我有这样的代码:
good = [x for x in mylist if x in goodvals]
bad = [x for x in mylist if x not in goodvals]
目标是根据mylist的内容是否满足条件,将它们拆分为另外两个列表。
我怎样才能做得更优雅呢?我能避免在mylist上做两个单独的迭代吗?我可以通过这样做来提高性能吗?
我有这样的代码:
good = [x for x in mylist if x in goodvals]
bad = [x for x in mylist if x not in goodvals]
目标是根据mylist的内容是否满足条件,将它们拆分为另外两个列表。
我怎样才能做得更优雅呢?我能避免在mylist上做两个单独的迭代吗?我可以通过这样做来提高性能吗?
当前回答
清晰快速
这个列表理解是简单的阅读和快速。这正是上级要求的。
set_good_vals = set(good_vals) # Speed boost.
good = [x for x in my_list if x in set_good_vals]
bad = [x for x in my_list if x not in set_good_vals]
我更喜欢一个列表理解而不是两个,但不像张贴的许多答案(其中一些相当巧妙),它是可读的和清晰的。这也是网页上最快的答案之一。
唯一(稍微)快一点的答案是:
set_good_vals = set(good_vals)
good, bad = [], []
for item in my_list:
_ = good.append(item) if item in set_good_vals else bad.append(item)
...还有它的变体。(见我的另一个答案)。但我觉得第一种方法更优雅,而且几乎一样快。
其他回答
还有另一个答案,简短但“邪恶”(用于理解列表的副作用)。
digits = list(range(10))
odd = [x.pop(i) for i, x in enumerate(digits) if x % 2]
>>> odd
[1, 3, 5, 7, 9]
>>> digits
[0, 2, 4, 6, 8]
def partition(pred, iterable):
'Use a predicate to partition entries into false entries and true entries'
# partition(is_odd, range(10)) --> 0 2 4 6 8 and 1 3 5 7 9
t1, t2 = tee(iterable)
return filterfalse(pred, t1), filter(pred, t2)
检查这个
手动迭代,使用条件选择每个元素将被追加到的列表:
good, bad = [], []
for x in mylist:
(bad, good)[x in goodvals].append(x)
清晰快速
这个列表理解是简单的阅读和快速。这正是上级要求的。
set_good_vals = set(good_vals) # Speed boost.
good = [x for x in my_list if x in set_good_vals]
bad = [x for x in my_list if x not in set_good_vals]
我更喜欢一个列表理解而不是两个,但不像张贴的许多答案(其中一些相当巧妙),它是可读的和清晰的。这也是网页上最快的答案之一。
唯一(稍微)快一点的答案是:
set_good_vals = set(good_vals)
good, bad = [], []
for item in my_list:
_ = good.append(item) if item in set_good_vals else bad.append(item)
...还有它的变体。(见我的另一个答案)。但我觉得第一种方法更优雅,而且几乎一样快。
之前的答案似乎并不能满足我所有的四种强迫症:
尽可能的懒惰, 只对原始Iterable求值一次 每个项只计算谓词一次 提供良好的类型注释(适用于python 3.7)
我的解决方案并不漂亮,我不认为我可以推荐使用它,但它是:
def iter_split_on_predicate(predicate: Callable[[T], bool], iterable: Iterable[T]) -> Tuple[Iterator[T], Iterator[T]]:
deque_predicate_true = deque()
deque_predicate_false = deque()
# define a generator function to consume the input iterable
# the Predicate is evaluated once per item, added to the appropriate deque, and the predicate result it yielded
def shared_generator(definitely_an_iterator):
for item in definitely_an_iterator:
print("Evaluate predicate.")
if predicate(item):
deque_predicate_true.appendleft(item)
yield True
else:
deque_predicate_false.appendleft(item)
yield False
# consume input iterable only once,
# converting to an iterator with the iter() function if necessary. Probably this conversion is unnecessary
shared_gen = shared_generator(
iterable if isinstance(iterable, collections.abc.Iterator) else iter(iterable)
)
# define a generator function for each predicate outcome and queue
def iter_for(predicate_value, hold_queue):
def consume_shared_generator_until_hold_queue_contains_something():
if not hold_queue:
try:
while next(shared_gen) != predicate_value:
pass
except:
pass
consume_shared_generator_until_hold_queue_contains_something()
while hold_queue:
print("Yield where predicate is "+str(predicate_value))
yield hold_queue.pop()
consume_shared_generator_until_hold_queue_contains_something()
# return a tuple of two generators
return iter_for(predicate_value=True, hold_queue=deque_predicate_true), iter_for(predicate_value=False, hold_queue=deque_predicate_false)
用下面的测试,我们从print语句中得到如下输出:
t,f = iter_split_on_predicate(lambda item:item>=10,[1,2,3,10,11,12,4,5,6,13,14,15])
print(list(zip(t,f)))
# Evaluate predicate.
# Evaluate predicate.
# Evaluate predicate.
# Evaluate predicate.
# Yield where predicate is True
# Yield where predicate is False
# Evaluate predicate.
# Yield where predicate is True
# Yield where predicate is False
# Evaluate predicate.
# Yield where predicate is True
# Yield where predicate is False
# Evaluate predicate.
# Evaluate predicate.
# Evaluate predicate.
# Evaluate predicate.
# Yield where predicate is True
# Yield where predicate is False
# Evaluate predicate.
# Yield where predicate is True
# Yield where predicate is False
# Evaluate predicate.
# Yield where predicate is True
# Yield where predicate is False
# [(10, 1), (11, 2), (12, 3), (13, 4), (14, 5), (15, 6)]