A Python one liner to group list of items into a dictionary of lists

groupby_to_dict.py

# One-liner

# items = [(1, 'a'), (2, 'b'), (3, 'c'), (1, 'd'), (3, 'e'), (1, 'f')]
reduce(lambda d, item: d.setdefault(item[0], []).append(item) or d, items, {})
# >>> {1: [(1, 'a'), (1, 'd'), (1, 'f')], 2: [(2, 'b')], 3: [(3, 'c'), (3, 'e')]}


# Featured function

def groupby_to_dict(iterable, key=None, value=None):
    """
    Group items from `iterable` into a dictionary of lists. The order of items is preserved.
    The `key` is a function computing a key value for each item. If not specified or is `None`, key defaults to an identity function and returns the item unchanged.
    The `value` is a function extracting a value for each item to be appended to the list. If not specified or is `None`, takes the item unchanged.
    """
    if key is None:
        key = lambda x: x
    if value is None:
        value = lambda x: x
    return reduce(lambda d, item: d.setdefault(key(item), []).append(value(item)) or d, iterable, {})


items = [(1, 'a'), (2, 'b'), (3, 'c'), (1, 'd'), (3, 'e'), (1, 'f')]
groupby_to_dict(items, lambda x: x[0])
# >>> {1: [(1, 'a'), (1, 'd'), (1, 'f')], 2: [(2, 'b')], 3: [(3, 'c'), (3, 'e')]}