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Cut sliceable objects into batches, eg for paging display on a website

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Splits any sliceable object into batches:

>>> from batcher import Batcher
>>> items = list('ABCDEFGHIJ')
>>> batcher = Batcher(items, 4)
>>> # How many batches are available?
>>> len(batcher)
3
>>> # Get contents of the first batch
>>> list(batcher[0])
['A', 'B', 'C', 'D']

Batches look like lists but know about their context too:

>>> batch = batcher[1]
>>> batch
<Batch #1>
>>> batcher[1].previous
<Batch #0>
>>> batcher[1].next
<Batch #2>
>>> batcher[2].next
None

A common requirement in web applications is to implement a pager widget. The range method can help when the user is viewing page n and we want to display a range of pages centered on n, adjusting endpoints to keep them in range:

>>> items = range(100)
>>> batcher = Batcher(items, 4)
>>> len(batcher)
25
>>> # The range of 5 batches centered on the given batch
>>> batcher[0].range(5)
[<Batch #0>, <Batch #1>, <Batch #2>, <Batch #3>, <Batch #4>]
>>> batcher[7].range(5)
[<Batch #5>, <Batch #6>, <Batch #7>, <Batch #8>, <Batch #9>]
>>> batcher[23].range(5)
[<Batch #20>, <Batch #21>, <Batch #22>, <Batch #23>, <Batch #24>]

If there aren’t enough batches available, the entire batch will be returned:

>>> items = range(10)
>>> batcher = Batcher(items, 4)
>>> len(batcher)
3
>>> batcher[1].range(5)
[<Batch #0>, <Batch #1>, <Batch #2>]

Because batches use python’s slicing API to retrieve data, we can interrogate the slice object to find out the indices of the first and last items in a batch, useful for showing data such as “Page 1 (items 1-10)”:

>>> items = range(25)
>>> batcher = Batcher(items, 10)
>>> batch = batcher[0]
>>> batch.slice
slice(0, 10, None)
>>> "Page %d (items %d-%d)" % (batch.index + 1,
...                            batch.slice.start + 1,
...                            batch.slice.stop)
'Page 1 (items 1-10)'

Note how python’s slice semantics mean that the indices are zero-based (so we add 1 when formatting for display) and that the stop index of the slice points to the item after the end of the series.

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