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, \'         


        

Answer 1

Use list.index(elem, start)! That uses a for loop in C (see it's 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))