lazr.batchnavigator 1.0

..
This file is part of lazr.batchnavigator.

lazr.batchnavigator is free software: you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation, version 3 of the License.

lazr.batchnavigator is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with lazr.batchnavigator. If not, see
<http://www.gnu.org/licenses/>.

Batch Navigation
****************

Batch navigation provides a way to navigate batch results in a web
page by providing URL links to the next, previous and numbered pages
of results.

It uses two arguments to control the batching:

- start: The first item we should show in current batch, defaulting to 0.
- batch: Controls the amount of items we are showing per batch. It will only
appear if it's different from the default value set when the batch
is created.

These values can be overriden in the request, unless you also pass
force_start=True, which will make the start argument (again, defaulting to 0)
always chosen.

Imports:

>>> from lazr.batchnavigator import BatchNavigator
>>> from zope.publisher.browser import TestRequest
>>> from zope.publisher.http import HTTPCharsets
>>> from zope.component import getSiteManager
>>> sm = getSiteManager()
>>> sm.registerAdapter(HTTPCharsets)

>>> def build_request(query_string_args=None, method='GET'):
... if query_string_args is None:
... query_string = ''
... else:
... if getattr(query_string_args, 'items', None) is not None:
... query_string_args = query_string_args.items()
... query_string = "&".join(
... ["%s=%s" % (k,v) for k,v in query_string_args])
... request = TestRequest(SERVER_URL='http://www.example.com/foo',
... method=method,
... environ={'QUERY_STRING': query_string})
... request.processInputs()
... return request

A dummy request object:

Some sample data.

>>> reindeer = ['Dasher', 'Dancer', 'Prancer', 'Vixen', 'Comet',
... 'Cupid', 'Donner', 'Blitzen', 'Rudolph']

You can create a batch and a batch navigator like so:

>>> reindeer_batch_navigator = BatchNavigator(
... reindeer, build_request(), size=3)

The BatchNavigator implements IBatchNavigator.

>>> from zope.interface.verify import verifyObject
>>> from lazr.batchnavigator.interfaces import IBatchNavigator

>>> verifyObject(IBatchNavigator, reindeer_batch_navigator)
True

The BatchNavigator class provides IBatchNavigatorFactory. This can be used
to register a batch navigator factory as a utility, for instance.

>>> from lazr.batchnavigator.interfaces import IBatchNavigatorFactory

>>> verifyObject(IBatchNavigatorFactory, BatchNavigator)
True

You can ask the navigator for the chunk of results currently being shown
(e.g. to iterate over them for rendering in ZPT):

>>> list(reindeer_batch_navigator.currentBatch())
['Dasher', 'Dancer', 'Prancer']

You can ask for the first, previous, next and last results' links:

>>> reindeer_batch_navigator.firstBatchURL()
''
>>> reindeer_batch_navigator.prevBatchURL()
''
>>> reindeer_batch_navigator.nextBatchURL()
'http://www.example.com/foo?start=3'
>>> reindeer_batch_navigator.lastBatchURL()
'http://www.example.com/foo?start=6'

The next link will be empty when there are no further results:

>>> request = build_request({"start": "3", "batch": "20"})
>>> last_reindeer_batch_navigator = BatchNavigator(reindeer, request=request)
>>> last_reindeer_batch_navigator.nextBatchURL()
''

The first and previous link should appear even when we start at a point between 0
and the batch size:

>>> request = build_request({"start": "2", "batch": "3"})
>>> last_reindeer_batch_navigator = BatchNavigator(reindeer, request=request)

Here, we can see too that the batch argument appears as part of the URL.
That's because the request asked for a different size than the default
one when we create the Batch object, by default, it's 5.

>>> last_reindeer_batch_navigator.firstBatchURL()
'http://www.example.com/foo?start=0&batch=3'

>>> last_reindeer_batch_navigator.prevBatchURL()
'http://www.example.com/foo?start=0&batch=3'

This all works with other values in the query string, too:

>>> request = build_request({'fnorb': 'bar',
... 'start': '3',
... 'batch': '3'})
>>> reindeer_batch_navigator_with_qs = BatchNavigator(
... reindeer, request, size=3)

In this case, we created the BatchNavigator with a default size of '3' and
the request is asking exactly that number of items per batch, and thus, we
don't need to show 'batch' as part of the URL.

>>> reindeer_batch_navigator_with_qs.firstBatchURL()
'http://www.example.com/foo?fnorb=bar&start=0'
>>> reindeer_batch_navigator_with_qs.prevBatchURL()
'http://www.example.com/foo?fnorb=bar&start=0'
>>> reindeer_batch_navigator_with_qs.nextBatchURL()
'http://www.example.com/foo?fnorb=bar&start=6'
>>> reindeer_batch_navigator_with_qs.lastBatchURL()
'http://www.example.com/foo?fnorb=bar&start=6'

You can ask for the links for each of the result pages:

>>> links = reindeer_batch_navigator.batchPageURLs()
>>> for link in links:
... label, url = link.items()[0]
... print label, url
[1] http://www.example.com/foo?start=0
2 http://www.example.com/foo?start=3
3 http://www.example.com/foo?start=6
_last_ http://www.example.com/foo?start=6

The ``force_start`` argument allows you to ignore the start value in the
request. This can be useful when, for instance, a filter has changed, and the
desired behavior is to restart at 0.

>>> reindeer_batch_navigator_with_qs = BatchNavigator(
... reindeer, request, size=3, force_start=True)
>>> reindeer_batch_navigator_with_qs.currentBatch().start
0
>>> reindeer_batch_navigator_with_qs.nextBatchURL()
'http://www.example.com/foo?fnorb=bar&start=3'
>>> reindeer[:3] == list(reindeer_batch_navigator_with_qs.currentBatch())
True

We ensure that batch arguments supplied in the URL are observed
for POST operations too:

>>> request = build_request({'fnorb': 'bar',
... 'start': '3',
... 'batch': '3'}, method='POST')
>>> reindeer_batch_navigator_post_with_qs = BatchNavigator(
... reindeer, request)

>>> reindeer_batch_navigator_post_with_qs.start
3
>>> reindeer_batch_navigator_post_with_qs.nextBatchURL()
'http://www.example.com/foo?fnorb=bar&start=6&batch=3'

We ensure that multiple size and batch arguments supplied in the URL don't
blow up the application. The first one is preferred.

>>> request = build_request(
... [('batch', '1'), ('batch', '7'), ('start', '2'), ('start', '10')])
>>> navigator = BatchNavigator(reindeer, request=request)
>>> navigator.nextBatchURL()
'http://www.example.com/foo?start=3&batch=1'

=============
Empty Batches
=============

You can also create an empty batch that will not have any items:

>>> null_batch_navigator = BatchNavigator(
... None, build_request(), size=3)
>>> null_batch_navigator.firstBatchURL()
''
>>> null_batch_navigator.nextBatchURL()
''
>>> null_batch_navigator.prevBatchURL()
''
>>> null_batch_navigator.lastBatchURL()
''

>>> null_batch_navigator = BatchNavigator(
... [], build_request(), size=3)
>>> null_batch_navigator.firstBatchURL()
''
>>> null_batch_navigator.nextBatchURL()
''
>>> null_batch_navigator.prevBatchURL()
''
>>> null_batch_navigator.lastBatchURL()
''

TODO:

- blowing up when start is beyond end
- orphans
- overlap

====================================
Supporting Results Without a __len__
====================================

Some result objects do not implement __len__ because generally Python code
assumes that __len__ is cheap. SQLObject and Storm result sets both have this
behavior, for instance, so that it is cleat that getting the length is a non-
trivial operation.

To support these objects, the batch looks for __len__ on the result set. If
it does not exist, it adapts the result to
zope.interface.common.sequence.IFiniteSequence and uses that __len__.

>>> class ExampleResultSet(object):
... def __init__(self, results):
... self.stub_results = results
... def count(self):
... # imagine this actually returned
... return len(self.stub_results)
... def __getitem__(self, ix):
... return self.stub_results[ix] # also works with slices
... def __iter__(self):
... return iter(self.stub_results)
...
>>> from zope.interface import implements
>>> from zope.component import adapts, getSiteManager
>>> from zope.interface.common.sequence import IFiniteSequence
>>> class ExampleAdapter(ExampleResultSet):
... adapts(ExampleResultSet)
... implements(IFiniteSequence)
... def __len__(self):
... return self.stub_results.count()
...
>>> sm = getSiteManager()
>>> sm.registerAdapter(ExampleAdapter)
>>> example = ExampleResultSet(reindeer)
>>> example_batch_navigator = BatchNavigator(
... example, build_request(), size=3)
>>> example_batch_navigator.currentBatch().total()
9

========================
Only Gets What Is Needed
========================

It's also important for performance of batching large result sets that the
batch only gets a slice of the results, rather than accessing the entirety.

>>> class ExampleResultSet(ExampleResultSet):
... def __init__(self, results):
... super(ExampleResultSet, self).__init__(results)
... self.getitem_history = []
... def __getitem__(self, ix):
... self.getitem_history.append(ix)
... return super(ExampleResultSet, self).__getitem__(ix)
...

>>> example = ExampleResultSet(reindeer)
>>> example_batch_navigator = BatchNavigator(
... example, build_request(), size=3)
>>> reindeer[:3] == list(example_batch_navigator.currentBatch())
True
>>> example.getitem_history
[slice(0, 3, None)]

=========================
Adding callback functions
=========================

Sometimes it is useful to have a function called with the batched
values once they have been determined. This is the case when there
are subsequent queries that are needed to be executed for each batch,
and it is undesirable or overly expensive to execute the query for
every value in the entire result set.

The callback function must define two parameters. The first is the
batch navigator object itself, and the second it the current batch.
The callback function is called once and only once when the
BatchNavigator is constructed, and the current batch is determined.

>>> def print_callback(context, batch):
... for item in batch:
... print item

>>> reindeer_batch_navigator = BatchNavigator(
... reindeer, build_request(), size=3, callback=print_callback)
Dasher
Dancer
Prancer

>>> request = build_request({"start": "3", "batch": "20"})
>>> last_reindeer_batch_navigator = BatchNavigator(
... reindeer, request=request, callback=print_callback)
Vixen
Comet
Cupid
Donner
Blitzen
Rudolph

Most likely, the callback function will be bound to a view class.
By providing the batch navigator itself as the context for the
callback allows the addition of extra member variables. This is
useful as the BatchNavigator becomes the context in page templates
that are batched.

>>> class ReindeerView:
... def constructReindeerFromAtoms(self, context, batch):
... # some significantly slow process
... view.built_reindeer = list(batch)
... def batchedReindeer(self):
... return BatchNavigator(
... reindeer, build_request(), size=3,
... callback=self.constructReindeerFromAtoms)

>>> view = ReindeerView()
>>> batch_navigator = view.batchedReindeer()
>>> print view.built_reindeer
['Dasher', 'Dancer', 'Prancer']
>>> print list(batch_navigator.currentBatch())
['Dasher', 'Dancer', 'Prancer']

==================
Maximum batch size
==================

Since the batch size is exposed in the URL, it's possible for users to
tweak the batch parameter to retrieve more results. Since that may
potentially exhaust server resources, an upper limit is put on the batch
size. If the requested batch parameter is higher than this, an
InvalidBatchSizeError is raised.

>>> class DemoBatchNavigator(BatchNavigator):
... max_batch_size = 5
...
>>> request = build_request({"start": "0", "batch": "20"})
>>> DemoBatchNavigator(reindeer, request=request )
Traceback (most recent call last):
...
InvalidBatchSizeError: Maximum for "batch" parameter is 5.

==============
URL parameters
==============

Normally, any parameters passed in the current page's URL are
reproduced in the batch navigator's links. A "transient" parameter is
one that was only relevant for the current page request and shouldn't be
passed on to subsequent ones.

In this next batch navigator, two parameters occur in the page's URL:
"noisy" and "quiet."

>>> request_parameters = {
... 'quiet': 'ssht',
... 'noisy': 'HELLO',
... }

>>> request_with_parameters = build_request(request_parameters)

One parameter, "quiet," is transient. There is another transient
parameter called "absent," but it's not passed in our ongoing page
request.

>>> navigator_with_parameters = BatchNavigator(
... reindeer, request_with_parameters, size=3,
... transient_parameters=['quiet', 'absent'])

Of these three parameters, only "noisy" recurs in the links produced by
the batch navigator.

>>> navigator_with_parameters.nextBatchURL()
'http://www.example.com/foo?noisy=HELLO&start=3'
>>> navigator_with_parameters.lastBatchURL()
'http://www.example.com/foo?noisy=HELLO&start=6'

The transient parameter is omitted, and the one that was never passed in
in the first place does not magically appear.

==============
Batch headings
==============

The batched values are usually one kind of object such as bugs. The
BatchNavigator's heading property contains a description of the objects
for display.

>>> navigator.heading
'results'

There is a special case for when there is only one item in the batch,
the singular version of the heading is returned.

>>> navigator = BatchNavigator(['only-one'], request=request)
>>> navigator.heading
'result'

The heading can be set by passing a singular and a plural version of
the heading. The batch navigation will return the appropriate
header based on the total items in the batch.

>>> navigator = BatchNavigator(reindeer, request=request)
>>> navigator.setHeadings('bug', 'bugs')
>>> navigator.heading
'bugs'

>>> navigator = BatchNavigator(['only-one'], request=request)
>>> navigator.setHeadings('bug', 'bugs')
>>> navigator.heading
'bug'

(Cleanup)

>>> sm.unregisterAdapter(HTTPCharsets)
True
>>> sm.unregisterAdapter(ExampleAdapter)
True

===============
Other Documents
===============

.. toctree::
:glob:

*
docs/*

============================
NEWS for lazr.batchnavigator
============================

1.0 (2009-03-24)
================

- Initial release on PyPI