Python 如何以相反的顺序读取文件？
如何使用python以相反的顺序读取文件？我想把一个文件从最后一行读到第一行Python 如何以相反的顺序读取文件？,python,file,reverse,Python,File,Reverse,如何使用python以相反的顺序读取文件？我想把一个文件从最后一行读到第一行 for line in reversed(open("filename").readlines()): print line.rstrip() 在Python 3中： for line in reversed(list(open("filename"))): print(line.rstrip()) with open('filename', 'r') as f: for line in r
for line in reversed(open("filename").readlines()):
print line.rstrip()
for line in reversed(list(open("filename"))):
print(line.rstrip())
with open('filename', 'r') as f:
for line in reversed(list(f.readlines())):
print(line)
for line in reversed(list(open("filename"))):
print(line.rstrip())
with open('filename', 'r') as f:
for line in reversed(list(f.readlines())):
print(line)
tac$ tac file
tac$ tac file
重新导入
def filerev（somefile，缓冲区=0x20000）：
somefile.seek（0，os.seek\u END）
size=somefile.tell（）
行=['']
rem=缓冲区大小%
位置=最大值（0，（大小//缓冲区-1）*缓冲区）
当pos>=0时：
somefile.seek（位置、操作系统seek\u集）
data=somefile.read（rem+buffer）+行[0]
雷姆=0
lines=re.findall（“[^\n]*\n？”，数据）
ix=长度（线）-2
当ix>0时：
屈服线[ix]
ix-=1
pos-=缓冲区
其他：
屈服线[0]
将open（sys.argv[1]，'r'）作为f：
对于文件中的行版本（f）：
系统标准输出写入（行）
# We need a command to reverse the line order of the file. On Linux this
# is 'tac', on OSX it is 'tail -r'
# 'tac' is not supported on osx, 'tail -r' is not supported on linux.
if sys.platform == "darwin":
command += "|tail -r"
elif sys.platform == "linux2":
command += "|tac"
else:
raise EnvironmentError('Platform %s not supported' % sys.platform)
不久前我不得不这样做，并使用了下面的代码。它通过管道与外壳相连。恐怕我没有完整的剧本了。如果您在unixish操作系统上，可以使用“tac”，但是在例如Mac OSX tac命令不起作用的情况下，请使用tail-r。下面的代码段测试您所处的平台，并相应地调整命令
# We need a command to reverse the line order of the file. On Linux this
# is 'tac', on OSX it is 'tail -r'
# 'tac' is not supported on osx, 'tail -r' is not supported on linux.
if sys.platform == "darwin":
command += "|tail -r"
elif sys.platform == "linux2":
command += "|tac"
else:
raise EnvironmentError('Platform %s not supported' % sys.platform)
在这里你可以找到我的我的实现，你可以通过改变“buffer”变量来限制ram的使用，有一个bug，程序在开始时打印一个空行 而且，如果没有超过缓冲区字节的新行，“leak”变量将增加，直到看到新行（“\n”）为止，ram使用量也可能会增加 这也适用于16GB的文件，比我的总内存大
import os,sys
buffer = 1024*1024 # 1MB
f = open(sys.argv[1])
f.seek(0, os.SEEK_END)
filesize = f.tell()
division, remainder = divmod(filesize, buffer)
line_leak=''
for chunk_counter in range(1,division + 2):
if division - chunk_counter < 0:
f.seek(0, os.SEEK_SET)
chunk = f.read(remainder)
elif division - chunk_counter >= 0:
f.seek(-(buffer*chunk_counter), os.SEEK_END)
chunk = f.read(buffer)
chunk_lines_reversed = list(reversed(chunk.split('\n')))
if line_leak: # add line_leak from previous chunk to beginning
chunk_lines_reversed[0] += line_leak
# after reversed, save the leakedline for next chunk iteration
line_leak = chunk_lines_reversed.pop()
if chunk_lines_reversed:
print "\n".join(chunk_lines_reversed)
# print the last leaked line
if division - chunk_counter < 0:
print line_leak
在这里您可以找到my my实现，您可以通过更改“buffer”变量来限制ram的使用，有一个错误，程序在开始时打印一个空行 而且，如果没有超过缓冲区字节的新行，“leak”变量将增加，直到看到新行（“\n”）为止，ram使用量也可能会增加 这也适用于16GB的文件，比我的总内存大
import os,sys
buffer = 1024*1024 # 1MB
f = open(sys.argv[1])
f.seek(0, os.SEEK_END)
filesize = f.tell()
division, remainder = divmod(filesize, buffer)
line_leak=''
for chunk_counter in range(1,division + 2):
if division - chunk_counter < 0:
f.seek(0, os.SEEK_SET)
chunk = f.read(remainder)
elif division - chunk_counter >= 0:
f.seek(-(buffer*chunk_counter), os.SEEK_END)
chunk = f.read(buffer)
chunk_lines_reversed = list(reversed(chunk.split('\n')))
if line_leak: # add line_leak from previous chunk to beginning
chunk_lines_reversed[0] += line_leak
# after reversed, save the leakedline for next chunk iteration
line_leak = chunk_lines_reversed.pop()
if chunk_lines_reversed:
print "\n".join(chunk_lines_reversed)
# print the last leaked line
if division - chunk_counter < 0:
print line_leak
作为生成器编写的正确、有效的答案
import os
def reverse_readline(filename, buf_size=8192):
"""A generator that returns the lines of a file in reverse order"""
with open(filename) as fh:
segment = None
offset = 0
fh.seek(0, os.SEEK_END)
file_size = remaining_size = fh.tell()
while remaining_size > 0:
offset = min(file_size, offset + buf_size)
fh.seek(file_size - offset)
buffer = fh.read(min(remaining_size, buf_size))
remaining_size -= buf_size
lines = buffer.split('\n')
# The first line of the buffer is probably not a complete line so
# we'll save it and append it to the last line of the next buffer
# we read
if segment is not None:
# If the previous chunk starts right from the beginning of line
# do not concat the segment to the last line of new chunk.
# Instead, yield the segment first
if buffer[-1] != '\n':
lines[-1] += segment
else:
yield segment
segment = lines[0]
for index in range(len(lines) - 1, 0, -1):
if lines[index]:
yield lines[index]
# Don't yield None if the file was empty
if segment is not None:
yield segment
作为生成器编写的正确、有效的答案
import os
def reverse_readline(filename, buf_size=8192):
"""A generator that returns the lines of a file in reverse order"""
with open(filename) as fh:
segment = None
offset = 0
fh.seek(0, os.SEEK_END)
file_size = remaining_size = fh.tell()
while remaining_size > 0:
offset = min(file_size, offset + buf_size)
fh.seek(file_size - offset)
buffer = fh.read(min(remaining_size, buf_size))
remaining_size -= buf_size
lines = buffer.split('\n')
# The first line of the buffer is probably not a complete line so
# we'll save it and append it to the last line of the next buffer
# we read
if segment is not None:
# If the previous chunk starts right from the beginning of line
# do not concat the segment to the last line of new chunk.
# Instead, yield the segment first
if buffer[-1] != '\n':
lines[-1] += segment
else:
yield segment
segment = lines[0]
for index in range(len(lines) - 1, 0, -1):
if lines[index]:
yield lines[index]
# Don't yield None if the file was empty
if segment is not None:
yield segment
像这样的怎么样：
import os
def readlines_reverse(filename):
with open(filename) as qfile:
qfile.seek(0, os.SEEK_END)
position = qfile.tell()
line = ''
while position >= 0:
qfile.seek(position)
next_char = qfile.read(1)
if next_char == "\n":
yield line[::-1]
line = ''
else:
line += next_char
position -= 1
yield line[::-1]
if __name__ == '__main__':
for qline in readlines_reverse(raw_input()):
print qline
由于文件是按相反的顺序逐字符读取的，因此即使在非常大的文件上也能工作，只要单个行适合内存。类似这样的内容如何：
import os
def readlines_reverse(filename):
with open(filename) as qfile:
qfile.seek(0, os.SEEK_END)
position = qfile.tell()
line = ''
while position >= 0:
qfile.seek(position)
next_char = qfile.read(1)
if next_char == "\n":
yield line[::-1]
line = ''
else:
line += next_char
position -= 1
yield line[::-1]
if __name__ == '__main__':
for qline in readlines_reverse(raw_input()):
print qline
由于文件是按相反顺序逐字符读取的，因此即使在非常大的文件上也可以使用，只要内存中有单独的行。创建第二个文件的简单函数（仅限linux）： 如何使用
tac('ordered.csv', 'reversed.csv')
f = open('reversed.csv')
创建第二个文件的简单函数（仅限linux）： 如何使用
tac('ordered.csv', 'reversed.csv')
f = open('reversed.csv')
在处理文件时，请始终将
与with open('filename', 'r') as f:
for line in reversed(f.readlines()):
print line
for line in reversed(list(open("filename"))):
print(line.rstrip())
with open('filename', 'r') as f:
for line in reversed(list(f.readlines())):
print(line)
在处理文件时，请始终将
与with open('filename', 'r') as f:
for line in reversed(f.readlines()):
print line
for line in reversed(list(open("filename"))):
print(line.rstrip())
with open('filename', 'r') as f:
for line in reversed(list(f.readlines())):
print(line)
您还可以使用python模块
文件\u向后读取pip安装文件\u向后读取（v1.2.1），
def previous_line(self, opened_file):
opened_file.seek(0, os.SEEK_END)
position = opened_file.tell()
buffer = bytearray()
while position >= 0:
opened_file.seek(position)
position -= 1
new_byte = opened_file.read(1)
if new_byte == self.NEW_LINE:
parsed_string = buffer.decode()
yield parsed_string
buffer = bytearray()
elif new_byte == self.EMPTY_BYTE:
continue
else:
new_byte_array = bytearray(new_byte)
new_byte_array.extend(buffer)
buffer = new_byte_array
yield None
opened_file = open(filepath, "rb")
iterator = self.previous_line(opened_file)
line = next(iterator) #one step
close(opened_file)
from collections import deque
fs = open("test.txt","rU")
fr = deque(fs)
fr.reverse() # reverse in-place, returns None
for li in fr:
print li
fs.close()