[ZODB-Dev] "Advanced ZODB for Python Programmers" article: volatile example

[Vincent Pelletier]

Hi.

I think the example on volatile attributes given in "Advanced ZODB for Python 
Programmers" article[1] shows a bad practice.

From the article:
    if hasattr(self, '_v_image'):
        return self._v_image

This should be rewritten as:
    try:
        return self._v_image
    except AttributeError:

This solves cases where _v_ attribute gets garbage-collected between hasattr 
call and "return" line. A way to make this obvious is to artificially set 
object cache size to 0.

Note that another broken pattern would be to use code like:
    self._v_image=expensive_calculation()
    return self._v_image

the article advertises the right pattern (using a local variable for return):
    image=expensive_calculation()
    self._v_image=image
    return image

I've been hitting problems with this code pattern many times in the past, and 
should have written about it earlier.

[1] http://zodb.org/documentation/articles/ZODB2.html
-- 
Vincent Pelletier