Question

I have a Python list of objects that could be pretty long. At particular times, I’m interested in all of the elements in the list that have a certain attribute, say flag, that evaluates to False. To do so, I’ve been using a list comprehension, like this:

objList = list()
# ... populate list
[x for x in objList if not x.flag]

Which seems to work well. After forming the sublist, I have a few different operations that I might need to do:

1. Subscript the sublist to get the element at index ind.
2. Calculate the length of the sublist (i.e. the number of elements that have flag == False).
3. Search the sublist for the first instance of a particular object (i.e. using the list’s .index() method).

I’ve implemented these using the naive approach of just forming the sublist and then using its methods to get at the data I want. I’m wondering if there are more efficient ways to go about these. #1 and #3 at least seem like they could be optimized, because in #1 I only need the first ind + 1 matching elements of the sublist, not necessarily the entire result set, and in #3 I only need to search through the sublist until I find a matching element.

Is there a good Pythonic way to do this? I’m guessing I might be able to use the () syntax in some way to get a generator instead of creating the entire list, but I haven’t happened upon the right way yet. I obviously could write loops manually, but I’m looking for something as elegant as the comprehension-based method.

Answer 1

If you need to do any of these operations a couple of times, the overhead of other methods will be higher, the list is the best way. It’s also probably the clearest, so if memory isn’t a problem, then I’d recommend just going with it.

If memory/speed is a problem, then there are alternatives – note that speed-wise, these might actually be slower, depending on the common case for your software.

For your scenarios:

#value = sublist[n]
value = nth(x for x in objList if not x.flag, n)

#value = len(sublist)
value = sum(not x.flag for x in objList)

#value = sublist.index(target)
value = next(dropwhile(lambda x: x != target, (x for x in objList if not x.flag)))

Using itertools.dropwhile() and the nth() recipe from the itertools docs.