groupby.py

from functools import partial

def iseven(n, d=2):
    """Example"""
    return n % d == 0

def groupby(key, seq):
    """Based on toolz groupby"""
    result = defaultdict(list)
    for item in seq:
        result[key(item)].append(item)
    return result

array = [1, 2, 3, 4, 5, 6, 7, 8]
groupby(iseven, array) # {False: [1, 3, 5, 7], True: [2, 4, 6, 8]}
groupby(partial(iseven, d=1), array) # {True: [1, 2, 3, 4, 5, 6, 7, 8]}
groupby(partial(iseven, d=3), array) # {False: [1, 2, 4, 5, 7, 8], True: [3, 6]}