python tips:檔案讀取——分行符號的問題

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問題：在windows系統中，換行的符號是‘\r\n‘。python在讀檔案的時候為了系統相容，會預設把‘\r‘，‘n‘，‘\r\n‘都視作換行。但是在windows檔案中，可能在同一行中同時存在‘\n‘，‘\r\n‘，‘\r‘。這個時候python的預設行為會將一行拆分成多行輸出，影響預期結果。

此時需要設定open函數的newline參數，修改python對換行的預設行為。

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（從檔案到程式），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‘，等於是顯示指定了分行符號，而且行中的符號不會發生轉換。

在輸出過程（從程式到檔案），newline用於指定‘\n‘的轉換符號：

1.如果newline為None，所有的‘\n‘都被轉換成系統分行符號。

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‘。

執行個體一：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‘]

執行個體二： 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‘]

執行個體三：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，分行符號就不見了，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‘]

執行個體四：輸入不指定newline，預設把所有的三種符號都當做分行符號，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‘]

執行個體五：輸入指定newline為‘‘，仍然把三種符號都當做分行符號，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‘]

執行個體六：輸入指定newline為‘\r‘，‘\n‘，‘\r\n‘，顯式指定了分行符號，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‘，指定分行符號只能是‘\r\n‘。

 

python tips:檔案讀取——分行符號的問題