fields.py 7.39 KB
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"""Pickle field implementation for Django."""

from copy import deepcopy
from base64 import b64encode, b64decode
from zlib import compress, decompress
try:
    from cPickle import loads, dumps
except ImportError:
    from pickle import loads, dumps

from django.db import models
from django.utils.encoding import force_unicode

from picklefield import DEFAULT_PROTOCOL

class PickledObject(str):
    """
    A subclass of string so it can be told whether a string is a pickled
    object or not (if the object is an instance of this class then it must
    [well, should] be a pickled one).

    Only really useful for passing pre-encoded values to ``default``
    with ``dbsafe_encode``, not that doing so is necessary. If you
    remove PickledObject and its references, you won't be able to pass
    in pre-encoded values anymore, but you can always just pass in the
    python objects themselves.

    """

class _ObjectWrapper(object):
    """
    A class used to wrap object that have properties that may clash with the 
    ORM internals.
    
    For example, objects with the `prepare_database_save` property such as 
    `django.db.Model` subclasses won't work under certain conditions and the
    same apply for trying to retrieve any `callable` object.
    """
    __slots__ = ('_obj',)

    def __init__(self, obj):
        self._obj = obj

def wrap_conflictual_object(obj):
    if hasattr(obj, 'prepare_database_save') or callable(obj):
        obj = _ObjectWrapper(obj)
    return obj

def dbsafe_encode(value, compress_object=False, pickle_protocol=DEFAULT_PROTOCOL):
    # We use deepcopy() here to avoid a problem with cPickle, where dumps
    # can generate different character streams for same lookup value if
    # they are referenced differently.
    # The reason this is important is because we do all of our lookups as
    # simple string matches, thus the character streams must be the same
    # for the lookups to work properly. See tests.py for more information.
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    #import time
    #print "bla enocde!!!\n"+str(time.time())
    #import traceback
    #traceback.print_stack()
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    if not compress_object:
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        value = b64encode(dumps(value, pickle_protocol))
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    else:
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        value = b64encode(compress(dumps(value, pickle_protocol)))
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    return PickledObject(value)


def dbsafe_decode(value, compress_object=False):
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    #import time
    #print "bla decode!!!\n"+str(time.time())
    #import traceback
    #traceback.print_stack()
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    if not compress_object:
        value = loads(b64decode(value))
    else:
        value = loads(decompress(b64decode(value)))
    return value


class PickledObjectField(models.Field):
    """
    A field that will accept *any* python object and store it in the
    database. PickledObjectField will optionally compress its values if
    declared with the keyword argument ``compress=True``.

    Does not actually encode and compress ``None`` objects (although you
    can still do lookups using None). This way, it is still possible to
    use the ``isnull`` lookup type correctly.
    """

    __metaclass__ = models.SubfieldBase

    def __init__(self, *args, **kwargs):
        self.compress = kwargs.pop('compress', False)
        self.protocol = kwargs.pop('protocol', DEFAULT_PROTOCOL)
        kwargs.setdefault('editable', False)
        super(PickledObjectField, self).__init__(*args, **kwargs)

    def get_default(self):
        """
        Returns the default value for this field.

        The default implementation on models.Field calls force_unicode
        on the default, which means you can't set arbitrary Python
        objects as the default. To fix this, we just return the value
        without calling force_unicode on it. Note that if you set a
        callable as a default, the field will still call it. It will
        *not* try to pickle and encode it.

        """
        if self.has_default():
            if callable(self.default):
                return self.default()
            return self.default
        # If the field doesn't have a default, then we punt to models.Field.
        return super(PickledObjectField, self).get_default()

    def to_python(self, value):
        """
        B64decode and unpickle the object, optionally decompressing it.

        If an error is raised in de-pickling and we're sure the value is
        a definite pickle, the error is allowed to propogate. If we
        aren't sure if the value is a pickle or not, then we catch the
        error and return the original value instead.

        """
        if value is not None:
            try:
                value = dbsafe_decode(value, self.compress)
            except:
                # If the value is a definite pickle; and an error is raised in
                # de-pickling it should be allowed to propogate.
                if isinstance(value, PickledObject):
                    raise
            else:
                if isinstance(value, _ObjectWrapper):
                    return value._obj
        return value

    def pre_save(self, model_instance, add):
        value = super(PickledObjectField, self).pre_save(model_instance, add)
        return wrap_conflictual_object(value)

    def get_db_prep_value(self, value, connection=None, prepared=False):
        """
        Pickle and b64encode the object, optionally compressing it.

        The pickling protocol is specified explicitly (by default 2),
        rather than as -1 or HIGHEST_PROTOCOL, because we don't want the
        protocol to change over time. If it did, ``exact`` and ``in``
        lookups would likely fail, since pickle would now be generating
        a different string.

        """
        if value is not None and not isinstance(value, PickledObject):
            # We call force_unicode here explicitly, so that the encoded string
            # isn't rejected by the postgresql_psycopg2 backend. Alternatively,
            # we could have just registered PickledObject with the psycopg
            # marshaller (telling it to store it like it would a string), but
            # since both of these methods result in the same value being stored,
            # doing things this way is much easier.
            value = force_unicode(dbsafe_encode(value, self.compress, self.protocol))
        return value

    def value_to_string(self, obj):
        value = self._get_val_from_obj(obj)
        return self.get_db_prep_value(value)

    def get_internal_type(self):
        return 'TextField'

    def get_db_prep_lookup(self, lookup_type, value, connection=None, prepared=False):
        if lookup_type not in ['exact', 'in', 'isnull']:
            raise TypeError('Lookup type %s is not supported.' % lookup_type)
        # The Field model already calls get_db_prep_value before doing the
        # actual lookup, so all we need to do is limit the lookup types.
        try:
            return super(PickledObjectField, self).get_db_prep_lookup(
                lookup_type, value, connection=connection, prepared=prepared)
        except TypeError:
            # Try not to break on older versions of Django, where the
            # `connection` and `prepared` parameters are not available.
            return super(PickledObjectField, self).get_db_prep_lookup(
                lookup_type, value)


# South support; see http://south.aeracode.org/docs/tutorial/part4.html#simple-inheritance
try:
    from south.modelsinspector import add_introspection_rules
except ImportError:
    pass
else:
    add_introspection_rules([], [r"^picklefield\.fields\.PickledObjectField"])