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I've previously used hasattr() to check for the existence of an attribute qua instance variable to lazy-load and cache that attribute

    def kdtree(self):
      if not hasattr(self, '_kdtree'):
        self._kdtree = scipy.spatial.KDTree(self.xi.T)
      return self._kdtree

like ruby's

    @foo ||= expensive_calculation

is there a better way?

3 comments

Well, you can do it this way:

    def kdtree(self):
        try:
            return self._kdtree
        except AttributeError:
            pass
        self._kdtree = scipy.spatial.KDTree(self.xi.T)
        return self._kdtree

And, of course, you could do all of that with a decorator.

In this particular case you should initialize self._kdtree to None in __init__, and just check if self._kdtree is None: <calculate>

Works great unless None is a valid value for the thing, in which case you'll recalculate it every time. If that was the case you should just use hasattr().

Or try and except AttributeError. Exceptions are a good thing.

I went with this decorator in the end, works a treat! https://github.com/jjgreen/mvpoly/blob/master/src/python/mvp...

Ah, nice, thanks

> ruby's

Perl has //= (http://perldoc.perl.org/perlop.html#Assignment-Operators)