python修改文件时，使用w模式会将原本的文件清空/覆盖。可以先用读（r）的方式打开，写到内存中,然后再用写（w）的方式打开。

1、替换文本中的taste 为 tasting

Yesterday when I was young
昨日当我年少轻狂
The taste of life was sweet
生命的滋味是甜的
As rain upon my tongue

#将文件读取到内存中
with open("./fileread.txt","r",encoding="utf-8") as f:
 lines = f.readlines() 
#写的方式打开文件
with open("./fileread.txt","w",encoding="utf-8") as f_w:
 for line in lines:
 if "taste" in line:
 #替换
 line = line.replace("taste","tasting")
 f_w.write(line)

2、全文中搜索替换或者单行替换

#文本内容
Yesterday when I was young
昨日当我年少轻狂
The taste of life was sweet
生命的滋味是甜的
As rain upon my tongue
taste
taste
taste
taste

#定义一个函数，带有4个参数
#x 表示要更新的文件名称
#y 表示要被替换的内容
#z 表示 替换后的内容
#s 默认参数为 1 表示只替换第一个匹配到的字符串
# 如果参数为 s = 'g' 则表示全文替换
def string_switch(x,y,z,s=1):
 with open(x, "r", encoding="utf-8") as f:
 #readlines以列表的形式将文件读出
 lines = f.readlines()
 
 with open(x, "w", encoding="utf-8") as f_w:
 #定义一个数字，用来记录在读取文件时在列表中的位置
 n = 0
 #默认选项，只替换第一次匹配到的行中的字符串
 if s == 1:
 for line in lines:
 if y in line:
 line = line.replace(y,z)
 f_w.write(line)
 n += 1
 break
 f_w.write(line)
 n += 1
 #将剩余的文本内容继续

测试

1)默认参数 1，只替换匹配到的第一行

string_switch("fileread.txt","taste","tasting")
 
#结果
Yesterday when I was young
昨日当我年少轻狂
The tasting of life was sweet
生命的滋味是甜的
As rain upon my tongue
taste
taste
taste
taste

2)全局替换

string_switch("fileread.txt","taste","tasting","g")
 
#结果
Yesterday when I was young
昨日当我年少轻狂
The tasting of life was sweet
生命的滋味是甜的
As rain upon my tongue
tasting
tasting
tasting
tasting