groupby.py

def group_by(source, key_func):
    acc = {}
    for item in source:
        acc.setdefault(key_func(item), []).append(item)
    return acc

if __name__ == '__main__':
    print group_by([(1, 2), (1, 3), (2, 3), (2, 4)], lambda x: x[0])
    print group_by([(1, 2), (1, 3), (2, 3), (2, 4)], lambda x: x[1])