replace，需要注意 replace 不会改变原 string 的内容

读取文件进行字符替代

filename='learning_python.txt'
with open (filename) as file_object:
    lines=file_object.readlines()
    for line in lines:
        line.replace('Python','C')
        print(line.strip())

运行结果不成功，是因为 replace 不会改变原 string 的内容
可以改成

filename='learning_python.txt'
with open (filename) as file_object:
    lines=file_object.readlines()
    for line in lines:
        print(line.replace('Python','C').strip())

或者

filename='learning_python.txt'
with open (filename) as file_object:
    lines=file_object.readlines()
    for line in lines:
        c=line.replace('Python','C').strip()
        print(c)