Debugging with mod_python (apache)

If somebody is going to create a patch for this, it must also mention that stderr is buffered in mod_python, so flushing the pipe each time is a necessity if you want to actually see the results in more-or-less realtime.

If talking about use of 'print', can you strongly discourage use of 'print' without explicitly directing the output to sys.stderr. Ie., discourage use of:

print 'debug text'

This is because the resulting code is technically not safe to run on all possible WSGI hosting solutions. This is because some WSGI hosting solutions, such as CGI, use sys.stdout to communicate with the core web server. Thus, if users use 'print' without directing it anywhere, the output from it will interleave with valid output being sent back to the client and screw up the response.

So, suggest use of:

print >> sys.stderr, ' debug text'

if anything.

But then as already pointed out, in mod_python this really needs to be:

print >> sys.stderr, ' debug text'
sys.stderr.flush()

What Django developers should perhaps instead consider to get around this, is in the Django mod_python adapter when it is imported, replace sys.stderr with a stream like object which buffers data until an EOL is encountered and then call mod_python.apache.log_error() explicitly. This will ensure that when sys.stderr is used that output appears promptly, it will also attach time stamps to each line of output. Note that if the output passed to the dummy stream object contains embedded EOL, then the line should be split and passed to log_error() one line at a time though.

By doing this sys.stderr replacement, it would avoid all these problems with output not appearing in Apache log in a timely manner. Although one could replace sys.stdout as well, as I said you probably want to discourage people outputing to sys.stdout directly or indirectly.

Done, incorporating suggestions of Malcolm and grahamd.

Patch to modpython.txt to clarify use of print statements

What about using standard python logging? Is there a way to get this to go to the error log (or elsewhere)?

Replying to dharris:

What about using standard python logging? Is there a way to get this to go to the error log (or elsewhere)?

I wrote a python logging Handler class, ApacheLogHandler?, to do just that. It even supports Apache virtual hosts. Use it the same way as the StreamHandler? 'console' object in the Python logging API docs.

I'm trying to make this a clean django.contrib.logging.* feature. Right now it's a Django Snippet.

Please don't anonymously bump something to "ready for checkin"; that status implies a certain level of review by an experienced triager.

This ticket was already accepted (Triage Stage).

Another quick solution to get direct output is to print into a tty. Get the tty device by typing 'tty' in a terminal (here /dev/pts/5), and use the small function bellow to print what you need to see:

def print_tty(message, dev='/dev/pts/5'):

    tty = open(dev, 'w')
    tty.write('%s' % message)
    tty.flush()

Example:

print_tty(request.POST)

updated patch, including new location of modpython.txt

I don't think we can expect to document every possible solution to debugging, but the current patch looks like a good addition.

I do not like the current patch's inclusion of a vague reference to corrupting client output in "some WSGI hosting solutions". This is text going into the page describing mod_python deployment, specifically. It should limit itself to what happens under mod_python. Sure, adding prints to stdout is a bad idea in other hosting environments too, but that's irrelevant to someone trying to figure out how to debug a problem under mod_python and more likely to cause confusion than anything else.