Duncan Booth wrote:
> One example of padding out iterators (although I didn't use map's fill-in
> to implement it) is turning a single column of items into a multi-column
> table with the items laid out across the rows first. The last row may have
> to be padded with some empty cells.
ANALYSIS
--------
This case relies on the side-effects of zip's implementation details --
the trick of windowing or data grouping with code like: zip(it(),
it(), it()). The remaining challenge is handling missing values when
the reshape operation produces a rectangular matrix with more elements
than provided by the iterable input.
The proposed function directly meets the challenge:
it = iter(iterable)
result = izip_longest(*[it]*group_size, pad='')
Alternately, the need can be met with existing tools by pre-padding the
iterator with enough extra values to fill any holes:
it = chain(iterable, repeat('', group_size-1))
result = izip_longest(*[it]*group_size)
Both approaches require a certain meaure of inventiveness, rely on
advacned tricks, and forgo readability to gain the raw speed and
conciseness afforded by a clever use of itertools. They are also a
challenge to review, test, modify, read, or explain to others.
In contrast, a simple generator is trivially easy to create and read,
albiet less concise and not as speedy:
it = iter(iterable)
while 1:
row = tuple(islice(it, group_size))
if len(row) == group_size:
yield row
else:
yield row + ('',) * (group_size - len(row))
break
The generator version is plain, simple, boring, and uninspirational.
But it took only seconds to write and did not require a knowledge of
advanced itertool combinations. It more easily explained than the
versions with zip tricks.
Raymond
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