Aaron Myles Landwehr added the comment:
Yeah, it is windows specific. The problem is that if you open conout$ and the
descriptor isn't 1, the buffer doesn't flush normally so the console won't
display anything unless you manually flush it.
Now, why would you want to redirect STDOUT to the console in the first place
like this? It's in the case where AllocConsole() has been called and since the
console is new, STDOUT is directed elsewhere or nowhere. So you can't dup it or
restore it or anything like that. In order, to get the normal behavior for the
windows console the redirect is needed for STDOUT with fileno=1. But, as far as
I can tell, it is impossible to actually do the redirect in python.
Anyway, let me discuss a bit the behavior between C, Python, and Perl w.r.t. to
opening files. It'll help clarify where python is lacking in comparison to perl
and c. You basically have two different 'flavors' of prototypes between the
languages:
1) handle fd = open(filename, mode)
2) open(filename, mode, fd)
In C, the first prototype matches 'fopen' and is used to open a file. And the
second prototype matches 'freopen' and is used to associate a different file
with the an existing file descriptor. Moreover, you can't use stdout as an
lvalue so you simply can't redefine it.
In code terms:
stdout = fopen("CONOUT$", "w"); //Invalid lvalue
FILE* file= fopen("CONOUT$", "w"); // Valid fileno=3 to N.
freopen("CONOUT$", "w", stdout); // Valid fileno=1.
freopen("CONOUT$", "w", file); //Valid IFF fd!=0, the value of fd is
whatever it was previously.
In perl the open function is similar to the second prototype mentioned above,
but the parameters are backwards. Moreover, It can either be used for new file
descriptors or for reusing file descriptors unlike C's version of the second
prototype.
In code terms:
open (STDOUT, ">", "CONOUT$"); //Valid fileno=1. Reuses the desciptor number.
open (FILE, ">", "CONOUT$"); //Valid fileno=3 to N. Reuses the fileno if a
valid descriptor. Otherwise, creates a new one.
In python, the os.open function matches the first prototype with the exception
that it can be used to redirect stdout unlike C's version.
In code terms:
sys.stdout = os.open("CONOUT$", "w"); // Valid fileno=3 to N.
So when you look at the behavior, you'll note that both C and Perl allow you to
redirect using an existing descriptor; where as, Python seems to be unable to
do this. Normally, it doesn't matter since you can just save stdout ahead of
time, but it matters in the case where you don't start with a console
(pythonw), or deallocated the console and allocated a new one later on.
As it stands, I don't believe this is fixable without an API extension/change
of some sort that would allow passing filehandles... either that or... just
making $CONOUT always allocate to 1 regardless. Though the latter would change
the behavior for anyone who currently uses CONOUT$ in that it currently needs
to be manually flushed to display to the console; where as, it wouldn't with
the change.
Boy that is long winded, but hopefully it describes the issue adequately.
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue22673>
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