Stopping list selection? [duplicate]

Answer 1

You can simply filter the tuples from the list as a generator expression and then you can stop taking the values from the generator expression when you get the first tuple whose second element is -1, like this

>>> s = [(0,-1), (1,0), (2,-1), (3,0), (4,0), (5,-1), (6,0), (7,-1)]
>>> from itertools import takewhile
>>> X = 3
>>> list(takewhile(lambda x: x[1] != -1, (item for item in s if item[0] >= X)))
[(3, 0), (4, 0)]

Here, the generator expression, (item for item in s if item[0] >= X) will give values one-by-one, on demand, (they are not generated all at once, so we save memory here) which are greater than or equal to X.

Then, we take values from that generator expression, only till we find a tuple whose second element is not equal to -1, with itertools.takewhile.

Answer 2

Here's a slightly hacky to implement takewhile as part of a generator expression:

def stop(): raise StopIteration

result = (
    stop() if item[1] == -1 else
    item
    for item in s
    if item[0] >= X
)

Or with different phrasing:

def until(cond):
    if cond: raise StopIteration
    return True

result = (
    item for item in s
    if item[0] >= X
    and until(item[1] == -1)
)