我是Python的新手（因为我通过Codecademy课程学习了它）并且可以使用一些帮助来搞清楚这一点。
我有一个文件'TestingDeleteLines.txt'，大约有300行文字。现在，我试图让它从该文件中打印出10条随机行，然后删除这些行。
所以，如果我的文件有10行：
Carrot
Banana
Strawberry
Canteloupe
Blueberry
Snacks
Apple
Raspberry
Papaya
Watermelon
我需要它从这些行中随机挑出，告诉我它是随机挑选的蓝莓，胡萝卜，西瓜和香蕉，然后删除这些行。
问题是，当Python读取文件时，它会读取该文件，一旦它到达结尾，它就不会返回并删除这些行。我目前的想法是，我可以将行写入列表，然后重新打开文件，将列表与文本文件匹配，如果找到匹配项，则删除行。
我目前的问题是双重的：
random.sample似乎不起作用，因为当我稍后使用每一行附加到URL时，我需要将这些行分开。import webbrowser
import random
"""url= 'http://www.google.com'
webbrowser.open_new_tab(url+myline)""" Eventually, I need a base URL + my 10 random lines opening in each new tab
def ShowMeTheRandoms():
x=1
DeleteList= []
lines=open('TestingDeleteLines.txt').read().splitlines()
for x in range(0,10):
myline=random.choice(lines)
print(myline) """debugging, remove later"""
DeleteList.append(myline)
x=x+1
print DeleteList """debugging, remove later"""
ShowMeTheRandoms()
我有一个文件'TestingDeleteLines.txt'，大约有300行文字。现在，我试图让它从该文件中打印出10条随机行，然后删除这些行。
#!/usr/bin/env python
import random
k = 10
filename = 'TestingDeleteLines.txt'
with open(filename) as file:
lines = file.read().splitlines()
if len(lines) > k:
random_lines = random.sample(lines, k)
print("\n".join(random_lines)) # print random lines
with open(filename, 'w') as output_file:
output_file.writelines(line + "\n"
for line in lines if line not in random_lines)
elif lines: # file is too small
print("\n".join(lines)) # print all lines
with open(filename, 'wb', 0): # empty the file
pass
O(n**2)算法是必要的can be improved（你不需要它来输入一个小文件）
要点是：你不要从文件中“删除”，而是用新内容重写整个文件（或另一个文件）。规范的方法是逐行读取原始文件，将要保留的行写回临时文件，然后用新文件替换旧文件。
with open("/path/to/source.txt") as src, open("/path/to/temp.txt", "w") as dest:
for line in src:
if should_we_keep_this_line(line):
dest.write(line)
os.rename("/path/to/temp.txt", "/path/to/source.txt")
那么list.pop呢 - 它会为你提供项目并一步更新列表。
lines = readlines()
deleted = []
indices_to_delete = random.sample(xrange(len(lines)), 10)
# sort to delete biggest index first
indices_to_delete.sort(reverse=True)
for i in indices_to_delete:
# lines.pop(i) delete item at index i and return the item
# do you need it or its index in the original file than
deleted.append((i, lines.pop(i)))
# write the updated *lines* back to the file or new file ?!
# and you have everything in deleted if you need it again
要从文件中选择随机行，您可以使用节省空间的单通道reservoir-sampling algorithm。要删除该行，您可以打印除所选行之外的所有内容：
#!/usr/bin/env python3
import fileinput
with open(filename) as file:
k = select_random_it(enumerate(file), default=[-1])[0]
if k >= 0: # file is not empty
with fileinput.FileInput(filename, inplace=True, backup='.bak') as file:
for i, line in enumerate(file):
if i != k: # keep line
print(line, end='') # stdout is redirected to filename
其中select_random_it() implements the reservoir-sampling algorithm：
import random
def select_random_it(iterator, default=None, randrange=random.randrange):
"""Return a random element from iterator.
Return default if iterator is empty.
iterator is exhausted.
O(n)-time, O(1)-space algorithm.
"""
# from https://stackoverflow.com/a/1456750/4279
# select 1st item with probability 100% (if input is one item, return it)
# select 2nd item with probability 50% (or 50% the selection stays the 1st)
# select 3rd item with probability 33.(3)%
# select nth item with probability 1/n
selection = default
for i, item in enumerate(iterator, start=1):
if randrange(i) == 0: # random [0..i)
selection = item
return selection
要从文件中打印k随机行并删除它们：
#!/usr/bin/env python3
import random
import sys
k = 10
filename = 'TestingDeleteLines.txt'
with open(filename) as file:
random_lines = reservoir_sample(file, k) # get k random lines
if not random_lines: # file is empty
sys.exit() # do nothing, exit immediately
print("\n".join(map(str.strip, random_lines))) # print random lines
delete_lines(filename, random_lines) # delete them from the file
其中reservoir_sample()使用与select_random_it()相同的算法但允许选择k项而不是一个：
import random
def reservoir_sample(iterable, k,
randrange=random.randrange, shuffle=random.shuffle):
"""Select *k* random elements from *iterable*.
Use O(n) Algorithm R https://en.wikipedia.org/wiki/Reservoir_sampling
If number of items less then *k* then return all items in random order.
"""
it = iter(iterable)
if not (k > 0):
raise ValueError("sample size must be positive")
sample = list(islice(it, k)) # fill the reservoir
shuffle(sample)
for i, item in enumerate(it, start=k+1):
j = randrange(i) # random [0..i)
if j < k:
sample[j] = item # replace item with gradually decreasing probability
return sample
和delete_lines()实用程序功能删除文件中选择的随机行：
import fileinput
import os
def delete_lines(filename, lines):
"""Delete *lines* from *filename*."""
lines = set(lines) # for amortized O(1) lookup
with fileinput.FileInput(filename, inplace=True, backup='.bak') as file:
for line in file:
if line not in lines:
print(line, end='')
os.unlink(filename + '.bak') # remove backup if there is no exception
qazxsw poi，qazxsw poi函数不会将整个文件加载到内存中，因此它们可以用于任意大文件。
让我们假设您有一个存储在reservoir_sample()中的文件中的行列表
delete_lines()从这里，您将覆盖以前的文本文件，其中items的内容与您的首选行结尾\ r \ n或\ n连接。 >>> items = ['a', 'b', 'c', 'd', 'e', 'f']
>>> choices = random.sample(items, 2) # select 2 items
>>> choices # here are the two
['b', 'c']
>>> for i in choices:
... items.remove(i)
...
>>> items # tee daa, no more b or c
['a', 'd', 'e', 'f']
不会删除行结尾，因此如果您使用该方法，则无需添加自己的行结尾。
也许你可以尝试使用0到300生成10个随机数
items然后通过使用列表推导复制从行数组中删除：
readlines()然后将行写回'TestingDeleteLines.txt'。
要查看上述复制代码的工作原理，这篇文章可能会有所帮助：
deleteLineNums = random.sample(xrange(len(lines)), 10)
编辑：要获得随机生成的索引的行，只需执行：
linesCopy = [line for idx, line in enumerate(lines) if idx not in deleteLineNums]
lines[:] = linesCopy
实际的行包含随机生成的行索引的实际行文本。
编辑：甚至更好，使用列表理解：
Remove items from a list while iterating