[英]Python 3 remove None values from end of sublist or exclude if sublist is entirely None values
我有一個列表列表,其中一些子列表完全由None
,有些子列表在字符串的末尾和字符串之間None
。 我需要做三件事:
如果有子列表,請從子列表的末尾刪除None
,將中間的用空字符串分隔開的那些替換。
排除完全為None
的子列表
用結果創建一個新的列表列表
我的嘗試獲得了預期的結果,但是我想知道是否有更快的方法可以做到這一點:
from itertools import islice
rows = [["row 1 index 0",None,"row 1 index 2",None,None],
[None,"row 2 index 1",None,None,None],
[None,None,None,None,None]]
data = []
for r in rows:
for i,c in enumerate(reversed(r)):
if c is not None:
data.append(["" if x is None else
str(x) for x in islice(r,0,len(r)-i)])
break
print (data)
所需的結果/輸出:
[['row 1 index 0', '', 'row 1 index 2'], ['', 'row 2 index 1']]
基准測試(據我所知):
from itertools import islice
import time
q = ["string",None,"string",None,"string"] + [None] * 95
rows = [q.copy() for i in range(500000)]
for z in range(1,6):
st = time.time()
data = []
for r in rows:
for i,c in enumerate(reversed(r)):
if c is not None:
data.append(["" if x is None else
str(x) for x in islice(r,0,len(r)-i)])
break
end = time.time()
print ("Run: " + str(z) + "| time: " + str(end-st))
結果(i5 ivybridge Windows 10):
Run: 1| time: 5.787390232086182
Run: 2| time: 5.802111387252808
Run: 3| time: 5.697156190872192
Run: 4| time: 5.38789963722229
Run: 5| time: 5.739344596862793
您可以通過列表理解逐步消除None
from itertools import dropwhile
rows = [["row 1 index 0",None,"row 1 index 2",None,None],
[None,"row 2 index 1",None,None,None],
[None,None,None,None,None]]
# remove lists with all Nones
rows1 = [row for row in rows if set(row) != {None}]
# remove trailing Nones
rows2 = [dropwhile(lambda x: x is None, reversed(row)) for row in rows1]
# replace None with ''
rows3 = [list(reversed([x if x is not None else '' for x in row])) for row in rows2]
print(rows3)
輸出:
[['row 1 index 0', '', 'row 1 index 2'], ['', 'row 2 index 1']]
性能取決於數據的性質! 請參閱以下時序,並傾向於您認為對預期遇到的數據集更好的時序。 我提出了一個部分就地的解決方案,該解決方案現在似乎在我的測試中具有最佳性能,但是希望可以很明顯地充實基准測試,以真實了解您的取舍。
首先,我進行了測試。
In [60]: rows = [["row 1 index 0",None,"row 1 index 2",None,None],
...: [None,"row 2 index 1",None,None,None],
...: [None,None,None,None,None]]
In [61]: rowsbig = [r*1000 for r in rows]
In [62]: rowsbig = [list(r) for _ in range(1000) for r in rowsbig]
In [63]: sum(len(r) for r in rowsbig)
Out[63]: 15000000
現在,有個小幫手可以保持衛生:
In [65]: def test_set(source=rowsbig):
...: return [list(r) for r in source]
...:
因此,讓我們將三種建議的方法包裝在函數中:
In [86]: def new_to_coding(rows):
...: data = []
...: for r in rows:
...: for i,c in enumerate(reversed(r)):
...: if c is not None:
...: data.append(["" if x is None else
...: str(x) for x in islice(r,0,len(r)-i)])
...: break
...: return data
...:
In [87]: def Bit(rows):
...: data = [list(map(lambda x: '' if x is None else x, row)) for row in rows]
...: data = [row[:max(i for i, e in enumerate(row, 1) if e is not '')] for row in data if set(row) != {''}]
...: return data
...:
In [88]: def taras(rows):
...: # remove lists with all Nones
...: rows1 = [row for row in rows if set(row) != {None}]
...: # remove trailing Nones
...: rows2 = [dropwhile(lambda x: x is None, reversed(row)) for row in rows1]
...: # replace None with ''
...: rows3 = [list(reversed([x if x is not None else '' for x in row])) for row in rows2]
...: return rows3
...:
快速檢查一下:
In [89]: taras(test_set()) == new_to_coding(test_set())
Out[89]: True
In [90]: Bit(test_set()) == new_to_coding(test_set())
Out[90]: True
現在,一些定時設置。 注意@new_to_coding始終使用timeit
模塊創建基准。 天真的time.time()
方法忽略了許多微妙之處,並且更加方便!
In [91]: from timeit import timeit
In [92]: setup = "from __main__ import new_to_coding, Bit, taras, test_set; testrows = test_set()"
現在,結果:
In [93]: # using OP's method
...: timeit('new_to_coding(testrows)', setup, number=5)
Out[93]: 5.416837869910523
In [94]: # using `Bit`
...: timeit('Bit(testrows)', setup, number=5)
Out[94]: 14.52187539380975
In [95]: # using `taras`
...: timeit('taras(testrows)', setup, number=5)
Out[95]: 3.7361009169835597
因此,似乎漸進方法是成功的! 當然,數據的確切性質可能會更改這些相對時間。 我懷疑“ None
”行的比例會影響這些方法的相對性能。 警告! 事實證明這是非常正確的! 查看編輯
我已經進行了微優化的@taras方法,確保所有名稱都在函數本地,因此無需全局查找,將list(reversed(alist))
替換為alist[::-1]
,並中間轉換生成器表達式,以便僅實現一個列表:
In [111]: def is_None(x): return x is None
...:
...: def taras_micro_op(rows, dropwhile=dropwhile, reversed=reversed, set=set, is_None=is_None):
...: # remove lists with all Nones
...: rows1 = (row for row in rows if set(row) != {None})
...: # remove trailing Nones
...: rows2 = (dropwhile(is_None, reversed(row)) for row in rows1)
...: # replace None with ''
...: rows3 = [[x if x is not None else '' for x in row][::-1] for row in rows2]
...: return rows3
...:
In [112]: taras_micro_op(test_set()) == taras(test_set())
Out[112]: True
In [113]: setup = "from __main__ import taras, taras_micro_op, test_set; testrows = test_set()"
In [114]: # using `taras`
...: timeit('taras(testrows)', setup, number=50)
Out[114]: 35.11660181987099
In [115]: # using `taras_micro_op`
...: timeit('taras_micro_op(testrows)', setup, number=50)
Out[115]: 33.70030225184746
In [116]: 33.70030225184746 / 35.11660181987099
Out[116]: 0.9596686611281929
不到5%的改善。 的確,如果只是為了提高內存效率,我會放棄“使用默認參數進行內聯”,而僅使用中間生成器表達式。
換句話說,我建議使用以下內容:
In [117]: def taras_memory_op(rows):
...: # remove lists with all Nones
...: rows1 = (row for row in rows if set(row) != {None})
...: # remove trailing Nones
...: rows2 = (dropwhile(lambda x: x is None, reversed(row)) for row in rows1)
...: # replace None with ''
...: rows3 = [[x if x is not None else '' for x in row][::-1] for row in rows2]
...: return rows3
...:
In [118]: setup = "from __main__ import taras, taras_memory_op, test_set; testrows = test_set()"
In [119]: # using `taras`
...: timeit('taras(testrows)', setup, number=50)
Out[119]: 35.10479677491821
In [120]: # using `taras`
...: timeit('taras_memory_op(testrows)', setup, number=50)
Out[120]: 34.00812040804885
In [121]: 34.00812040804885/35.10479677491821
Out[121]: 0.9687599283396816
因為實際上大多數已經很小的改進實際上都來自使用生成器表達式!
因此,我使用op提供的測試集進行了嘗試:
In [3]: q = ["string",None,"string",None,"string"] + [None] * 95
...: rows = [q.copy() for i in range(500000)]
...:
In [4]: sum(len(r) for r in rows)
Out[4]: 50000000
注意,在我最初的測試集中,大約有33%的“所有None
”行。 但是,在上面, 沒有所有行都為None
行 。 事實證明,這肯定會影響性能。
In [7]: def test_set(source=rows):
...: return [list(r) for r in source]
...:
In [8]: setup = "from __main__ import new_to_coding, taras_memory_op, test_set; testrows = test_set()"
In [9]: # using OP's method
...: timeit('new_to_coding(testrows)', setup, number=5)
Out[9]: 14.014577565016225
In [10]: # using `taras`
...: timeit('taras_memory_op(testrows)', setup, number=5)
Out[10]: 33.28037207596935
因此,我提出了另一種解決方案。 警告! 以下解決方案可就地更改內部列表 :
In [14]: def sanitize(rows):
...: result = []
...: for row in rows:
...: tail = True
...: maxidx = len(row) - 1
...: for i, item in enumerate(reversed(row)):
...: if item is None:
...: if tail:
...: row.pop()
...: else:
...: row[maxidx - i] = ''
...: else:
...: tail = False
...: if row:
...: result.append(row)
...: return result
...:
In [15]: setup = "from __main__ import new_to_coding, taras_memory_op, sanitize, test_set; testrows = test_set()"
In [16]: # using `sanitize`
...: timeit('sanitize(testrows)', setup, number=5)
Out[16]: 8.261458976892754
In [17]: sanitize(test_set()) == new_to_coding(test_set())
Out[17]: True
因此,使用我最初制作的測試集:
In [18]: rows = [["row 1 index 0",None,"row 1 index 2",None,None],
...: [None,"row 2 index 1",None,None,None],
...: [None,None,None,None,None]]
In [19]:
In [19]: rows = [r*1000 for r in rows]
In [20]: rowsbig = [list(r) for _ in range(1000) for r in rows]
In [21]: rows = rowsbig
In [22]: del rowsbig
In [23]: def test_set(source=rows):
...: return [list(r) for r in source]
...:
In [24]: setup = "from __main__ import new_to_coding, taras_memory_op, sanitize, test_set; testrows = test_set()"
In [25]: # using `taras`
...: timeit('taras_memory_op(testrows)', setup, number=10)
Out[25]: 6.563127358909696
In [26]: # using OP's method
...: timeit('new_to_coding(testrows)', setup, number=10)
Out[26]: 10.173962660133839
In [27]: # using `sanitize`
...: timeit('sanitize(testrows)', setup, number=10)
Out[27]: 6.3629974271170795
我敢肯定有一種更緊湊的方法,但這是我對列表理解的看法:
data = [list(map(lambda x: '' if x is None else x, row)) for row in rows]
data = [row[:max(i for i, e in enumerate(row, 1) if e is not '')] for row in data if set(row) != {''}]
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