IT-Swarm.Net

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?

我想对输入和输出文件使用with open(...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')
174
Disnami

Python允许在单个with中放置多个open()语句。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,你通过在函数末尾加上一个明确的return来获得任何东西。您可以使用return提前退出,但最后会有它,并且该函数将在没有它的情况下退出。 (当然,对于返回值的函数,可以使用return指定要返回的值。)

在引入with语句时,或者在Python 2.6中,Python 2.5中不支持使用带with的多个open()项,但Python 2.7和Python 3.1或更新版本支持它。

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

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语句嵌套为建议的其他答案或使用 contextlib.nested

270
steveha

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
24
RanRag

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)

这比你的版本更好,因为你保证即使你的代码遇到异常也会关闭outfile。显然你可以用try/finally做到这一点,但是with是正确的方法。

或者,正如我刚才所了解的那样,你可以在一个with语句中有多个上下文管理器 由@steveha 描述。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。

10
David Heffernan

有时,您可能希望打开可变数量的文件并将每个文件视为相同,您可以使用contextlib执行此操作

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())                   
1
brother-bilo