Update to my previous Google AppEngine auto-updating properties post.
In my last post I was talking about:
[...] however there is still an issue, for now the MagicProperty is only updated the first time the property it affects is set
I fixed that issue, it has a local cache that is added to the model, so if the value does change, the cache would no longer be valid and the value is recreated, otherwise the already computed value is returned. There was however another issue I had to deal with, and that was the fact that the cache was not being written to the datastore, this meant that upon getting the data back from the datastore we recomputed it anyway. Now the programmer has to do some extra work, they have to add a cache unindexed property to the model, and pass it into the MagicProperty to be used as the caching variable. Within the save() they have to prime the cache by setting a temporary variable equal to the to be computed variable just in case the cache variables have not been primed, this has not yet been documented other than in my SVN history.
We still don't allow setting of the magic property, however upon returning from
the datastore it calls
make_value_from_datastore, which returns a
_MagicDatastore, as long as we set with an instance of
_MagicDatastore it will
allow it to go through, so we can get the value from the datastore without
having to recompute the value.
Read the code for an example on how to use this.
Code is added below, feel free to use it as you wish under the license that is attached.
### # Copyright (c) 2010 Bert JW Regeer; # # Permission to use, copy, modify, and distribute this software for any # purpose with or without fee is hereby granted, provided that the above # copyright notice and this permission notice appear in all copies. # # THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES # WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF # MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR # ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES # WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN # ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF # OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. # ### import logging import hashlib from google.appengine.ext import db def MagicProperty(prop, magic_func=None, cache_prop=None, pass_instance=False, *args, **kw): if magic_func: # No pants required. return _MagicProperty(prop, magic_func, cache_prop, pass_instance, *args, **kw) else: # We are putting some pants on the function. def pants(magic_func): return _MagicProperty(prop, magic_func, cache_prop, pass_instance, *args, **kw) return pants class _MagicDatastore(): """This is an internal class for _MagicProperty.""" def __init__(self, val): self.value = val def retval(self): return self.value class _MagicProperty(db.Property): """MagicProperty which will modify output based on a function that it is given. This has several ways in which it may be called: In this example we create the model, no caching is done, so any data that is returned from the datastore will be erased the first time that the MagicProperty is accessed and recomputed. class MagicTest(db.Model): title = db.StringProperty(required=True) chars = utils.MagicProperty(title, len, required=True) mytest = MagicTest(title="It was for the good of the school!") >>> print mytest.title It was for the good of the school! >>> print mytest.chars 34 mytest = MagicTest.all().get() >>> print mytest.title Hello >>> print mytest.chars 5 # Do note, this is recalculated the first time it is called, as long as title does not change it won't be recomputed. In this example we create the model, and we also create a caching property so that even when we get values back from the datastore we use the cached computed value rather than running the function again. Do note that this requires overriding put() to prime the cache as there is currently no way to specify that certain properties should be "saved" before others. class MagicTesting(db.Model): title = db.StringProperty(required=True) cache = db.UnindexedProperty() chars = utils.MagicProperty(title, len, cache_prop=cache, required=True) def put(self, *args, **kw): prime_cache = self.chars super(MagicTesting).put(*args, **kw) mytesting = MagicTesting(title="Get the pocket knife out of my boot.") >>> print mytesting.title Get the pocket knife out of my boot. >>> print mytesting.chars 36 mytesting = MagicTest.all().get() >>> print mytesting.title How are you? >>> print mytesting.chars 12 # This is not recomputed so long as the title has not changed, however it uses more datastore space to store a hash. Inspired by: http://appengine-cookbook.appspot.com/recipe/custom-model-properties-are-cute http://code.google.com/appengine/articles/extending_models.html http://googleappengine.blogspot.com/2009/07/writing-custom-property-classes.html """ def __init__(self, prop, magic_func, cache_prop, pass_instance, *args, **kw): """ Extra parameters you can give this initializer. prop = Property to be acted upon magic_func = The function to be called when the property is accessed cache_prop = The property that can hold our cache, I suggest it is an db.UnindexedProperty() since it just stores sha1 hashes """ super(_MagicProperty, self).__init__(*args, **kw) self.magic_func = magic_func self.magic_prop = prop self.magic_cache = cache_prop self.magic_pass = pass_instance def get_cache_val(self, model_instance, class_instance): if self.magic_cache is not None: return self.magic_cache.__get__(model_instance, class_instance) return getattr(model_instance, self.attr_name() + "orig", None) def set_cache_val(self, model_instance, val): val = hashlib.sha1(val).hexdigest() if self.magic_cache is not None: self.magic_cache.__set__(model_instance, val) setattr(model_instance, self.attr_name() + "orig", val) def attr_name(self): # In google.appengine.ex.db there is an explicit warning not to use this method, so we test for it first. if self._attr_name: return self._attr_name() else: return "_" + self.name def __get__(self, model_instance, class_instance): if model_instance is None: return self cur = self.magic_prop.__get__(model_instance, class_instance) cur = cur.encode('utf-8') last = self.get_cache_val(model_instance, class_instance) if last == hashlib.sha1(cur).hexdigest(): logging.info("Cache hit: %s" % (cur)) return getattr(model_instance, self.attr_name(), None) logging.info("Cache miss: %s" % (cur)) magic_done = u"" if self.magic_pass: magic_done = self.magic_func(model_instance, cur) else: magic_done = self.magic_func(cur) # Set the attribute in the model setattr(model_instance, self.attr_name(), magic_done) self.set_cache_val(model_instance, cur) return magic_done def __set__(self, model_instance, value): if isinstance(value, _MagicDatastore): setattr(model_instance, self.attr_name(), value.retval()) else: raise db.DerivedPropertyError("MagicProperty is magic. Magic may not be modified.") def make_value_from_datastore(self, value): return _MagicDatastore(value)