Split a sequence or generator into two iterators, each iterating over the elements that either pass or fail a predicate function.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
from collections import deque
def splitby(pred, seq):
trues = deque()
falses = deque()
iseq = iter(seq)
def pull(source, pred, thisval, thisbuf, otherbuf):
while 1:
while thisbuf:
yield thisbuf.popleft()
newitem = next(source)
# uncomment next line to show that source is processed only once
# print "pulled", newitem
if pred(newitem) == thisval:
yield newitem
else:
otherbuf.append(newitem)
true_iter = pull(iseq, pred, True, trues, falses)
false_iter = pull(iseq, pred, False, falses, trues)
return true_iter, false_iter
Sometimes a sequence of items must be split into 2 groups depending on the evaluation of a predicate function. Using groupby to do this requires that the sequence be sorted, then grouped. Using filter/filterfalse requires 2 passes over the input sequence, which fails if the input is actually a generator. Paul Moore suggests using tee to create a split iterator over the sequence, and then passing the tee'd iterators to filter and filterfalse - this is simpler code, but it does evaluate the predicate twice for each element in the sequence; this recipe does the evaluation once per element and pushes the value to the corresponding output match/no-match list/generator.
The splitby recipe only traverses the input sequence once, so it will work for a generator, and it will only pull from the generator as values are pulled from the two output iterators.
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4