An method to detect the last item in a loop.
Python, 38 lines
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Example (and doctest):
>>> for n, islast in iter_islast(range(4)):
... if islast:
... print "last but not least:",
... print n
...
0
1
2
last but not least: 3
>>> list(iter_islast(''))
[]
>>> list(iter_islast('1'))
[('1', True)]
>>> list(iter_islast('12'))
[('1', False), ('2', True)]
>>> list(iter_islast('123'))
[('1', False), ('2', False), ('3', True)]
>>>
"""
def iter_islast(iterable):
""" iter_islast(iterable) -> generates (item, islast) pairs
Generates pairs where the first element is an item from the iterable
source and the second element is a boolean flag indicating if it is the
last item in the sequence.
"""
it = iter(iterable)
prev = it.next()
for item in it:
yield prev, False
prev = item
yield prev, True
|
A question that comes up every once in a while on comp.lang.python is how to detect the last item on a for loop. The proposed solution is usually to use range(len(List)) or enumerate and check the index for len(List)-1. Sometimes the reason for wanting to detect the last item is to avoid printing a trailing comma or other separator. This specific case is best handled with str.join but it is not a general solution.
This recipe makes the code easy to read and works for non-indexable sources like files, generators, itertools objects, etc.
Tags: algorithms
