import sys

class print_and_save(object):
    def __init__(self, filepath):
        self.f = open(filepath, 'w')

    def __enter__(self):
        self.old=sys.stdout #將當前系統輸出儲存到臨時變數
        sys.stdout=self
    
    def write(self, message):
        self.old.write(message)
        self.f.write(message)

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.f.close()
        sys.stdout=self.old


with print_and_save("a.txt"):
    print("Hello world")

# 輸出：Hello world
# 檔案a.txt 內容也寫入了 Hello world