def alter(file,old_str,new_str):
  """
  替换文件中的字符串
  :param file:文件名
  :param old_str:就字符串
  :param new_str:新字符串
  :return:
  """
  file_data = ""
  with open(file, "r", encoding="utf-8") as f:
    for line in f:
      if old_str in line:
        line = line.replace(old_str,new_str)
      file_data += line
  with open(file,"w",encoding="utf-8") as f:
    f.write(file_data)

alter("file1", "09876", "python")

import os
def alter(file,old_str,new_str):
  """
  将替换的字符串写到一个新的文件中，然后将原文件删除，新文件改为原来文件的名字
  :param file: 文件路径
  :param old_str: 需要替换的字符串
  :param new_str: 替换的字符串
  :return: None
  """
  with open(file, "r", encoding="utf-8") as f1,open("%s.bak" % file, "w", encoding="utf-8") as f2:
    for line in f1:
      if old_str in line:
        line = line.replace(old_str, new_str)
      f2.write(line)
  os.remove(file)
  os.rename("%s.bak" % file, file)

alter("file1", "python", "测试")

import re,os
def alter(file,old_str,new_str):

  with open(file, "r", encoding="utf-8") as f1,open("%s.bak" % file, "w", encoding="utf-8") as f2:
    for line in f1:
      f2.write(re.sub(old_str,new_str,line))
  os.remove(file)
  os.rename("%s.bak" % file, file)
alter("file1", "admin", "password")