Debugging with mod_python (apache)

[Manfred Wassmann <manolo@…>]

This is what the docs say:

If you’re the type of programmer who debugs using scattered print statements, note that print statements
have no effect in mod_python; they don’t appear in the Apache log, as one might expect. If you have the
need to print debugging information in a mod_python setup, either do this:

assert False, the_value_i_want_to_see

Or add the debugging information to the template of your page.

This is not completely true. Everything you write to sys.stderr ends up in the apache error log, like this

import sys
sys.stderr.write('This goes to the apache error log')

[Malcolm Tredinnick]

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.

[Graham Dumpleton]

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.

[Nick Efford]

Done, incorporating suggestions of Malcolm and grahamd.

[Nick Efford]

Patch to modpython.txt to clarify use of print statements

[Doug Harris]

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

[adroffner@…]

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.

[James Bennett]

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

[Thomas Güttler]

This ticket was already accepted (Triage Stage).

[Camille Harang]

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)

[Tim Graham]

updated patch, including new location of modpython.txt

[Tim Graham]

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

[Karen Tracey]

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.

[Russell Keith-Magee]

Deferring due to the absence of a trunk-ready patch.

[Daniel Roseman]

Updated patch removing references to WSGI.

[Daniel Roseman]

Updated patch removing references to WSGI.

[Tim Graham]

updated danielr's patch to apply cleanly trunk

[Simon Meers]

(In [14059]) Fixed #5677 -- update modpython stdout documentation. Thanks to Manfred Wassmann for the report, nickefford for the initial patch and Graham Dumpleton for the expert advice.

[Simon Meers]

(In [14060]) [1.2.X] Fixed #5677 -- update modpython stdout documentation. Thanks to Manfred Wassmann for the report, nickefford for the initial patch and Graham Dumpleton for the expert advice.

Backported of r14059 from trunk.