Do Python lists have an equivalent to dict.get?

Question

I have a list of integers. I want to know whether the number 13 appears in it and, if so, where. Do I have to search the list twice, as in the code below?

if 13 in intList:
   i = intList.index(13)

In the case of dictionaries, there's a get function which will ascertain membership and perform look-up with the same search. Is there something similar for lists?

Answer 1

You answered it yourself, with the index() method. That will throw an exception if the index is not found, so just catch that:

def getIndexOrMinusOne(a, x):
  try:
    return a.index(x)
  except ValueError:
    return -1

Answer 2

It looks like you'll just have to catch the exception...

try:
    i = intList.index(13)
except ValueError:
    i = some_default_value

Answer 3

Just put what you got in a function and use it:)

You can either use if i in list: return list.index(i) or the try/except, depending on your preferences.

Answer 4

No, there isn't a direct match for what you asked for. There was a discussion a while back on the Python mailing list about this, and people reached the conclusion that it was probably a code smell if you needed this. Consider using a dict or set instead if you need to test membership that way.

Answer 5

You can catch the ValueError exception, or you can do:

i = intList.index(13) if 13 in intList else -1

(Python 2.5+)

BTW. if you're going to do a big batch of similar operations, you might consider building inverse dictionary value -> index.

intList = [13,1,2,3,13,5,13]
indexDict = defaultdict(list)
for value, index in zip(intList, range(len(intList))):
   indexDict[value].append(index)

indexDict[13]
[0, 4, 6]