应评论者的要求发布我对类似问题的回答的答案，其中使用相同的技术来改变文件的最后一行，而不仅仅是获取它。
对于较大的文件，mmap这是执行此操作的最佳方法。为了改进现有mmap答案，此版本可在 Windows 和 Linux 之间移植，并且应该运行得更快（尽管如果不对 32 位 Python 与 GB 范围内的文件进行一些修改，它将无法工作，请参阅其他答案以获取有关处理此问题的提示，并用于修改以在 Python 2 上工作）。
import io # Gets consistent version of open for both Py2.7 and Py3.x
import itertools
import mmap
def skip_back_lines(mm, numlines, startidx):
'''Factored out to simplify handling of n and offset'''
for _ in itertools.repeat(None, numlines):
startidx = mm.rfind(b'\n', 0, startidx)
if startidx < 0:
break
return startidx
def tail(f, n, offset=0):
# Reopen file in binary mode
with io.open(f.name, 'rb') as binf, mmap.mmap(binf.fileno(), 0, access=mmap.ACCESS_READ) as mm:
# len(mm) - 1 handles files ending w/newline by getting the prior line
startofline = skip_back_lines(mm, offset, len(mm) - 1)
if startofline < 0:
return [] # Offset lines consumed whole file, nothing to return
# If using a generator function (yield-ing, see below),
# this should be a plain return, no empty list
endoflines = startofline + 1 # Slice end to omit offset lines
# Find start of lines to capture (add 1 to move from newline to beginning of following line)
startofline = skip_back_lines(mm, n, startofline) + 1
# Passing True to splitlines makes it return the list of lines without
# removing the trailing newline (if any), so list mimics f.readlines()
return mm[startofline:endoflines].splitlines(True)
# If Windows style \r\n newlines need to be normalized to \n, and input
# is ASCII compatible, can normalize newlines with:
# return mm[startofline:endoflines].replace(os.linesep.encode('ascii'), b'\n').splitlines(True)
这假设拖尾的行数足够小，您可以一次安全地将它们全部读入内存；您还可以将其设为生成器函数，并通过将最后一行替换为以下内容来手动读取一行：
mm.seek(startofline)
# Call mm.readline n times, or until EOF, whichever comes first
# Python 3.2 and earlier:
for line in itertools.islice(iter(mm.readline, b''), n):
yield line
# 3.3+:
yield from itertools.islice(iter(mm.readline, b''), n)
最后，以二进制模式读取（必须使用mmap），因此它给str出行（Py2）和bytes行（Py3）；如果您想要unicode（Py2）或str（Py3），可以调整迭代方法以为您解码和/或修复换行符：
lines = itertools.islice(iter(mm.readline, b''), n)
if f.encoding: # Decode if the passed file was opened with a specific encoding
lines = (line.decode(f.encoding) for line in lines)
if 'b' not in f.mode: # Fix line breaks if passed file opened in text mode
lines = (line.replace(os.linesep, '\n') for line in lines)
# Python 3.2 and earlier:
for line in lines:
yield line
# 3.3+:
yield from lines
注意：我在无法访问 Python 进行测试的机器上输入了这一切。如果我输入任何错误，请告诉我；这与我认为应该可以使用的其他答案非常相似，但是调整（例如处理 an ）可能会导致细微的错误。如果有任何错误，请在评论中告诉我。offset