On 11/16/2017 8:56 AM, bunslow wrote:
For taking values alternately from a series of iterables, there's two
primary functions:
builtin.zip
itertools.zip_longest
These bunch together the nth items of each iterable, while
itertools.cycle does not.
...
def roundrobin(*iterables):
"roundrobin('ABC', 'D', 'EF') --> A D E B F C"
pending = len(iterables)
nexts = cycle(iter(it).__next__ for it in iterables)
while pending:
try:
for next in nexts:
yield next()
except StopIteration:
pending -= 1
nexts = cycle(islice(nexts, pending))
Things that strike me as unpythonic:
1) requiring the total number of input iterable, > 2) making gratuitous use of
`next`,
I disagree that 1 and 2 are problems.
3) using a while loop in code dealing with iterables,
I agree that this is not necessary, and give a replacement below.
4) combining loops, exceptions, and composed itertools functions in
non-obvious ways that make control flow difficult to determine
I agree that the correctness of the last statement is slightly opaque.
But this nicely demonstrates a non-trivial use of cycle.
Now, I get it, looking at the "roughly equivalent to" code for
zip_longest in the docs, there doesn't seem to be much way around it for
generally similar goals, and as I said above, unpythonic is fine when
necessary (practicality beats purity), but in this case, for being a
"recipe" in the itertools docs, it should *make use* of the zip_longest
which already does all the unpythonic stuff for you (though honestly I'm
not convinced either that the zip_longest code in the docs is the most
possible pythonic-ness). Instead, the following recipe (which I also
submitted to the StackOverflow question, and which is generally similar
to several other later answers, all remarking that they believe it's
more pythonic) is much cleaner and more suited to demonstrating the
power of itertools to new developers than the mess of a "recipe" pasted
above.
def roundrobin(*iters):
"roundrobin('ABC', 'D', 'EF') --> A D E B F C"
# Perhaps "flat_zip_nofill" is a better name, or something similar
sentinel = object()
for tup in it.zip_longest(*iters, fillvalue=sentinel):
yield from (x for x in tup if x is not sentinel)
This adds and then deletes grouping and fill values that are not wanted.
To me, this is an 'algorithm smell'. One of the principles of
algorithm design is to avoid unnecessary calculations. For an edge case
such as roundrobin(1000000*'a', ''), the above mostly does unnecessary work.
The following combines 3 statements into one for statement.
def roundrobin(*iterables):
"roundrobin('ABC', 'D', 'EF') --> A D E B F C"
nexts = cycle(iter(it).__next__ for it in iterables)
for reduced_len in reversed(range(1, len(iterables))):
try:
for next in nexts:
yield next()
except StopIteration:
nexts = cycle(islice(nexts, reduced_len))
In particular, this is just an extremely thin wrapper around
zip_longest, whose primary purpose is to eliminate the
otherwise-mandatory "fillvalues" that zip_longest requires to produce
uniform-length tuples.
But we do not want tuples or fill values.
It's also an excellent example of how to make
best pythonic use of iterables in general, and itertools in particular,
and as such a much better implementation to be demonstrated in
documentation.
I disagree. [I have mostly stopped using 'pythonic' because there is
too much disagreement on particulars, and its use seems to inhibit as
much as facilitate insight.]
[snip more]
I realize at the end of the day this is a pretty trivial and ultimately
meaningless nit to pick, but I've never contributed before and have a
variety of similar minor pain points in the docs/stdlib, and I'm trying
to gauge 1) how well this sort of minor QoL improvement is wanted,
We constantly improve the docs.
2) even if it is wanted, am I going about it the right way.
Typos and trivial grammar issues can be filed as a PR with no issue
required. Clarifications usually require an issue and perhaps
discussion. Since this is more about philosophy of algorithm design,
python-ideas was a good place to start.
If the
answers to both of these questions are positive regarding this
particular case, then I'll look into making a BPO issue and pull request
on GitHub, which IIUC is the standard path for contributions.
Since I have a competing 'improvement', I would hold off on a PR until
Raymond Hettinger, the itertools author, comments.
--
Terry Jan Reedy
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