如何使用open with语句打开文件

问题:如何使用open with语句打开文件

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)读入另一个文件中,同时对照文件中的名称检查名称并将文本附加到文件中。该代码有效。可以做得更好吗?

我想对with open(...输入文件和输出文件都使用该语句,但是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')

I’m looking at how to do file input and output in Python. I’ve written the following code to read a list of names (one per line) from a file into another file while checking a name against the names in the file and appending text to the occurrences in the file. The code works. Could it be done better?

I’d wanted to use the with open(... statement for both input and output files but can’t see how they could be in the same block meaning I’d need to store the names in a temporary location.

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')

回答 0

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

Python allows putting multiple open() statements in a single with. You comma-separate them. Your code would then be:

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')

And no, you don’t gain anything by putting an explicit return at the end of your function. You can use return to exit early, but you had it at the end, and the function will exit without it. (Of course with functions that return a value, you use the return to specify the value to return.)

Using multiple open() items with with was not supported in Python 2.5 when the with statement was introduced, or in Python 2.6, but it is supported in Python 2.7 and Python 3.1 or newer.

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

If you are writing code that must run in Python 2.5, 2.6 or 3.0, nest the with statements as the other answers suggested or use contextlib.nested.


回答 1

这样使用嵌套块

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

Use nested blocks like this,

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

回答 2

您可以将块嵌套。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outfile即使您的代码遇到异常,您也可以保证将其关闭。显然,您可以通过try / finally进行操作,但这with是正确的方法。

或者,正如我刚学到的,您可以在@steveha描述的with语句中包含多个上下文管理器。在我看来,这比嵌套是更好的选择。

对于您的最后一个小问题,退货没有实际目的。我会删除它。

You can nest your with blocks. Like this:

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)

This is better than your version because you guarantee that outfile will be closed even if your code encounters exceptions. Obviously you could do that with try/finally, but with is the right way to do this.

Or, as I have just learnt, you can have multiple context managers in a with statement as described by @steveha. That seems to me to be a better option than nesting.

And for your final minor question, the return serves no real purpose. I would remove it.


回答 3

有时,您可能想打开不同数量的文件,并对待每个文件相同,可以使用 contextlib

from contextlib import ExitStack
filenames = [file1.txt, file2.txt, file3.txt]

with open('outfile.txt', 'a') as outfile:
    with ExitStack() as stack:
        file_pointers = [stack.enter_context(open(file, 'r')) for file in filenames]                
            for fp in file_pointers:
                outfile.write(fp.read())                   

Sometimes, you might want to open a variable amount of files and treat each one the same, you can do this with contextlib

from contextlib import ExitStack
filenames = [file1.txt, file2.txt, file3.txt]

with open('outfile.txt', 'a') as outfile:
    with ExitStack() as stack:
        file_pointers = [stack.enter_context(open(file, 'r')) for file in filenames]                
            for fp in file_pointers:
                outfile.write(fp.read())