python - Fastest way to find Indexes of item in list?

Question

If one was to attempt to find the indexes of an item in a list you could do it a couple different ways here is what I know to be the fastest:

aList = [123, 'xyz', 'zara','xyz', 'abc']; 
indices = [i for i, x in enumerate(aList) if x == "xyz"]
print(indices)

Another way not pythonic and slower:

count = 0
indices = []
aList = [123, 'xyz', 'zara','xyz', 'abc'];
for i in range(0,len(aList):
    if 'xyz' == aList[i]:
        indices.append(i)
print(indices)

The first method is undoubtedly faster however what if you wanted to go faster, is there a way? For the first index using method:

aList = [123, 'xyz', 'zara','xyz', 'abc'];             
print "Index for xyz : ", aList.index( 'xyz' ) 

is very fast but can't handle multiple indexes.
How might one go about speeding things up?

Answer 1

Use list.index(elem, start)! That uses a for loop in C (see its implementation list_index_impl function in the source of CPython's listobject.c). Avoid looping through all the elements in Python, it is slower than in C.

def index_finder(lst, item):
    """A generator function, if you might not need all the indices"""
    start = 0
    while True:
        try:
            start = lst.index(item, start)
            yield start
            start += 1
        except ValueError:
            break

import array
def index_find_all(lst, item, results=None):
    """ If you want all the indices.
    Pass results=[] if you explicitly need a list,
    or anything that can .append(..)
    """
    if results is None:
        length = len(lst)
        results = (array.array('B') if length <= 2**8 else
                   array.array('H') if length <= 2**16 else
                   array.array('L') if length <= 2**32 else
                   array.array('Q'))
    start = 0
    while True:
        try:
            start = lst.index(item, start)
            results.append(start)
            start += 1
        except ValueError:
            return results

# Usage example
l = [1, 2, 3, 4, 5, 6, 7, 8] * 32

print(*index_finder(l, 1))
print(*index_find_all(l, 1))