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，以将文件列表中的名称列表（每行一个）读入另一个文件，同时对照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。该代码有效。可以做得更好吗？

我想对with open(...输入和输出文件都使用该语句，但看不到它们如何位于同一块中，这意味着我需要将名称存储在一个临时位置。

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。您用逗号分隔。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，通过return在函数的末尾放置一个显式字符不会获得任何收益。您可以使用return提前退出，但最后要退出，并且该函数将在没有退出的情况下退出。（当然，对于返回值的函数，您可以使用return来指定要返回的值。）

引入该语句时，Python 2.5或Python 2.6不支持open()与一起使用多个项目，但Python 2.7和Python
3.1或更高版本with支持使用多个项目with。

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

如果要编写必须在Python
2.5、2.6或3.0中运行的代码，则将with语句嵌套为建议的其他答案或使用contextlib.nested。