Writing to stdout and a log file

[Mike]

I would like my 'print' statements to send its output to the user's
screen and a log file.

This is my initial attempt:

class StdoutLog(file) :
def __init__(self, stdout, name='/tmp/stdout.log',
mode='w',bufsiz e=-1):
super(StdoutLog , self).__init__( name,mode,bufsi ze)
self.stdout = stdout
def write(self, data):
self.stdout.wri te(data)
self.write(data )

import sys
sys.stdout = StdoutLog(sys.s tdout)
print 'STDOUT', sys.stdout

When the program is run the string is written to the log file but
nothing appears on my screen. Where's the screen output?

It looks like the superclass's write() method is getting called instead
of the StdoutLog instance's write() method.

The python documentation says 'print' should write to
sys.stdout.writ e() but that doesn't seem to be happening.

Any idea what's going one?
Or ideas on how to debug this?

Thanks, Mike

[Jeff Epler]

This variation works:
#------------------------------------------------------------------------
class Tee:
def __init__(self, *args):
self.files = args

def write(self, data):
for f in self.files:
result = f.write(data)
return result

def writelines(self , seq):
for i in seq: self.write(i)

import sys
sys.stdout = Tee(sys.stdout, open("/tmp/stdout.log", "w"))

print 'STDOUT', sys.stdout
#------------------------------------------------------------------------

It appears that the 'print' statement always uses file.write if
isinstance(sys. stdout, file). I don't know whether this has been
reported as a bug before, or if there's a reason for the current
behavior. It may be an accidental behavior that is left over from the
days when builtin types were not subclassable.

Jeff

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.2.6 (GNU/Linux)

iD8DBQFCZaMDJd0 1MZaTXX0RAmdYAJ 9GiKYMT/1HjCfa/8DdpPXNSIvJrACe OOK6
mCfiTD9mqTqNpUu dqvm/Log=
=2JZ/
-----END PGP SIGNATURE-----

[Mike]

Thanks.

I should've mentioned I want StdoutLog to subclass the 'file' type
because I need all the file attributes available.

I could add all the standard file methods and attributes to StdoutLog
without subclassing 'file' but I'd rather avoid this if I can.

[Jeff Epler]

In that case, it looks like you won't be able to get what you want
without modifying CPython. PRINT_ITEM calls PyFile_SoftSpac e,
PyFile_WriteStr ing, and PyFile_WriteObj ect, which all use
PyFile_Check(). It might be as simple as changing these to
PyFile_CheckExa ct() calls in PyFile_WriteStr ing / PyFile_WriteObj ect,
but I have no idea whether the test suite still works after this change
is made. It does make this program work (it prints things from X.write):

class X(file):
def write(self, s):
print "X.write", `s`
return file.write(self , s)

import sys
x = X("/tmp/out.txt", "w")
print >>x, 42

I don't care to be the champion of this patch, or to submit it to
sourceforge; I suspect there should be a better review of PyFile_Check
vs PyFile_CheckExa ct uses in fileobject.c, instead of just picking the
few spots that make this usage work. Before being submitted as a patch,
a testcase should be added too. Feel free to run with this if you feel
strongly about it.

Jeff

Index: Objects/fileobject.c
=============== =============== =============== =============== =======
RCS file: /cvsroot/python/python/dist/src/Objects/fileobject.c,v
retrieving revision 2.193
diff -u -u -r2.193 fileobject.c
--- Objects/fileobject.c 7 Nov 2004 14:15:28 -0000 2.193
+++ Objects/fileobject.c 20 Apr 2005 02:41:32 -0000
@@ -2012,7 +2012,7 @@
PyErr_SetString (PyExc_TypeErro r, "writeobjec t with NULL file");
return -1;
}
- else if (PyFile_Check(f )) {
+ else if (PyFile_CheckEx act(f)) {
FILE *fp = PyFile_AsFile(f );
#ifdef Py_USING_UNICOD E
PyObject *enc = ((PyFileObject* )f)->f_encoding;
@@ -2082,7 +2082,7 @@
"null file for PyFile_WriteStr ing");
return -1;
}
- else if (PyFile_Check(f )) {
+ else if (PyFile_CheckEx act(f)) {
FILE *fp = PyFile_AsFile(f );
if (fp == NULL) {
err_closed();
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.2.6 (GNU/Linux)

iD8DBQFCZcHmJd0 1MZaTXX0RAh4eAJ 4wjWlguTzoHWLbj ltZ2f+cXEfa9ACd ErTX
C5ebIIE+I3NrgC3 cIUO9W9o=
=WN4p
-----END PGP SIGNATURE-----

[Wolfram Kraus]

Mike wrote:

I would like my 'print' statements to send its output to the user's
screen and a log file.

This is my initial attempt:

class StdoutLog(file) :
def __init__(self, stdout, name='/tmp/stdout.log',
mode='w',bufsiz e=-1):
super(StdoutLog , self).__init__( name,mode,bufsi ze)
self.stdout = stdout
def write(self, data):
self.stdout.wri te(data) What happens when you do a self.stdout.flu sh() here?
self.write(data )

import sys
sys.stdout = StdoutLog(sys.s tdout)
print 'STDOUT', sys.stdout

When the program is run the string is written to the log file but
nothing appears on my screen. Where's the screen output?

It looks like the superclass's write() method is getting called instead
of the StdoutLog instance's write() method.

The python documentation says 'print' should write to
sys.stdout.writ e() but that doesn't seem to be happening.

Any idea what's going one?
Or ideas on how to debug this?

Thanks, Mike

I had the same problem (writing to file and stdout with print) and my
solution was *not* to subclass file and instead add a
self.outfile=fi le(...) to the constructor.

HTH,
Wolfram

[Mike]

flushing stdout has no effect.

I've got an implementation that does not subclass file. It's not as
nice but it works.

[Michael Hoffman]

Mike wrote:

I should've mentioned I want StdoutLog to subclass the 'file' type
because I need all the file attributes available.

You might use a surrogate pattern. Here's one I use for this kind of
situation, where I want to subclass but that can't be done for some reason.

class SurrogateNotIni tedError(except ions.AttributeE rror):
pass

class Surrogate(objec t):
def __init__(self, data):
self._data = data

def __getattr__(sel f, name):
if name == "_data":
raise SurrogateNotIni tedError, name
else:
try:
return getattr(self._d ata, name)
except SurrogateNotIni tedError:
raise SurrogateNotIni tedError, name

I'll leave it as an exercise to the reader to make this work when
self._data is actually a list of objects instead of a single object.
You'll obviously need special logic for different methods, like write(),
since for some of them you will want to call every object in self._data,
and others only a single object.
--
Michael Hoffman

[Mike]

Perfect. This is what I"ll use. Thanks! Mike