我知道如何为TXT文件执行此操作，但现在我在为CSV文件执行操作时遇到了一些麻烦。
如何在Python中从底部读取CSV文件？
答案 0 :(得分：20)
与文本文件几乎相同：将整个内容读入列表然后向后：
import csv
with open('test.csv', 'r') as textfile:
for row in reversed(list(csv.reader(textfile))):
print ', '.join(row)
如果你想得到花哨的话，你可以编写很多代码来读取从文件末尾开始并向后工作的块，一次发出一行，然后将其提供给csv.reader，但是只能用于可以搜索的文件，即磁盘文件，但不能用于标准输入。
我们中的一些人的文件不适合内存，任何人都可以使用不需要将整个文件存储在内存中的解决方案吗？
这有点棘手。幸运的是，所有csv.reader期望的是一个类似迭代器的对象，每次调用next()时返回一个字符串（行）。所以我们抓住Darius Bacon在“Most efficient way to search the last x lines of a file in python”中提出的技术来向后读取文件的行，而不必拉入整个文件：
import os
def reversed_lines(file):
"Generate the lines of file in reverse order."
part = ''
for block in reversed_blocks(file):
for c in reversed(block):
if c == '\n' and part:
yield part[::-1]
part = ''
part += c
if part: yield part[::-1]
def reversed_blocks(file, blocksize=4096):
"Generate blocks of file's contents in reverse order."
file.seek(0, os.SEEK_END)
here = file.tell()
while 0 < here:
delta = min(blocksize, here)
here -= delta
file.seek(here, os.SEEK_SET)
yield file.read(delta)
并将reversed_lines提供给代码，以便在到达csv.reader之前反转行，从而无需reversed和list：
import csv
with open('test.csv', 'r') as textfile:
for row in csv.reader(reversed_lines(textfile)):
print ', '.join(row)
可能有更多的Pythonic解决方案，它不需要在内存中逐个字符地反转块（提示：只需获取块中有行结束的索引列表，反转它，以及使用它来切片块），并使用chain中的itertools将连续块中的线簇粘合在一起，但这仍然是读者的练习。
值得注意的是，只有CSV文件中的列不包含换行符时，上面的reversed_lines（）惯用法才有效。
AARGH！总有一些东西。幸运的是，解决这个问题并不算太糟糕：
def reversed_lines(file):
"Generate the lines of file in reverse order."
part = ''
quoting = False
for block in reversed_blocks(file):
for c in reversed(block):
if c == '"':
quoting = not quoting
elif c == '\n' and part and not quoting:
yield part[::-1]
part = ''
part += c
if part: yield part[::-1]
当然，如果您的CSV方言不使用"，则需要更改引号字符。
答案 1 :(得分：0)
Building on @mike-desimone 's answer. Here's a solution that provides the same structure as a python file object but is read in reverse, line by line:
import os
class ReversedFile(object):
def __init__(self, f, mode='r'):
"""
Wraps a file object with methods that make it be read in reverse line-by-line
if ``f`` is a filename opens a new file object
"""
if mode != 'r':
raise ValueError("ReversedFile only supports read mode (mode='r')")
if not type(f) == file:
# likely a filename
f = open(f)
self.file = f
self.lines = self._reversed_lines()
def _reversed_lines(self):
"Generate the lines of file in reverse order."
part = ''
for block in self._reversed_blocks():
for c in reversed(block):
if c == '\n' and part:
yield part[::-1]
part = ''
part += c
if part: yield part[::-1]
def _reversed_blocks(self, blocksize=4096):
"Generate blocks of file's contents in reverse order."
file = self.file
file.seek(0, os.SEEK_END)
here = file.tell()
while 0 < here:
delta = min(blocksize, here)
here -= delta
file.seek(here, os.SEEK_SET)
yield file.read(delta)
def __getattribute__(self, name):
"""
Allows for the underlying file attributes to come through
"""
try:
# ReversedFile attribute
return super(ReversedFile, self).__getattribute__(name)
except AttributeError:
# self.file attribute
return getattr(self.file, name)
def __iter__(self):
"""
Creates iterator
"""
return self
def seek(self):
raise NotImplementedError('ReversedFile does not support seek')
def next(self):
"""
Next item in the sequence
"""
return self.lines.next()
def read(self):
"""
Returns the entire contents of the file reversed line by line
"""
contents = ''
for line in self:
contents += line
return contents
def readline(self):
"""
Returns the next line from the bottom
"""
return self.next()
def readlines(self):
"""
Returns all remaining lines from the bottom of the file in reverse
"""
return [x for x in self]
答案 2 :(得分：0)
去吧。这是一个简单的程序来反转CSV文件中的行。
import csv
BC_file = open('Master.csv', 'rb')
BC_reader = csv.reader(BC_file)
next(BC_reader)
for row in reversed(list(BC_reader)):
print row[0]