t.py

with open(r'C:\Users\cc\Desktop\909090.txt','w',encoding='utf-8') as file:
    file.write("嘻嘻嘻嘻哦")

with open(r'C:\Users\cc\Desktop\909090.txt','r',encoding='utf-8') as file:
    h=file.read()
#data=''
with open(r'C:\Users\cc\Desktop\909090.txt','r',encoding='utf-8') as file:
    for ad in file:
        if "哦" in ad:
            new=ad.replace("哦","y")
        #data+=
    print(new)