[mod_python] Detecting Back Button Usage

Kenneth Loafman kenneth at loafman.com
Tue Mar 20 14:18:39 EST 2007

Clodoaldo wrote:
> 2007/3/20, Kenneth Loafman <kenneth at loafman.com>:
>> Clodoaldo wrote:
>> > 2007/3/20, Kenneth Loafman <kenneth at loafman.com>:
>> >> I'm using mod_python under Ubuntu Edgy 64, doing some basic plots of
>> >> data and allowing users to select a point to center on a region and 
>> gen
>> >> the next plot.  All of that works as long as the user moves forward.
>> >> The problem comes in the use of the back button.  The browser merely
>> >> displays the last page rather than rebuilding it, or at least 
>> restoring
>> >> the variables.  This leads to a lot of confusion because the server is
>> >> on one set of plot values and the browser is now on another.  This 
>> seems
>> >> to be a common problem.  Is there a solution, or should I just warn 
>> the
>> >> user not to use the browser back button?
>> >
>> > How does the user select the point? Entering it in a form field? If so
>> > save the plot values (the page state) on the page using a hidden field
>> > inside the form:
>> > <input type="hidden" name="variable_1" value="99.9">
>> > <input type="hidden" name="variable_2" value="33.3">
>> >
>> > When the form is submitted those values will be available to your 
>> script.
>> What I am doing is this:
>> def regen_plot(req):
>>     <plot logic snipped>
>> return """\
>> %s
>> <div align="center">
>> <a href="/demo/index/display_hit">
>> <img src="/demo/%s.gif" ismap border=0></a>
>> <p>X range is %d through %d seconds</p>
>> </div>
>> %s
>> """ % (_head, dbname, xrng[0], xrng[1], _tail(req))
>> Then when the user hits a point on the plot, display_hit is called.  I
>> translate the x,y args from the URL into plot space, center the point
>> around a smaller region, and plot again.  Going forward, it all works.
>> Session() vars work, but since I'm not in a form, and there's no submit
>> button, the hidden values don't work.  req.form.get('x') fails.
>> Is it possible to get <form> to use an <img ... ismap> tag in lieu of
>> the submit button?
> I don't know. What if you just set the the values in the URL? This way
> the user could even bookmark each frame of his interest.:
>> <a href="/demo/index/display_hit?var_1=99.9&var_2=33.3">

I tried that and the values just go away.  The <img...ismap> tag appends 
the x,y values of the mouse to the url, so what you see in the status 
bar is:


When the mouse is clicked, the first ?... goes away before being passed.



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