python tips:文件读取——换行符的问题

Posted 2020-12-30 luoheng23

问题：在windows系统中，换行的符号是‘ ‘。python在读文件的时候为了系统兼容，会默认把‘ ‘，‘n‘，‘ ‘都视作换行。但是在windows文件中，可能在同一行中同时存在‘ ‘，‘ ‘，‘ ‘。这个时候python的默认行为会将一行拆分成多行输出，影响预期结果。

此时需要设置open函数的newline参数，修改python对换行的默认行为。

open(file, mode=‘r‘, buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)

newline有五种取值：None，‘‘，‘ ‘，‘ ‘，‘ ‘。

在输入过程（从文件到程序），newline用于定义换行的符号：

1.如果newline为None，碰到‘ ‘，‘ ‘，‘ ‘都算行尾，而且这些符号都会被转换成‘ ‘。

2.如果newline为‘‘，也是碰到‘ ‘，‘ ‘，‘ ‘都算行尾，但是这些符号不会发生转换。

3.如果newline为‘ ‘，‘ ‘，‘ ‘，等于是显示指定了换行符，而且行中的符号不会发生转换。

在输出过程（从程序到文件），newline用于指定‘ ‘的转换符号：

1.如果newline为None，所有的‘ ‘都被转换成系统换行符。

2.如果newline为‘‘，‘ ‘，不会发生转换。

3.如果newline为‘ ‘，‘ ‘，所有的‘ ‘会被转换成‘ ‘或者‘ ‘。

实例一：输出不指定newline，所有的‘ ‘都被替换成‘ ‘，即使是‘ ‘中的‘ ‘也不例外。

def file_seperator_test1():
    # output
    with open("medical.txt", "w") as f:
        f.write("I am a
 good
 boy.
")
    #input
    with open("medical.txt", "r", newline="
") as f:
        print(list(f))

if __name__ == "__main__":
    file_seperator_test1()

输出结果：

[‘I am a
 good
‘, ‘ boy.

‘]

实例二： 输出指定newline为‘‘或‘ ‘，不会转换

def file_seperator_test2():
    # output
    with open("medical.txt", "w", newline="") as f:
        f.write("I am a
 good
 boy.
")
    with open("medical2.txt", "w", newline="
") as f:
        f.write("I am a
 good
 boy.
")

    #input
    with open("medical.txt", "r", newline="
") as f:
        print(list(f))
    with open("medical2.txt", "r", newline="
") as f:
        print(list(f))

if __name__ == "__main__":
    file_seperator_test2()

输出结果：

[‘I am a
 good
 boy.
‘]
[‘I am a
 good
 boy.
‘]

实例三：输出指定newline为‘ ‘或‘ ‘，所有的‘ ‘都被替换了，当所有‘ ‘都被替换成‘ ‘时，在windows中，换行符就不见了，所有的行变成了一行

def file_seperator_test3():
    # output
    with open("medical.txt", "w", newline="
") as f:
        f.write("I am a
 good
 boy.
 where should
 I change the line ?
")
        f.write("I can‘t stop
")
    with open("medical2.txt", "w", newline="
") as f:
        f.write("I am a
 good
 boy.
")

    #input
    with open("medical.txt", "r", newline="
") as f:
        print(list(f))
    with open("medical2.txt", "r", newline="
") as f:
        print(list(f))


if __name__ == "__main__":
    file_seperator_test3()　

输出结果：

["I am a
 good
 boy.

 where should

 I change the line ?

I can‘t stop

"]
[‘I am a
 good
‘, ‘ boy.

‘]

实例四：输入不指定newline，默认把所有的三种符号都当做换行符，而且全都转换成‘ ‘

def file_seperator_test4():
    # output
    with open("medical.txt", "w", newline="") as f:
        f.write("I am a
 good
 boy.
")
    #input
    with open("medical.txt", "r") as f:
        print(list(f))


if __name__ == "__main__":
    file_seperator_test4()　

输出结果：

[‘I am a
‘, ‘ good
‘, ‘ boy.
‘]

实例五：输入指定newline为‘‘，仍然把三种符号都当做换行符，但是不转换

def file_seperator_test5():
    # output
    with open("medical.txt", "w", newline="") as f:
        f.write("I am a
 good
 boy.
")
    #input
    with open("medical.txt", "r", newline="") as f:
        print(list(f))

if __name__ == "__main__":
    file_seperator_test5()

输出结果：

[‘I am a
‘, ‘ good
‘, ‘ boy.
‘]

实例六：输入指定newline为‘ ‘，‘ ‘，‘ ‘，显式指定了换行符，只有碰到这几个符号才会换行

def file_seperator_test6():
    # output
    with open("medical.txt", "w", newline="") as f:
        f.write("I am a
 good
 boy.
 where should
 I change the line ?
")
        f.write("I can‘t stop
")
    with open("medical2.txt", "w", newline="") as f:
        f.write("I am a
 good
 boy.
 where should
 I change the line ?
")
        f.write("I can‘t stop
")
    with open("medical3.txt", "w", newline="") as f:
        f.write("I am a
 good
 boy.
 where should
 I change the line ?
")
        f.write("I can‘t stop
")

    #input
    with open("medical.txt", "r", newline="
") as f:
        print(list(f))
    with open("medical2.txt", "r", newline="
") as f:
        print(list(f))
    with open("medical3.txt", "r", newline="
") as f:
        print(list(f))

if __name__ == "__main__":
    file_seperator_test6()

输出结果：

[‘I am a
‘, ‘ good
 boy.
‘, ‘
 where should
‘, ‘
 I change the line ?
‘, "
I can‘t stop
", ‘
‘]
[‘I am a
 good
‘, ‘ boy.
‘, ‘ where should
‘, ‘ I change the line ?
‘, "I can‘t stop
"]
[‘I am a
 good
 boy.
‘, ‘ where should
‘, ‘ I change the line ?
‘, "I can‘t stop
"]

结论：

1.如果要写入带‘ ‘的行，可以把newline设定为‘‘或者‘ ‘，避免python更改‘ ‘

2.如果要读入带‘ ‘的行，可以把newline设定为‘ ‘，指定换行符只能是‘ ‘。