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\x 00\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-fil...s.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