I feel like Python ought to have a built-in to do this. Take a list of items
and turn them into a dictionary mapping keys to a list of items with that key
in common.
It's easy enough to do:
# using defaultdict
lookup = collections.defaultdict(list)
for item in items:
lookup[key(item)].append(item)
# or, using plain dict
lookup = {}
for item in items:
lookup.setdefault(key(item), []).append(item)
But this is frequent enough of a use case that a built-in function would be
nice. I could implement it myself, as such:
def grouped(iterable, key):
result = {}
for item in iterable:
result.setdefault(key(item), []).append(item)
return result
lookup = grouped(items, key)
This is different than `itertools.groupby` in a few important ways. To get the
same result from `groupby`, you'd have to do this, which is a little ugly:
lookup = dict((k, list(v)) for k, v in groupby(sorted(items, key=key), key))
Some examples:
>>> items = range(10)
>>> grouped(items, lambda x: x % 2)
{0: [0, 2, 4, 6, 8], 1: [1, 3, 5, 7, 9]}
>>> items = 'hello stack overflow how are you'.split()
>>> grouped(items, len)
{8: ['overflow'], 3: ['how', 'are', 'you'], 5: ['hello', 'stack']}
Is there a better way?
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