Python itertools.groupby trap

groupby.py

data = [{'id': 1, 'value': 1}, {'id': 2, 'value': 2}, {'id': 2, 'value': 3}, {'id': 1, 'value': 4}]

for id, ele in groupby(data, key=lambda e: e['id']):
    print "id: %d" % id
    print "values: %s" % list(ele)
# => id: 1
# => values: [{'id': 1, 'value': 1}]
# => id: 2
# => values: [{'id': 2, 'value': 2}, {'id': 2, 'value': 3}]
# => id: 1
# => values: [{'id': 1, 'value': 4}]


{key: list(value) for key, value in groupby(data, key=lambda x: x['id'])}            # Dangerous!
# => {1: [{'id': 1, 'value': 4}], 2: [{'id': 2, 'value': 2}, {'id': 2, 'value': 3}]} # missing the first element!

{key: list(value) for key, value in groupby(sorted(data, key=lambda x: x['id']), key=lambda x: x['id'])}
# => {1: [{'id': 1, 'value': 1}, {'id': 1, 'value': 4}],
#     2: [{'id': 2, 'value': 2}, {'id': 2, 'value': 3}]}

# According to the official document, https://docs.python.org/2/library/itertools.html#itertools.groupby
#
# It generates a break or new group every time the value of the key function changes (which is why it is
#   usually necessary to have sorted the data using the same key function)
#
# (╯‵□′)╯︵┻━┻

sorted_groupby.py

def sorted_groupby(iterable, key=None):
	"""
	# https://docs.python.org/2/library/itertools.html#itertools.groupby
	The built-in `itertools.groupby` has a trap that it generates a break or new
	group every time the value of the key function changes, that is, it requires
	developers to sort the data using the same key function firstly. This function
	acts as a wrapper to sort automatically so that from now on you can always
	group data safely
	"""
	iterable = sorted(iterable, key=key)
	return itertools.groupby(iterable=iterable, key=key)

def groupby_to_dict(iterable, key=None):
	"""
	Sort and group `iterable` with provided key function, and then return a dict
	represents the grouped data
	"""
	return {k: list(v) for k, v in sorted_groupby(iterable=iterable, key=key)}

for k, v in sorted_groupby(data, key=lambda x: x['id']):
	print "key: %d" % k
	print "value: %s" % list(v)
# => key: 1
# => value: [{'id': 1, 'value': 1}, {'id': 1, 'value': 4}]
# => key: 2
# => value: [{'id': 2, 'value': 2}, {'id': 2, 'value': 3}]

groupby_to_dict(data, key=lambda x: x['id'])
# => {1: [{'id': 1, 'value': 1}, {'id': 1, 'value': 4}],
# =>  2: [{'id': 2, 'value': 2}, {'id': 2, 'value': 3}]}

data = [1, 2, 3, 2, 3, 4, 1, 1, 2]

groupby_to_dict(data)
# => {1: [1, 1, 1], 2: [2, 2, 2], 3: [3, 3], 4: [4]}