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Oct 31, 2009, 2:07 AM
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[issue7244] itertools.zip_longest behaves strangely with an iterable class
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New submission from Daniel Urban <urban.dani+py [at] gmail>:
I'm trying to write an iterable class, and it behaves strangely with
itertools.zip_longest. The following example demonstrates this:
class Repeater: # this class is similar to itertools.repeat
def __init__(self, o, t):
self.o = o
self.t = int(t)
def __iter__(self): # its iterator is itself
return self
def __next__(self):
if self.t > 0:
self.t -= 1
return self.o
else:
raise StopIteration
(Of course this is a trivial class, which could be substituted with
itertools.repeat, but I wanted to keep it simple for this example.)
The following code shows my problem:
>>> r1 = Repeater(1, 3)
>>> r2 = Repeater(2, 4)
>>> for i, j in zip_longest(r1, r2, fillvalue=0):
... print(i, j)
...
1 2
1 2
1 2
0Traceback (most recent call last):
File "<stdin>", line 2, in <module>
File "zip_longest_test_case.py", line 30, in __next__
raise StopIteration
StopIteration
>>>
It seems, that zip_longest lets through the StopIteration exception,
which it shouldn't.
The strange thing is, that if I use the python implementation of
zip_longest, as it is in the documentation [1], I get the expected
result:
# zip_longest as it is in the documentation:
def zip_longest(*args, fillvalue=None):
# zip_longest('ABCD', 'xy', fillvalue='-') --> Ax By C- D-
def sentinel(counter = ([fillvalue]*(len(args)-1)).pop):
yield counter() # yields the fillvalue, or raises IndexError
fillers = repeat(fillvalue)
iters = [chain(it, sentinel(), fillers) for it in args]
try:
for tup in zip(*iters):
yield tup
except IndexError:
pass
Test code again:
>>> r1 = Repeater(1, 3)
>>> r2 = Repeater(2, 4)
>>> for i, j in zip_longest(r1, r2, fillvalue=0):
... print(i, j)
...
1 2
1 2
1 2
0 2
I would think, that this is the expected behaviour.
Also, Matthew Dixon Cowles discovered, that if using list(), the C
implementation of itertools.zip_longest also works fine:
>>> r1=Repeater(1,3)
>>> r2=Repeater(2,5)
>>> list(itertools.zip_longest(r1,r2,fillvalue=0))
[(1, 2), (1, 2), (1, 2), (0, 2), (0, 2)]
This is strange, and I think it really shouldn't work this way.
(Thanks for Matthew Dixon Cowles' help on the python-help mailing list.)
I'm attaching a test script, which tries all 4 variations (library
zip_longest with and without list(), and the documentation's zip_longest
impplementation with and without list()).
And another thing: it works fine in 2.6.4 (with izip_longest).
----------
components: Extension Modules
files: zip_longest_test_case.py
messages: 94746
nosy: durban
severity: normal
status: open
title: itertools.zip_longest behaves strangely with an iterable class
type: behavior
versions: Python 3.1
Added file: http://bugs.python.org/file15238/zip_longest_test_case.py
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<http://bugs.python.org/issue7244>
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