[mod_python] Caching issues

Graham Dumpleton graham.dumpleton at gmail.com
Sun Jan 11 23:39:08 EST 2009

2009/1/12 Tim Valenta <tonightslastsong at gmail.com>:
> Okay, that makes sense.  My PythonPath system var definitely includes
> my www directory.

That in itself may not be enough to cause problem. You should actually
see a warning in Apache error log complaining you have done this. That
is, overlapped sys.path with directories that mod_python module
importer uses.

The real problem with doing this overlapping is that a module which
wasn't one managed by mod_python, ie., standard Python module, imports
something from the www directory, which also happens to be a handler
module, there will actually be too copies in memory and so can give
unexpected results.


> I had been using a forced path in my apache conf
> file to point to '~', for simple reference for the time being.
> I think my problem is simply that the modules I'm trying to import
> aren't 'candidates' for reloading.  Other than that, all other modules
> should already have been chained together with these
> apache.load_module calls.  My test case was pretty simple-- just two
> scripts, the first loading the second.
> Tim
> On Sun, Jan 11, 2009 at 8:25 PM, Graham Dumpleton
> <graham.dumpleton at gmail.com> wrote:
>> 2009/1/12 Tim Valenta <tonightslastsong at gmail.com>:
>>> Apologies for more trouble on the caching issue-- I've been adapting
>>> some of my scripts over to this apache.load_module function, but I
>>> don't think I'm seeing resolution to the issue.  If I've read the
>>> documentation properly, it seems that the PythonOption for
>>> auto-reloading is set to On by default, so to be clear, I haven't put
>>> a line for that in my httpd.conf file.  Are there any other tricks, or
>>> have I simply misread the documentation?  I still have to restart the
>>> Apache service in order to push the changes.
>> Reloading does not work for any Python modules/packages installed in
>> standard Python module search path. Thus, if you have set PythonPath
>> to some directory where you store your modules, or if they are in
>> system site-packages directory, they are not candidates for reloading.
>> Where are the modules you are modifying and expecting to be reloading
>> located? Is that location on sys.path, or have you explicitly setup
>> mod_python module importer path to tell it where they are?
>> From within running application, print out __name__ from the module
>> you have loaded and are expecting to be reloadable, What is it? Does
>> it look like a normal module name or something magic?
>> How was that module imported? There needs to be a chain from
>> mod_python handler for imports right down through to all modules you
>> want reloadable. If there aren't, for example, you are using
>> import_module() from a normal Python module which isn't a candidate
>> for reloading, you will not see the depth checking for reloading
>> occurring as you might expect.
>> Graham
>>> Tim
>>> On Sun, Jan 11, 2009 at 2:32 PM, Tim Valenta <tonightslastsong at gmail.com> wrote:
>>>> Despite the jab at Windows, the site will be deployed on a Unix
>>>> machine.  I just prefer Windows for ease of use, especially since my
>>>> wife has zero experience on a unix terminal or in the dearth of truly
>>>> professional software found on the Unix/Linux platform.  So yes, in
>>>> the end it will be on a "real" operating system,  But for now I'm
>>>> willingly sticking to a more user-friendly environment for the simple
>>>> folk.
>>>> Given the characteristics of PHP that allow it to circumvent this
>>>> caching stuff, I still think a nice little friendly red-flag reminder
>>>> on the main mod_python tutorial should point out this limitation of an
>>>> Apache module like this, for those of us like me, who would have gone
>>>> many months more before having every accidentally found this manual
>>>> import method.
>>>> On Sun, Jan 11, 2009 at 2:23 PM, Graham Dumpleton
>>>> <graham.dumpleton at gmail.com> wrote:
>>>>> 2009/1/12 Tim Valenta <tonightslastsong at gmail.com>:
>>>>> >> Still not short enough, so much so I stopped part way through.
>>>>> >
>>>>> > And for that I apologize.  I do appreciate the straight answer given,
>>>>> > though I would appreciate much more the addition of that fact in some
>>>>> > point-blank documentation that was easy to find.  I have scoured the
>>>>> > mod_python archives by search engine and came out utterly empty
>>>>> > handed.  I knew what the problem was, but I could find nothing to
>>>>> > remedy the issue.  I haven't encountered anything quite like this in
>>>>> > my adventures with PHP, perl, or other similar scripting languages
>>>>> > piloted by Apache.  It raises a valid concern to which I ideally
>>>>> > should have found answers very quickly.
>>>>> >
>>>>> > I hope this cures the subsequent issue given in the latter half of my message.
>>>>> PHP is specifically designed for web applications and it purposely
>>>>> throws away all code at the end of each request and thus is reloading
>>>>> it all on every request. Have a read of:
>>>>>  http://blog.ianbicking.org/2008/01/12/what-php-deployment-gets-right/
>>>>> I would be very surprised if mod_perl auto reloaded code as it is
>>>>> similar to Python in that you are applying a non web language to the
>>>>> web. Only way perl code would be reload is if you were running them as
>>>>> CGI scripts. If you run Python as CGI scripts you will get reloading
>>>>> as well, but as with anything CGI, much slower.
>>>>> BTW, maybe read:
>>>>>  http://blog.dscpl.com.au/2008/12/using-modwsgi-when-developing-django.html
>>>>> For WSGI hosted Python web application, this provides auto reloading
>>>>> on code changes. You do need to use a real operating system though,
>>>>> and not Windows.
>>>>> Graham
>>>>> > Tim
>>>>> >
>>>>> > On Sat, Jan 10, 2009 at 10:23 PM, Graham Dumpleton
>>>>> > <graham.dumpleton at gmail.com> wrote:
>>>>> >>
>>>>> >> 2009/1/11 Tim Valenta <tonightslastsong at gmail.com>:
>>>>> >> > Hello all-- I've been experiencing a caching issue from the
>>>>> >> > very beginning of my use of mod_python...  It's been at least 2 months now,
>>>>> >> > and I keep running into actual issues that prevent me from coding.
>>>>> >> > I have a knack for over-explaining, so I'll try to keep this concise yet
>>>>> >> > descriptive.
>>>>> >>
>>>>> >> Still not short enough, so much so I stopped part way through.
>>>>> >>
>>>>> >> The simple matter of it is that mod_python does not do deep checking
>>>>> >> of code for changes, nor does it automatically restart the process
>>>>> >> when code is changed. Thus the need to restart Apache when you make
>>>>> >> code changes to anything imported from sys.path is expected and normal
>>>>> >> behaviour.
>>>>> >>
>>>>> >> The only time any code is automatically reloaded is the direct code
>>>>> >> files imported by mod_python using its own special module importer.
>>>>> >> This is documented under 'import_module()' function in:
>>>>> >>
>>>>> >>  http://www.modpython.org/live/current/doc-html/pyapi-apmeth.html
>>>>> >>
>>>>> >> Graham
>>>>> >>
>>>>> >> > I'm developing a site on my local machine, Windows Vista, using Apache 2.2.x
>>>>> >> > and mod_python 3.3.1 .  I'm a programmer for a living, and I'm not quick to
>>>>> >> > point the finger at the language, *but* (you knew that was coming, eh?) I
>>>>> >> > know my code isn't to blame for the issue:
>>>>> >> > I write some basic code for an 'index.py' file, using the
>>>>> >> > mod_python.publisher handler.  'index.py' includes other modules which I've
>>>>> >> > coded from that same location, etc, etc.  Nothing fancy.  Any changes I make
>>>>> >> > in the 'index.py' file will be reflected immediately on my local web server.
>>>>> >> >  On the other hand, any changes I make to the modules included via import
>>>>> >> > from within 'index.py' are completely ignored by the web server.  It took me
>>>>> >> > a while to realize that my pages didn't reflect my code.  I also discovered
>>>>> >> > that the .pyc files had nothing to do with it.  I finally just restarted the
>>>>> >> > apache web service (which in fact runs as a service on my machine), and then
>>>>> >> > my code finally gets pushed through to the web server.
>>>>> >> > I've been looking around practically every other day for documentation on
>>>>> >> > how to make apache/mod_python simply cut it out and stop caching my python
>>>>> >> > code, but I've found nothing.  You can imagine the annoyance this presents,
>>>>> >> > since I have to restart my web server every single time I make even the
>>>>> >> > slightest change to a 'utility.py' file, etc.
>>>>> >> > I've been coping with the problem for a while now, but then I've found far
>>>>> >> > more annoying issues recently.  To abbreviate the problem into short terms,
>>>>> >> > I've got a main module 'MAIN' which imports another module for a class
>>>>> >> > 'CLASS'.  CLASS also has a few imports, such as 'backend' stuff for
>>>>> >> > interfacing with various databases, etc.  We'll call the 'backend' module
>>>>> >> > "BACKEND".  Given the setup, any changes I make to CLASS or BACKEND require
>>>>> >> > an apache restart in order to take effect.
>>>>> >> > I have a function in CLASS which calls a function from it's imported BACKEND
>>>>> >> > module.  I tried adding a parameter to the BACKEND function in question, and
>>>>> >> > properly passed said parameter while in CLASS, yet the mod_python debugger
>>>>> >> > spits out an error about me having passed 3 arguments, when the BACKEND
>>>>> >> > function takes exactly 2.  This is outright false, since my function in
>>>>> >> > BACKEND looks like:
>>>>> >> > def getUsers(self, req, terms):
>>>>> >> >
>>>>> >> > and I'm calling it with
>>>>> >> > self.backend.getUsers(self.req, search)
>>>>> >> >
>>>>> >> > In reality, my code dictates that I'm passing 3 (including the implicit
>>>>> >> > 'self' argument), and BACKEND's 'getUsers' does in fact take exactly 3
>>>>> >> > arguments.  Yet, the debugger is telling me that it takes only 2.
>>>>> >> > I was trying to pass it 'req' because I wanted to investigate a little error
>>>>> >> > in the code by printing something to the output HTML.  So, my attempt is
>>>>> >> > foiled, since somewhere something isn't being updated to what my most
>>>>> >> > current code actually says.
>>>>> >> > Just to test, I made the 'getUsers' function return immediately with a
>>>>> >> > string of gibberish, like 'return "adsfasdfadsfa"'.  this should make my
>>>>> >> > other code spin wildly out of control and encounter errors, yet when I
>>>>> >> > restart apache and test it... lo and behold, it's completely ignoring my
>>>>> >> > goofy 'return' statement.  The 'getUsers' function is still somehow
>>>>> >> > returning valid data, as if the 'return' wasn't there at all!
>>>>> >> > So then I tried causing actual syntax errors.  The debugger caught this,
>>>>> >> > much to my inner joy.  So I tried causing a semantic error instead:
>>>>> >> > referencing a non-existent attribute of a non-existent variable:
>>>>> >> > madeUpVar.moo = 42
>>>>> >> > Syntactically, nothing wrong, but at run time it should most definitely
>>>>> >> > encounter a NameError or something equally as realistic.  But I restart
>>>>> >> > apache, and... nothing.  The line is completely ignored.
>>>>> >> > Which leads me to believe that it's not actually being 'ignored' per se, but
>>>>> >> > rather the code being compiled is not the same as the code in play within
>>>>> >> > the web server.  When I delete my .pyc files and restart apache and visit
>>>>> >> > the URL that triggers my python code, my .py files are in fact being
>>>>> >> > recompiled down to their byte code .pyc files.  And clearly the interpreter
>>>>> >> > is processing my code, since it flags me on improper syntax.  Yet, no matter
>>>>> >> > what kind of syntactically-sound nonsense I put into my code, the changes
>>>>> >> > aren't being reflected in my web server.
>>>>> >> > These problems come and go, and I've go better explanation than over zealous
>>>>> >> > caching.  I imagine that by tomorrow sometime when I start my computer up,
>>>>> >> > the problem will have disappeared for the time being.
>>>>> >> > I've cursed this computer up and down as I've tried to figure out ANYTHING
>>>>> >> > that I can do to alleviate the issue, by to no avail.
>>>>> >> > Anybody with counsel to spare my tired brain is welcome to share...
>>>>> >> > _______________________________________________
>>>>> >> > Mod_python mailing list
>>>>> >> > Mod_python at modpython.org
>>>>> >> > http://mailman.modpython.org/mailman/listinfo/mod_python
>>>>> >> >
>>>>> >> >
>>>>> >
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