1. indico.util.fossilize – “Serializing” elaborate Python objects to dictionaries and lists

fossilize allows us to “serialize” complex python objects into dictionaries and lists. Such operation is very useful for generating JSON data structures from business objects. It works as a wrapper around zope.interface.

Some of the features are:
  • Different “fossil” types for the same source class;
  • Built-in inheritance support;
class indico.util.fossilize.Fossilizable

Bases: object

Base class for all the objects that can be fossilized

classmethod clearCache()

Clears the fossil attribute cache

classmethod fossilizeIterable(target, interface, useAttrCache=False, filterBy=None, **kwargs)

Fossilizes an object, be it a ‘direct’ fossilizable object, or an iterable (dict, list, set);

classmethod fossilize_obj(obj, interfaceArg=None, useAttrCache=False, mapClassType=None, **kwargs)

Fossilizes the object, using the fossil provided by interface.

  • interfaceArg (IFossil, NoneType, or dict) – the target fossile type
  • useAttrCache (boolean) – use caching of attributes if same fields are repeated for a fossil
interface indico.util.fossilize.IFossil

Fossil base interface. All fossil classes should derive from this one.

exception indico.util.fossilize.InvalidFossilException

Bases: exceptions.Exception

The fossil name doesn’t follow the convention I(w+)Fossil or has an invalid method name and did not declare a .name tag for it

exception indico.util.fossilize.NonFossilizableException

Bases: exceptions.Exception

Object is not fossilizable (doesn’t implement Fossilizable)

indico.util.fossilize.addFossil(klazz, fossils)

Declares fossils for a class

  • klazz – a class object
  • fossils – a fossil class (or a list of fossil classes)

Shortcut for Fossilizable.clearCache()

indico.util.fossilize.fossilize(target, interfaceArg=None, useAttrCache=False, **kwargs)

Method that allows the “fossilization” process to be called on data structures (lists, dictionaries and sets) as well as normal Fossilizable objects.

  • target (Fossilizable) – target object to be fossilized
  • interfaceArg (IFossil, NoneType, or dict) – target fossil type
  • useAttrCache (boolean) – use the attribute caching

Simple wrapper around ‘implements’

1.1. Example

A simple example class:

class User(Fossilizable):

    fossilizes(ISimpleUserFossil, IComplexUserFossil)

    def __init__(self, id, name, friends = []):
        self.id = id
        self.name = name
        self.friends = friends

    def getId(self):
        return self.id

    def getName(self):
        return self.name

    def getFriends(self):
        return self.friends

(note that the code above will fail if the fossils below are not declared first)

A simple example Fossil:

class ISimpleUserFossil(IFossil):
    """ A simple user fossil """

    def getId(self):
        """ The ID of the user """

    def getName(self):
        """ The name, in uppercase """
    getName.convert = lambda x: x.upper()

A slightly more complex Fossil:

class IComplexUserFossil(IFossil):
    """ A complex user fossil """

    def getId(self):
        """ The ID of the user """
    getId.name = 'identityNumber'

    def getFriends(self):
        """ His/her friends """
    getFriends.result = ISimpleUserFossil


>>> u1 = User(1,'john')

>>> u1.fossilize(ISimpleUserFossil)
{'id': 1, 'name': 'JOHN', '_type': 'User', '_fossil': 'simpleUserFossil'}

>>> u2 = User(2,'bob')

>>> u3 = User(3, 'lisa', friends=[u1,u2])

>>> u3.fossilize(IComplexUserFossil)
{'friends': [{'identityNumber': 1, 'name': 'JOHN', '_type': 'User', '_fossil': 'simpleUserFossil'},
             {'id': 2, 'name': 'BOB', '_type': 'User', '_fossil': 'simpleUserFossil'}],
             'id': 3, '_type': 'User', '_fossil': 'complexUserFossil'}

>>> fossilize([u1, u2, u3], ISimpleUserFossil)
[{'id': 1, 'name': 'JOHN', '_type': 'User', '_fossil': 'simpleUserFossil'},
 {'id': 2, 'name': 'BOB', '_type': 'User', '_fossil': 'simpleUserFossil'},
 {'id': 3, 'name': 'LISA', '_type': 'User', '_fossil': 'simpleUserFossil'}]

1.2. Advanced topics

1.2.1. Valid fossil names. Fossil base class

Valid fossil names have to start with I (from “interface”) and finish with Fossil, i.e. they have to comply with the regular expression:: ^I(\w+)Fossil$ .

Also, fossils have to always inherit directly or indirectly from the IFossil fossil, which in turns inherits from zope.interface.Interface.

1.2.2. _type and _fossil

All of the fossilized objects produced will have a _type attribute, with the name of the original object’s class, and a _fossil attribute with the name of the fossil used:

>>> u = User(1, 'john')
>>> u.fossilize(u, ISimpleUserFossil)
{'id': 1, 'name': 'JOHN', '_type': 'User', '_fossil': 'simpleUserFossil'}

1.2.3. Valid method names

A fossil’s method names have to be in the get* form, has* form, or is* form. Otherwise, the name tag is needed. Example:

class ISomeFossil(IFossil):
    """ A complex user fossil """

    def getName(self):
        """ The name of the user """

    def hasChildren(self):
        """ Returns if the user has chidlren or not """

    def isMarried(self):
        """ Returns if the user is married or not """

    def requiresAccomodation(self):
        """ Returns if the user requires accomodation or not """
    requiresAccomodation.name = 'requiresAcc'

Fossilizing an imaginary user object with this fossil would result in:

>>> u.fossilize(ISomeFossil)
{ 'name': 'bob', 'hasChildren': False, 'isMarried': True, 'requiresAcc': True, '_type': 'User', '_fossil': 'someFossil'}

As shown, the getXyz methods correspond to a xwz attribute, the hasXwz methods correspond to a xwz attribute, and so on... The other methods need a name tag or an InvalidFossilException will be thrown.

1.2.4. Method tags

As seen in the example, it is possible to apply valued tags to the fossil methods:

  • name tag: overrides the normal name that would be given to the attribute by the fossilizing engine.
  • convert tag: applies a function to the result of the object’s method. Useful to covert datetime objects into strings, capitalize strings, etc.
  • result tag: when the result of an object’s method is another object that might be fossilized, you can specify which interface (fossil) to use with the result tag.

1.2.5. Different ways of specifying the fossil to use

Let’s take the User class from the first example, and an additional group class. We will not write their methods:

class User(Fossilizable):
    """ Class for a User. A User has an id and a name """
    fossilizes(ISimpleUserFossil, IComplexUserFossil)

class Group(Fossilizable):
    """ Class for a Group. A Group has an id and a groupName """
    fossilizes(ISimpleGroupFossil, IComplexGroupFossil)

The normal way to specify which fossil to use is to just write the fossil class:

>>> u = User(1, 'john')
>>> u.fossilize(u, ISimpleUserFossil)
{'id': 1, 'name': 'JOHN', '_type': 'User', '_fossil': 'simpleUserFossil'}

This way should be used whenever we are sure that the object we are fossilizing is of a given class.

However, in some cases we are not sure of the interface that should be used. Or, we may be fossilizing a list of heteregenous objects and we cannot or we do not want to use the same fossil for all of them.

In this case, there are currently two options:

  • Use None as the interface (or leaving the interface argument empty). In this case, the “default” fossil will be used for each object, which means the first fossil declared with the fossilizes declaration in the object’s class. If the object’s class does not invoke fossilizes but one of its super-classes does, the first fossil from that super-class will be used. Example:

    >>> friends = [User(1, 'john'), Group(5, 'family')]
    >>> fossilize(friends)
    [{'id': 1, 'name': 'JOHN', '_type': 'User', '_fossil': 'simpleUserFossil'},
     {'id': 5, 'groupName': 'family', '_type': 'Group', '_fossil': 'simpleGroupFossil'}
  • Use a dictionary to specify which fossil should be used depending on the object’s class. The keys of the dictionary can be: class objects, class names as strings, full class names as strings, or a class object corresponding to an object’s super class. Examples:

    >>> friends = [User(1, 'john'), Group(5, 'family')]
    >>> fossilize(friends, {User: ISimpleUserFossil, Group: ISimpleGroupFossil})
    [{'id': 1, 'name': 'JOHN', '_type': 'User', '_fossil': 'simpleUserFossil'},
     {'id': 5, 'groupName': 'family', '_type': 'Group', '_fossil': 'simpleGroupFossil'}
    >>> fossilize(friends, {"User": ISimpleUserFossil, "Group": ISimpleGroupFossil})
    (same output)
    >>> fossilize(friends, {"package.subpackage.User": ISimpleUserFossil, "package.subpackage.Group": ISimpleGroupFossil})
    (same output)

1.2.6. Changing a fossil in execution time

If for some reason you need to change a fossil behaviour in execution time (i.e. after it has been imported), know that it is possible, but please, avoid doing this unless you have a very good reason for it. All fossils inherit from zope.interface.Interface, which defines methods so that this is possible.

Example: change the ‘name’ tag of a given method of a fossil:

>>> IComplexUserFossil.get('getFriends').setTaggedValue('name', 'myFriends')