如何清除弦乐对象? [英] how do I clear a stringio object?

问题描述

我创建了一个stringio对象，并且其中包含一些文本.我想清除其现有值并重用它，而不是调用它.反正有这样做吗?

I have a stringio object created and it has some text in it. I'd like to clear its existing values and reuse it instead of recalling it. Is there anyway of doing this?

推荐答案

TL; DR

不必费心清除它，只需创建一个新的文件即可-更快.

TL;DR

Don't bother clearing it, just create a new one—it’s faster.

这是我将如何发现此类情况的方法:

Here's how I would find such things out:

>>> from StringIO import StringIO
>>> dir(StringIO)
['__doc__', '__init__', '__iter__', '__module__', 'close', 'flush', 'getvalue', 'isatty', 'next', 'read', 'readline', 'readlines', 'seek', 'tell', 'truncate', 'write', 'writelines']
>>> help(StringIO.truncate)
Help on method truncate in module StringIO:

truncate(self, size=None) unbound StringIO.StringIO method
    Truncate the file's size.

    If the optional size argument is present, the file is truncated to
    (at most) that size. The size defaults to the current position.
    The current file position is not changed unless the position
    is beyond the new file size.

    If the specified size exceeds the file's current size, the
    file remains unchanged.

因此，您需要.truncate(0).但是初始化一个新的StringIO可能更便宜(更容易).请参阅下面的基准.

So, you want .truncate(0). But it's probably cheaper (and easier) to initialise a new StringIO. See below for benchmarks.

(感谢 tstone2077 用于指出差异.)

>>> from io import StringIO
>>> dir(StringIO)
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__enter__', '__eq__', '__exit__', '__format__', '__ge__', '__getattribute__', '__getstate__', '__gt__', '__hash__', '__init__', '__iter__', '__le__', '__lt__', '__ne__', '__new__', '__next__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setstate__', '__sizeof__', '__str__', '__subclasshook__', '_checkClosed', '_checkReadable', '_checkSeekable', '_checkWritable', 'close', 'closed', 'detach', 'encoding', 'errors', 'fileno', 'flush', 'getvalue', 'isatty', 'line_buffering', 'newlines', 'read', 'readable', 'readline', 'readlines', 'seek', 'seekable', 'tell', 'truncate', 'writable', 'write', 'writelines']
>>> help(StringIO.truncate)
Help on method_descriptor:

truncate(...)
    Truncate size to pos.

    The pos argument defaults to the current file position, as
    returned by tell().  The current file position is unchanged.
    Returns the new absolute position.

重要的是要注意，现在当前文件的位置不变，而将其截断为零将重置Python 2变体中的位置.

It is important to note with this that now the current file position is unchanged, whereas truncating to size zero would reset the position in the Python 2 variant.

因此，对于Python 2，您只需要

Thus, for Python 2, you only need

>>> from cStringIO import StringIO
>>> s = StringIO()
>>> s.write('foo')
>>> s.getvalue()
'foo'
>>> s.truncate(0)
>>> s.getvalue()
''
>>> s.write('bar')
>>> s.getvalue()
'bar'

如果在Python 3中执行此操作，则不会获得预期的结果:

If you do this in Python 3, you won't get the result you expected:

>>> from io import StringIO
>>> s = StringIO()
>>> s.write('foo')
3
>>> s.getvalue()
'foo'
>>> s.truncate(0)
0
>>> s.getvalue()
''
>>> s.write('bar')
3
>>> s.getvalue()
'\x00\x00\x00bar'

因此在Python 3中，您还需要重置位置:

So in Python 3 you also need to reset the position:

>>> from cStringIO import StringIO
>>> s = StringIO()
>>> s.write('foo')
3
>>> s.getvalue()
'foo'
>>> s.truncate(0)
0
>>> s.seek(0)
0
>>> s.getvalue()
''
>>> s.write('bar')
3
>>> s.getvalue()
'bar'

如果在Python 2代码中使用truncate方法，则同时调用seek(0)更为安全(之前或之后都没关系)，这样，当您不可避免地移植代码时，代码不会中断到Python3.还有另一个原因就是您应该只创建一个新的StringIO对象！

If using the truncate method in Python 2 code, it's safer to call seek(0) at the same time (before or after, it doesn't matter) so that the code won't break when you inevitably port it to Python 3. And there's another reason why you should just create a new StringIO object!

>>> from timeit import timeit
>>> def truncate(sio):
...     sio.truncate(0)
...     return sio
... 
>>> def new(sio):
...     return StringIO()
... 

为空时，使用StringIO:

When empty, with StringIO:

>>> from StringIO import StringIO
>>> timeit(lambda: truncate(StringIO()))
3.5194039344787598
>>> timeit(lambda: new(StringIO()))
3.6533868312835693

使用StringIO输入3KB数据:

With 3KB of data in, with StringIO:

>>> timeit(lambda: truncate(StringIO('abc' * 1000)))
4.3437709808349609
>>> timeit(lambda: new(StringIO('abc' * 1000)))
4.7179079055786133

与cStringIO相同:

And the same with cStringIO:

>>> from cStringIO import StringIO
>>> timeit(lambda: truncate(StringIO()))
0.55461597442626953
>>> timeit(lambda: new(StringIO()))
0.51241087913513184
>>> timeit(lambda: truncate(StringIO('abc' * 1000)))
1.0958449840545654
>>> timeit(lambda: new(StringIO('abc' * 1000)))
0.98760509490966797

因此，忽略潜在的内存问题(del oldstringio)，截断StringIO.StringIO的速度更快(空的速度快3％，3KB的数据速度快8％)，但创建速度更快(也更快")一个新的cStringIO.StringIO(对于空文件，速度快8％，对于3KB数据，速度快10％).因此，我建议只使用最简单的一种-假设您正在使用CPython，请使用cStringIO并创建新的.

So, ignoring potential memory concerns (del oldstringio), it's faster to truncate a StringIO.StringIO (3% faster for empty, 8% faster for 3KB of data), but it's faster ("fasterer" too) to create a new cStringIO.StringIO (8% faster for empty, 10% faster for 3KB of data). So I'd recommend just using the easiest one—so presuming you're working with CPython, use cStringIO and create new ones.

相同的代码，只是插入了seek(0).

The same code, just with seek(0) put in.

>>> def truncate(sio):
...     sio.truncate(0)
...     sio.seek(0)
...     return sio
... 
>>> def new(sio):
...     return StringIO()
...

为空时:

>>> from io import StringIO
>>> timeit(lambda: truncate(StringIO()))
0.9706327870007954
>>> timeit(lambda: new(StringIO()))
0.8734330690022034

其中包含3KB数据:

>>> timeit(lambda: truncate(StringIO('abc' * 1000)))
3.5271066290006274
>>> timeit(lambda: new(StringIO('abc' * 1000)))
3.3496507499985455

因此，对于Python 3，创建一个新的而不是重复使用一个空白的速度快11％，而创建一个新的而不是重复使用3K的速度快5％.同样，创建一个新的StringIO而不是截断并查找.

So for Python 3 creating a new one instead of reusing a blank one is 11% faster and creating a new one instead of reusing a 3K one is 5% faster. Again, create a new StringIO rather than truncating and seeking.

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