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，从一个文件读取一个名称列表(每行一个)到另一个文件，同时根据文件中的名称检查一个名称，并将文本追加到文件中出现的名称。代码可以工作。还能做得更好吗?

我想对输入和输出文件都使用with open(...语句，但不能看到它们如何在同一个块中，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')
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小开

你可以用积木来筑巢。是这样的:

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outfile也会被关闭。显然你可以用try/finally来实现，但是with才是正确的方法。

或者，正如我刚刚学到的，你可以在with语句中有多个上下文管理器由@steveha描述。在我看来，这似乎是一个比嵌套更好的选择。

至于你的最后一个小问题，回报没有任何实际意义。我会移除它。

小开

像这样使用嵌套块，

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

Python允许在一个with中放入多个open()语句。用逗号分隔。你的代码将是:

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，在函数末尾显式地放置return不会得到任何东西。你可以使用return提前退出，但你在最后有它，函数将没有它退出。(当然，对于返回值的函数，您可以使用return来指定要返回的值。)

在Python 2.5或Python 2.6中引入with语句时，不支持在with中使用多个open()项，但在Python 2.7和Python 3.1或更新版本中支持。

< a href = " http://docs.python.org/reference/compound_stmts.html " with语句中> http://docs.python.org/reference/compound_stmts.html with语句中 http://docs.python.org/release/3.1/reference/compound_stmts.html#the-with-statement < / p >

如果你编写的代码必须在Python 2.5、2.6或3.0中运行，则按照其他答案的建议嵌套with语句或使用contextlib.nested。

小开

有时，你可能想要打开不同数量的文件，并对每个文件都一视同仁，你可以使用contextlib来做到这一点

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())

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