問題：在windows系統中，換行的符號是'\r\n'。python在讀文件的時候為了系統兼容，會默認把'\r'，'n'，'\r\n'都視作換行。但是在windows文件中，可能在同一行中同時存在'\n'，'\r\n'，'\r'。這個時候python的默認行為會將一行拆分成多行輸出，影響預期結果。
此時需要設置open函數的newline參數，修改python對換行的默認行為。
open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)
newline有五種取值：None，''，'\n'，'\r'，'\r\n'。
在輸入過程（從文件到程序），newline用於定義換行的符號：
1.如果newline為None，碰到'\r'，'\n'，'\r\n'都算行尾，而且這些符號都會被轉換成'\n'。
2.如果newline為''，也是碰到'\r'，'\n'，'\r\n'都算行尾，但是這些符號不會發生轉換。
3.如果newline為'\r'，'\n'，'\r\n'，等於是顯示指定了換行符，而且行中的符號不會發生轉換。
在輸出過程（從程序到文件），newline用於指定'\n'的轉換符號：
1.如果newline為None，所有的'\n'都被轉換成系統換行符。
2.如果newline為''，'\n'，不會發生轉換。
3.如果newline為'\r'，'\r\n'，所有的'\n'會被轉換成'\r'或者'\r\n'。
實例一：輸出不指定newline，所有的'\n'都被替換成'\r\n'，即使是'\r\n'中的'\n'也不例外。
def file_seperator_test1():
# output
with open("medical.txt", "w") as f:
f.write("I am a\r good\n boy.\r\n")
#input
with open("medical.txt", "r", newline="\r\n") as f:
print(list(f))
if __name__ == "__main__":
file_seperator_test1()
輸出結果：
['I am a\r good\r\n', ' boy.\r\r\n']
實例二： 輸出指定newline為''或'\n'，不會轉換
def file_seperator_test2():
# output
with open("medical.txt", "w", newline="") as f:
f.write("I am a\r good\n boy.\r\n")
with open("medical2.txt", "w", newline="\n") as f:
f.write("I am a\r good\n boy.\r\n")
#input
with open("medical.txt", "r", newline="\r\n") as f:
print(list(f))
with open("medical2.txt", "r", newline="\r\n") as f:
print(list(f))
if __name__ == "__main__":
file_seperator_test2()
輸出結果：
['I am a\r good\n boy.\r\n'] ['I am a\r good\n boy.\r\n']
實例三：輸出指定newline為'\r'或'\r\n'，所有的'\n'都被替換了，當所有'\n'都被替換成'\r'時，在windows中，換行符就不見了，所有的行變成了一行
def file_seperator_test3():
# output
with open("medical.txt", "w", newline="\r") as f:
f.write("I am a\r good\n boy.\r\n where should\r\n I change the line ?\r\n")
f.write("I can't stop\r\n")
with open("medical2.txt", "w", newline="\r\n") as f:
f.write("I am a\r good\n boy.\r\n")
#input
with open("medical.txt", "r", newline="\r\n") as f:
print(list(f))
with open("medical2.txt", "r", newline="\r\n") as f:
print(list(f))
if __name__ == "__main__":
file_seperator_test3()
輸出結果：
["I am a\r good\r boy.\r\r where should\r\r I change the line ?\r\rI can't stop\r\r"] ['I am a\r good\r\n', ' boy.\r\r\n']
實例四：輸入不指定newline，默認把所有的三種符號都當做換行符，而且全都轉換成'\n'
def file_seperator_test4():
# output
with open("medical.txt", "w", newline="") as f:
f.write("I am a\r good\n boy.\r\n")
#input
with open("medical.txt", "r") as f:
print(list(f))
if __name__ == "__main__":
file_seperator_test4()
輸出結果：
['I am a\n', ' good\n', ' boy.\n']
實例五：輸入指定newline為''，仍然把三種符號都當做換行符，但是不轉換
def file_seperator_test5():
# output
with open("medical.txt", "w", newline="") as f:
f.write("I am a\r good\n boy.\r\n")
#input
with open("medical.txt", "r", newline="") as f:
print(list(f))
if __name__ == "__main__":
file_seperator_test5()
輸出結果：
['I am a\r', ' good\n', ' boy.\r\n']
實例六：輸入指定newline為'\r'，'\n'，'\r\n'，顯式指定了換行符，只有碰到這幾個符號才會換行
def file_seperator_test6():
# output
with open("medical.txt", "w", newline="") as f:
f.write("I am a\r good\n boy.\r\n where should\r\n I change the line ?\r\n")
f.write("I can't stop\r\n")
with open("medical2.txt", "w", newline="") as f:
f.write("I am a\r good\n boy.\r\n where should\r\n I change the line ?\r\n")
f.write("I can't stop\r\n")
with open("medical3.txt", "w", newline="") as f:
f.write("I am a\r good\n boy.\r\n where should\r\n I change the line ?\r\n")
f.write("I can't stop\r\n")
#input
with open("medical.txt", "r", newline="\r") as f:
print(list(f))
with open("medical2.txt", "r", newline="\n") as f:
print(list(f))
with open("medical3.txt", "r", newline="\r\n") as f:
print(list(f))
if __name__ == "__main__":
file_seperator_test6()
輸出結果：
['I am a\r', ' good\n boy.\r', '\n where should\r', '\n I change the line ?\r', "\nI can't stop\r", '\n'] ['I am a\r good\n', ' boy.\r\n', ' where should\r\n', ' I change the line ?\r\n', "I can't stop\r\n"] ['I am a\r good\n boy.\r\n', ' where should\r\n', ' I change the line ?\r\n', "I can't stop\r\n"]
結論：
1.如果要寫入帶'\n'的行，可以把newline設定為''或者'\n'，避免python更改'\n'
2.如果要讀入帶'\n'的行，可以把newline設定為'\r\n'，指定換行符只能是'\r\n'。