列表推导式不会复制元素
我花了一段时间才弄明白这个问题。请看下面的示例代码,尝试使用不同的方法进行实验。
代码
您可以指定列表元素复制所需的时间以及评估所需的时间。对于列表推导式来说,复制时间是无关紧要的,事实证明如此。
import time
import timeit
import numpy as np
def ObjectFactory(time_eval, time_copy):
"""
Creates a class
Parameters
----------
time_eval : float
time to evaluate (True or False, i.e. keep in list or not) an object
time_copy : float
time to (shallow-) copy an object. Used by list comprehension.
Returns
-------
New class with defined copy-evaluate performance
"""
class Object:
def __init__(self, id_, keep):
self.id_ = id_
self._keep = keep
def __repr__(self):
return f"Object({self.id_}, {self.keep})"
@property
def keep(self):
time.sleep(time_eval)
return self._keep
def __copy__(self):
time.sleep(time_copy)
return self.__class__(self.id_, self._keep)
return Object
def remove_items_from_list_list_comprehension(lst):
return [el for el in lst if el.keep]
def remove_items_from_list_new_list(lst):
new_list = []
for el in lst:
if el.keep:
new_list += [el]
return new_list
def remove_items_from_list_new_list_by_ind(lst):
new_list_inds = []
for ee in range(len(lst)):
if lst[ee].keep:
new_list_inds += [ee]
return [lst[ee] for ee in new_list_inds]
def remove_items_from_list_del_elements(lst):
"""WARNING: Modifies lst"""
new_list_inds = []
for ee in range(len(lst)):
if lst[ee].keep:
new_list_inds += [ee]
for ind in new_list_inds[::-1]:
if not lst[ind].keep:
del lst[ind]
if __name__ == "__main__":
ClassSlowCopy = ObjectFactory(time_eval=0, time_copy=0.1)
ClassSlowEval = ObjectFactory(time_eval=1e-8, time_copy=0)
keep_ratio = .8
n_runs_timeit = int(1e2)
n_elements_list = int(1e2)
lsts_to_tests = dict(
list_slow_copy_remove_many = [ClassSlowCopy(ii, np.random.rand() > keep_ratio) for ii in range(n_elements_list)],
list_slow_copy_keep_many = [ClassSlowCopy(ii, np.random.rand() > keep_ratio) for ii in range(n_elements_list)],
list_slow_eval_remove_many = [ClassSlowEval(ii, np.random.rand() > keep_ratio) for ii in range(n_elements_list)],
list_slow_eval_keep_many = [ClassSlowEval(ii, np.random.rand() > keep_ratio) for ii in range(n_elements_list)],
)
for lbl, lst in lsts_to_tests.items():
print()
for fct in [
remove_items_from_list_list_comprehension,
remove_items_from_list_new_list,
remove_items_from_list_new_list_by_ind,
remove_items_from_list_del_elements,
]:
lst_loc = lst.copy()
t = timeit.timeit(lambda: fct(lst_loc), number=n_runs_timeit)
print(f"{fct.__name__}, {lbl}: {t=}")
输出
remove_items_from_list_list_comprehension, list_slow_copy_remove_many: t=0.0064229519994114526
remove_items_from_list_new_list, list_slow_copy_remove_many: t=0.006507338999654166
remove_items_from_list_new_list_by_ind, list_slow_copy_remove_many: t=0.006562008995388169
remove_items_from_list_del_elements, list_slow_copy_remove_many: t=0.0076057760015828535
remove_items_from_list_list_comprehension, list_slow_copy_keep_many: t=0.006243691001145635
remove_items_from_list_new_list, list_slow_copy_keep_many: t=0.007145451003452763
remove_items_from_list_new_list_by_ind, list_slow_copy_keep_many: t=0.007032064997474663
remove_items_from_list_del_elements, list_slow_copy_keep_many: t=0.007690364996960852
remove_items_from_list_list_comprehension, list_slow_eval_remove_many: t=1.2495998149970546
remove_items_from_list_new_list, list_slow_eval_remove_many: t=1.1657221479981672
remove_items_from_list_new_list_by_ind, list_slow_eval_remove_many: t=1.2621939050004585
remove_items_from_list_del_elements, list_slow_eval_remove_many: t=1.4632593330024974
remove_items_from_list_list_comprehension, list_slow_eval_keep_many: t=1.1344162709938246
remove_items_from_list_new_list, list_slow_eval_keep_many: t=1.1323430630000075
remove_items_from_list_new_list_by_ind, list_slow_eval_keep_many: t=1.1354237199993804
remove_items_from_list_del_elements, list_slow_eval_keep_many: t=1.3084568729973398
timeit
重新测试了您的代码,而不是使用cProfile
。我能够复制您的结果。filter
是最快的。我需要深入挖掘并弄清楚为什么 cProfile 显示 filter 是最快的。我删除了我的答案,因为基于 cProfile vs. timeit 之间的差异只会增加未来知识寻求者的困惑 ;) 唯一可以确定的结论是list.remove
很糟糕。 - Praveen Gollakotac = [x for x in b if tokeep(x)]
花费的时间相同,这很奇怪。 - jfsfilter()
在Python 3中返回一个迭代器。调用结果上的list(),以获取项目。 - jfs