python中文件操作实现全文或单行替换方法

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输出
			for i in range(n,len(lines)):
				f_w.write(lines[i])
		#全局匹配替换
		elif s == 'g':
			for line in lines:
				if y in line:
					line = line.replace(y,z)
				f_w.write(line)

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

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