typedef int (*funcptr)();

An engineers technical notebook

Magic Properties on Google AppEngine

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)

Auto-updating Properties in Google AppEngine Models

EDIT: I have an updated version of this code available in a newer post, please go to Magic Properties on Google AppEngine for the updated information. For the generic information that has not changed, and the idea behind the code please continue below.

In Google AppEngine a model may contain many different properties, from this model you can generate a form using djangoforms. This is all good and well until you have a required property on a model that needs to have part of it be calculated based upon something the user types in. For example, a blog entry generally has a "slug" which is generated from the title, now in Google AppEngine's models, if a property is required it has to be set at object creation time.

Djangoforms has a form.save(committed=False) that allows one to get back an unsaved Model, however since slug is not in the form (since it is auto generated) it cannot be set at model creation time. Even adding a hidden field to the form with the name of the property is not going to satisfy the model since a value of None is considered to be empty, which means it does not meet the required=True status.

After trying a multitude of different options I ran across an article about extending models that specifically talked about adding custom properties that were saved into the DB, and could be searched on, this got me digging and I found a blog post on the Google AppEngine blogspot about the exact same thing, using yet again a different example. The one that got me on the right track was the following post by Rodrigo Moraes on his custom model properties.

I came up with the following fairly quickly after also taking a look at googleappengine.ext.db within the Google AppEngine framework, however there is still an issue, for now the MagicProperty is only updated the first time the property it affects is set. I am not yet sure how to work around that, I will have to do some more digging, maybe there is a way to get notified if another property gets modified or updated.

import logging
from google.appengine.ext import db


def MagicProperty(prop, magic_func=None, *args, **kw):
    if magic_func:
        # No pants required.
        return _MagicProperty(prop, magic_func, *args, **kw)
    else:
        # We are putting some pants on the function.
        def pants(magic_func):
            return _MagicProperty(prop, magic_func, *args, **kw)
        return pants


class _MagicProperty(db.Property):
    """MagicProperty which will modify output based on a function that it is given.

    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, *args, **kw):
        super(_MagicProperty, self).__init__(*args, **kw)
        self.magic_func = magic_func
        self.magic_prop = prop

    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

        # TODO: If the property that this one is functioning on is changed, we need a way to know that so that we recalculate

        # Original __get__ in google.appengine.ext.db has getattr to retrieve the value from the model instance
        magic_done = getattr(model_instance, self.attr_name(), None)
        if magic_done is None:
            magic_done = self.magic_func(self.magic_prop.__get__(model_instance, class_instance))

            # Set the attribute in the model
            setattr(model_instance, self._attr_name(), magic_done)
        return magic_done


    def __set__(self, *args):
        raise db.DerivedPropertyError("MagicProperty is magic. Magic may not be modified.")</pre>

The approach that the DateTimeProperty takes is one that I can't use because my class is dependant upon an external property, not whether or not a value was set when the object was instantiated.

Google AppEngine is posing some rather interesting challenges. Although, for slugs it may not be entirely bad that it can only be set once, as that means title changes on the article won't cause the slug to change possibly breaking old links. This definitely requires more research and more thinking.

Property class on AppEngine does not support a tuple for choices

Google AppEngine does not allow one to set the choices field of a model property to a tuple; this workaround will have have a form give the user the choices with the user friendly items but have the backend store the non-user friendly version. 

# Set up our non-user friendly and user friendly choices
REDIRECT_CHOICES = (
    (301, "301 - Permanent Redirect"),
    (302, "302 - Temporary Redirect"),
    (403, "403 - Access Denied"),
    (404, "404 - File Not Found"),
    (500, "500 - Internal Server Error")
)

# Sample model that has the required Property with the choices set to the non-user friendly values
class Shorturl(db.Model):
    uripath = db.StringProperty(verbose_name="Path", required=True)
    httpcode = db.IntegerProperty(verbose_name="HTTP code", required=True, default=301, choices=[x[0] for x in REDIRECT_CHOICES])
    location = db.StringProperty(verbose_name="Location")

# This is our djangoform that has the choices set as required so that the user gets the user friendly version and not just simply
# the numbers.
class ShorturlForm(djangoforms.ModelForm):
    httpcode = forms.IntegerField(widget=forms.Select(choices=REDIRECT_CHOICES), required=True, label='HTTP code')

    class Meta:
        model = Shorturl

Given the above tuple it will set the choices in the model for Google AppEngine and pass it to the field on the Django form allowing user friendly output. This will allow the user to pick the right redirect choice without having to know that various different redirect codes or have to look them up.