__peter__ at web
Sep 27, 2008, 1:02 AM
Post #2 of 2
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Re: Negative block sizes with file-like objects
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Steven D'Aprano wrote:
> I have a proxy class that wraps an arbitrary file-like object fp and
> reads blocks of data from it. Is it safe to assume that fp.read(-1) will
> read until EOF? I know that's true for file.read() and StringIO.read(),
> but is it a reasonable assumption to make for arbitrary file-like objects?
>
> To put it in more concrete terms, I have a class like this:
>
> class C(object):
> # Much simplified version.
> def __init__(self, fp):
> self.fp = fp
> def read(self, size=-1):
> return self.fp.read(size)
>
>
> Should I re-write the read() method like this?
>
> def read(self, size=-1):
> if size < 0:
> return self.fp.read()
> else:
> return self.fp.read(size)
Grepping through the python source shows that both -1 and None are used as
the default size.
# python2.5.2
>>> import tarfile
>>> open("sample", "w").write("contents-of-sample")
>>> t = tarfile.open("sample.tar", "w")
>>> t.add("sample")
>>> t.close()
>>> t = tarfile.open("sample.tar")
>>> t.extractfile("sample").read()
'contents-of-sample'
>>> t.extractfile("sample").read(-1)[:50]
'contents-of-sample\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
So you may fare better with the rewrite. But even that may fail:
http://docs.python.org/lib/bltin-file-objects.html#l2h-295
"""
read( [size])
[...] If the size argument is negative or omitted, read all data until EOF
is reached. [...]
Also note that when in non-blocking mode, less data than what was requested
may be returned, even if no size parameter was given.
"""
Peter
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