OpenStack Study: filters.py

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# Copyright (c) 2011-2012 OpenStack Foundation

# All Rights Reserved.


# Licensed under the Apache License, Version 2.0 (the "License"); you may

# not use this file except in compliance with the License. You may obtain

# a copy of the License at


# http://www.apache.org/licenses/LICENSE-2.0


# Unless required by applicable law or agreed to in writing, software

# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT

# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the

# License for the specific language governing permissions and limitations

# under the License.


Filter support


from nova import loadables

from nova.openstack.common.gettextutils import _

from nova.openstack.common import log as logging

LOG = logging.getLogger(__name__)

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class BaseFilter(object):

"""Base class for all filter classes."""

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    def _filter_one(self, obj, filter_properties):

        """Return True if it passes the filter, False otherwise.

        Override this in a subclass.


        return True

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    def filter_all(self, filter_obj_list, filter_properties):

        """Yield objects that pass the filter.

        Can be overridden in a subclass, if you need to base filtering

        decisions on all objects.  Otherwise, one can just override

        _filter_one() to filter a single object.


        for obj in filter_obj_list:

            if self._filter_one(obj, filter_properties):

                yield obj

    # Set to true in a subclass if a filter only needs to be run once

    # for each request rather than for each instance

    run_filter_once_per_request = False

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    def run_filter_for_index(self, index):

        """Return True if the filter needs to be run for the "index-th"

        instance in a request.  Only need to override this if a filter

        needs anything other than "first only" or "all" behaviour.


        if self.run_filter_once_per_request and index > 0:

            return False


            return True

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class BaseFilterHandler(loadables.BaseLoader):

"""Base class to handle loading filter classes.

This class should be subclassed where one needs to use filters.


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    def get_filtered_objects(self, filter_classes, objs,

            filter_properties, index=0):

        list_objs = list(objs)

        LOG.debug(_("Starting with %d host(s)"), len(list_objs))

        for filter_cls in filter_classes:

            cls_name = filter_cls.__name__

            filter = filter_cls()

            if filter.run_filter_for_index(index):

                objs = filter.filter_all(list_objs,


                if objs is None:

                    LOG.debug(_("Filter %(cls_name)s says to stop filtering"),

                          {'cls_name': cls_name})


                list_objs = list(objs)

                if not list_objs:

                    LOG.info(_("Filter %s returned 0 hosts"), cls_name)


                LOG.debug(_("Filter %(cls_name)s returned "

                            "%(obj_len)d host(s)"),

                          {'cls_name': cls_name, 'obj_len': len(list_objs)})

        return list_objs