicate returns True. Once the predicate returns False for the first time, it yields that element and all subsequent elements unconditionally.

from itertools import dropwhile

numbers = [1, 2, 3, 4, 5, 3, 2, 1]

# Drop while less than 4
result = list(dropwhile(lambda x: x < 4, numbers))
print(result)

# Practical: skip whitespace at start
lines = ['  ', '  ', 'content', 'more']
content = list(dropwhile(str.isspace, lines))
print(content)

[4, 5, 3, 2, 1]
['content', 'more']