[Zope] Re: Returning Image Data from a Product

Tue, 26 Feb 2002 15:35:52 -0800

Terry Hancock wrote:
> * I want to return the PNG Image (i.e. a block of data *
> * with mime type 'image/png'), NOT an HTML image tag   *
> * containing a reference to result_image .             *

> Is it possible that it matters whether I'm using __getattr__ or
> a python method to return this?  

OKAY. This is the critical difference -- I just found
that out by testing.

However, I still think it would be very edifying to hear
someone explain why. :)  I can see that there *is* a
distinction here between the return value for the __getattr__
method and a fixed Python method, but I don't really
understand it.

To recap and (hopefully) clarify, the problem:

class MyTestClass(OFS.SimpleItem.SimpleItem):

  def __getattr__(self, keystr):
       "Access my data by catching the attribute lookup"
       # Decode keystr and catch a URL:
       try:
           if keystr != 'Foo' : raise AttribError

	   # omitted PIL code to read and convert an image
	   # which is now stored in self.result_image

           return self.result_image
       except:
           OFS.SimpleItem.SimpleItem.__getattr__(keystr)

       def Foo2(self):
           "Direct access by Python method"

     	   # omitted PIL code to read and convert an image
	   # which is now stored in self.result_image
           # (i.e. same as above)

           return self.result_image       

If I do this, pointing my browser at an instance, called, say
"TestObject":

http://myzope/TestObject/Foo

returns a PNG image (i.e. a block of data with type 'image/png'
which is rendered by the browser), while:

http://myzope/TestObject/Foo2

returns an HTML-coded image tag, containing
src="http://myzope/TestObject/result_image". This is
not rendered by the browser, unless I wrap it in 
<html><body>...</body></html> tags.

So why is that?

Why do I care?  Well, mainly because the browser will try
to cache "result_image" if it's always the same URL, whereas
the __getattr__ approach has a unique URL (in the example
it doesn't matter, but actually I want to use some coded
URL, like "s50" = "scaled to 50x50 pixels".

Thanks!

Terry

-- 
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Terry Hancock
hancock@anansispaceworks.com       
Anansi Spaceworks                 
http://www.anansispaceworks.com 
P.O. Box 60583                     
Pasadena, CA 91116-6583
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