Python equivalent to Scala groupby

Question

I have a list of objects, and I'd like a function that can take that list along with a function operating on the items in that list, and produce a dict with keys from the result of applying that function to the item, and values being a list of items with that key.

Example:

def group_by(iterable: Iterable[A], f: Callable[A, B]) -> Dict[B, List[A]]:
    ???

lst = [(1,2), (3,4), (1,3)]
result = group_by(lst, lambda i: i[0])
result == {1: [(1,2), (1,3)],
           3: [(3,4)]}

itertools.groupby is close, but I don't want to require that my input be sorted.

Answer 1

Here's an approach with defaultdict :

from collections import defaultdict
def group_by(iterable, f):
    results = defaultdict(list)
    for x in iterable:
        results[f(x)].append(x)
    return results

Answer 2

you are looking for itertools.groupby

from itertools import groupby
groups = []
uniquekeys = []
data = sorted(data, key=keyfunc)
for k, g in groupby(data, keyfunc):
    # watch out! g is an iterator here, you must iterate it
    groups.append(list(g))      # Store group iterator as a list
    uniquekeys.append(k)

def my_groupby(fn,datum):
    d = {}
    for data in datum:
        d.setdefault(fn(d),[]).append(data)
    return d