如何使用open with语句打开文件

Question

我正在研究如何在Python中进行文件输入和输出.我编写了以下代码来读取文件中的名称列表(每行一个)到另一个文件,同时根据文件中的名称检查名称,并将文本附加到文件中的出现位置.代码有效.可以做得更好吗？

我想对with open(...输入和输出文件使用该语句,但无法看到它们在同一块中的含义,这意味着我需要将名称存储在临时位置.

def filter(txt, oldfile, newfile):
    '''\
    Read a list of names from a file line by line into an output file.
    If a line begins with a particular name, insert a string of text
    after the name before appending the line to the output file.
    '''

    outfile = open(newfile, 'w')
    with open(oldfile, 'r', encoding='utf-8') as infile:
        for line in infile:
            if line.startswith(txt):
                line = line[0:len(txt)] + ' - Truly a great person!\n'
            outfile.write(line)

    outfile.close()
    return # Do I gain anything by including this?

# input the name you want to check against
text = input('Please enter the name of a great person: ')    
letsgo = filter(text,'Spanish', 'Spanish2')

Answer 1

Python允许将多个open()语句放在一个语句中with.你可以用逗号分隔它们.您的代码将是:

def filter(txt, oldfile, newfile):
    '''\
    Read a list of names from a file line by line into an output file.
    If a line begins with a particular name, insert a string of text
    after the name before appending the line to the output file.
    '''

    with open(newfile, 'w') as outfile, open(oldfile, 'r', encoding='utf-8') as infile:
        for line in infile:
            if line.startswith(txt):
                line = line[0:len(txt)] + ' - Truly a great person!\n'
            outfile.write(line)

# input the name you want to check against
text = input('Please enter the name of a great person: ')    
letsgo = filter(text,'Spanish', 'Spanish2')

不,你没有通过return在你的功能结束时明确地获得任何东西.您可以使用return提前退出,但最后退出,函数将在没有它的情况下退出.(当然,对于返回值的函数,您可以使用return指定要返回的值.)

在引入语句时,或者在Python 2.6中,Python 2.5中不支持使用多个open()项目,但Python 2.7和Python 3.1或更新版本支持它.withwith

http://docs.python.org/reference/compound_stmts.html#the-with-statement http://docs.python.org/release/3.1/reference/compound_stmts.html#the-with-statement

如果您正在编写必须在Python 2.5,2.6或3.0中运行的代码,请将with语句嵌套为建议或使用的其他答案contextlib.nested.

Answer 2

使用这样的嵌套块,

with open(newfile, 'w') as outfile:
    with open(oldfile, 'r', encoding='utf-8') as infile:
        # your logic goes right here

Answer 3

你可以嵌套你的块.像这样:

with open(newfile, 'w') as outfile:
    with open(oldfile, 'r', encoding='utf-8') as infile:
        for line in infile:
            if line.startswith(txt):
                line = line[0:len(txt)] + ' - Truly a great person!\n'
            outfile.write(line)

这比您的版本更好,因为您保证outfile即使您的代码遇到异常也将关闭.显然你可以用try/finally with做到这一点,但这是正确的方法.

或者,正如我刚刚学到的,您可以在@steveha所描述的with语句中拥有多个上下文管理器.在我看来,这比嵌套更好.

对于你的最后一个小问题,回报没有任何实际意义.我会删除它.