如果你使用inplace=1参数，Python标准库的fileinput模块将会inplace重写文件:

import sys
import fileinput

# replace all occurrences of 'sit' with 'SIT' and insert a line after the 5th
for i, line in enumerate(fileinput.input('lorem_ipsum.txt', inplace=1)):
    sys.stdout.write(line.replace('sit', 'SIT'))  # replace 'sit' and write
    if i == 4: sys.stdout.write('\n')  # write a blank line after the 5th line

写了一个小类来干净地做这件事。

import tempfile

class FileModifierError(Exception):
    pass

class FileModifier(object):

    def __init__(self, fname):
        self.__write_dict = {}
        self.__filename = fname
        self.__tempfile = tempfile.TemporaryFile()
        with open(fname, 'rb') as fp:
            for line in fp:
                self.__tempfile.write(line)
        self.__tempfile.seek(0)

    def write(self, s, line_number = 'END'):
        if line_number != 'END' and not isinstance(line_number, (int, float)):
            raise FileModifierError("Line number %s is not a valid number" % line_number)
        try:
            self.__write_dict[line_number].append(s)
        except KeyError:
            self.__write_dict[line_number] = [s]

    def writeline(self, s, line_number = 'END'):
        self.write('%s\n' % s, line_number)

    def writelines(self, s, line_number = 'END'):
        for ln in s:
            self.writeline(s, line_number)

    def __popline(self, index, fp):
        try:
            ilines = self.__write_dict.pop(index)
            for line in ilines:
                fp.write(line)
        except KeyError:
            pass

    def close(self):
        self.__exit__(None, None, None)

    def __enter__(self):
        return self

    def __exit__(self, type, value, traceback):
        with open(self.__filename,'w') as fp:
            for index, line in enumerate(self.__tempfile.readlines()):
                self.__popline(index, fp)
                fp.write(line)
            for index in sorted(self.__write_dict):
                for line in self.__write_dict[index]:
                    fp.write(line)
        self.__tempfile.close()

那么你可以这样使用它:

with FileModifier(filename) as fp:
    fp.writeline("String 1", 0)
    fp.writeline("String 2", 20)
    fp.writeline("String 3")  # To write at the end of the file

这取决于你想做什么。你可以用"a"打开它:

 with open("foo.txt", "a") as f:
     f.write("new line\n")

如果你想要preprend一些东西，你必须先从文件中读取:

with open("foo.txt", "r+") as f:
     old = f.read() # read everything in the file
     f.seek(0) # rewind
     f.write("new line\n" + old) # write the new line before

不幸的是，没有办法在不重写文件的情况下插入到文件中间。正如之前的海报所指出的，你可以使用seek附加到一个文件或覆盖它的一部分，但如果你想在开始或中间添加东西，你必须重写它。

这是操作系统的问题，不是Python的问题。这在所有语言中都是一样的。

我通常会从文件中读取，进行修改，并将其写入一个名为myfile.txt.tmp的新文件或类似的文件。这比将整个文件读入内存要好，因为文件可能太大了。临时文件完成后，我将其重命名为与原始文件相同的名称。

这是一种很好的、安全的方法，因为如果文件写入由于任何原因崩溃或中止，您仍然拥有原始文件。

正如Adam所提到的，在决定是否有足够的内存将其全部读入内存，替换部分并重新写入之前，必须考虑系统的限制。

如果你正在处理一个小文件或没有内存问题，这可能会有帮助:

选择1) 将整个文件读入内存，对整行或部分行执行正则表达式替换，并将其替换为该行加上额外的行。你需要确保“中间行”在文件中是唯一的，或者如果你在每一行上都有时间戳，这应该是相当可靠的。

# open file with r+b (allow write and binary mode)
f = open("file.log", 'r+b')   
# read entire content of file into memory
f_content = f.read()
# basically match middle line and replace it with itself and the extra line
f_content = re.sub(r'(middle line)', r'\1\nnew line', f_content)
# return pointer to top of file so we can re-write the content with replaced string
f.seek(0)
# clear file content 
f.truncate()
# re-write the content with the updated content
f.write(f_content)
# close file
f.close()

选择2) 算出中线，把它替换成这条线加上另外一条线。

# open file with r+b (allow write and binary mode)
f = open("file.log" , 'r+b')   
# get array of lines
f_content = f.readlines()
# get middle line
middle_line = len(f_content)/2
# overwrite middle line
f_content[middle_line] += "\nnew line"
# return pointer to top of file so we can re-write the content with replaced string
f.seek(0)
# clear file content 
f.truncate()
# re-write the content with the updated content
f.write(''.join(f_content))
# close file
f.close()

就地重写文件通常是通过用修改过的名称保存旧副本来完成的。Unix用户添加了一个~来标记旧的。Windows用户会做各种各样的事情——添加.bak或.old——或者完全重命名文件，或者把~放在名称的前面。

import shutil
shutil.move(afile, afile + "~")

destination= open(aFile, "w")
source= open(aFile + "~", "r")
for line in source:
    destination.write(line)
    if <some condition>:
        destination.write(<some additional line> + "\n")

source.close()
destination.close()

您可以使用下面的代码来代替shutil。

import os
os.rename(aFile, aFile + "~")