Simple implementation of an iterable version of str.find()

finditer.py

def finditer(S, sub, start=0, end=None, overlap=False):
    """Iterate over the indices of substring sub in S.

    Returns an iterable, in ascending order, of the indices in S where
    substring sub is found such that sub is contained within S[start:end].
    Optional arguments start and end are interpreted as in slice notation. A
    True value for the optional argument overlap will allow overlapping
    matches.

    """
    increment = 1 if overlap else max(1, len(sub))
    start = S.find(sub, start, end)
    while start != -1:
        yield start
        start = S.find(sub, start+increment, end)
    
def rfinditer(S, sub, start=0, end=None, overlap=False):
    """Iterate over descending indices of substring sub in S.

    Returns an iterable, in descending order, of the indices in S where
    substring sub is found such that sub is contained within S[start:end].
    Optional arguments start and end are interpreted as in slice notation. A
    True value for the optional argument overlap will allow overlapping
    matches.

    """
    if end is None:
        end = len(S)
    increment = len(sub)-1 if overlap else 0
    end = S.rfind(sub, start, end)
    while end != -1:
        yield end
        end = S.rfind(sub, start, end+increment)