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If you pass the 'U' (Universal newlines) flag into gzip.open(), the flag
gets passed into the file open command used to open the gzip file
itself. As the 'U' flag can cause changes in teh data (Lineffed
translation), when it is used with a binary file open, the data is
corrupted, and all can go to heck.
In virtually all of my code that reads text files, I use the 'U' flag to
open files, it really helps not having to deal with newline issues. Yes,
they are fewer now that the Macintosh uses \n, but they can still be a pain.
Anyway, we added such support to some matplotlib methods, and found that
gzip file reading broken We were passing the flags though into either
file() or gzip.open(), and passing 'U' into gzip.open() turns out to be
fatal.
1) It would be nice if the gzip module (and the zip lib module)
supported Universal newlines -- you could read a compressed text file
with "wrong" newlines, and have them handled properly. However, that may
be hard to do, so at least:
2) Passing a 'U' flag in to gzip.open shouldn't break it -- it shuld be
ignored or raise an exeption.
I took a look at the Python SVN (2.5.4 and 2.6.1) for the gzip lib. I
see this:
 # guarantee the file is opened in binary mode on platforms
 # that care about that sort of thing
 if mode and 'b' not in mode:
 mode += 'b'
 if fileobj is None:
 fileobj = self.myfileobj = __builtin__.open(filename, mode
or 'rb')
this is going to break for 'U' == you'll get 'rUb'. I tested
file(filename, 'rUb'), and it looks like it does universal newline
translation.
So:
* Either gzip should be a bit smarter, and remove the 'U' flag (that's
what we did in the MPL code), or force 'rb' or 'wb'.
* Or: file opening should be a bit smarter -- what does 'rUb' mean? a
file can't be both Binary and Universal Text. Should it raise an
exception? Somehow I think it would be better to ignore the 'U', but
maybe that's only because of the issue I happen to be looking at now.
That later seems a better idea -- this issue could certainly come up in
other places than the gzip module, but maybe it would break a bunch of
code -- who knows?
I haven't touched py3 yet, so I have not idea if this issue is different
there. 
NOTE: passing in the 'U' flag doesn't guarantee that gzi will break. The
right combination of bytes needs to be there. In fact, when I first
tested this with a small test file, it worked just fine -- I though gzip
was ignoring the flag. However, when tested with a larger (real) gz
file, it did break.
very simple patch:
Add:
mode.replace('U', '')
to the above code before opeing the file 
But we may want to do something smarter...
see the (limited) discussion at:
http://mail.python.org/pipermail/python-dev/2009-January/085662.html 

Seems like this should be fairly easy to do right. 'U' needs to be
removed from the flags but then applied to the lines read from the
stream.

Here's a patch against trunk. Extra test case and minor doc tweak
included.

Same bug in 2.5, I don't know if the patch applies to 2.5

Unfortunately universal newlines are more complicated than replace() can
handle. See io.py, you may be able to use one of those classes to the
the universal new line handling on the cheap (or at least easy).

The problem appears to be that the gzip module simply doesn't support
universal newlines yet.
I'm currently working on the zipfile module's universal newline support
(issue6759) so if nobody else is working on this, I'll do it.
I'm not sure if file object's open() behavior when presented with 'rUb'
is correct or not.
>>> f = open("test.txt", "w").write("blah\r\nblah\rblah\nblah\r\n")
>>> f = open("test.txt", "rUb")
>>> f.read()
'blah\nblah\nblah\nblah\n'
Since 'U' and 'b' are conceptually mutually exclusive on platforms where
'b' matters, I can see this being confusing.

New changeset e647229c422b by Nadeem Vawda in branch '2.7':
Issue #5148: Ignore 'U' in mode given to gzip.open() and gzip.GzipFile().
http://hg.python.org/cpython/rev/e647229c422b 

The data corruption issue is now fixed in the 2.7 branch.
In 3.x, using a mode containing 'U' results in an exception rather than silent data corruption. Additionally, gzip.open() has supported text modes ("rt"/"wt"/"at") and newline translation since 3.3 [issue 13989].