Derek Peschel wrote:
> Here are two functions.
>
> def invert_dict_to_lists(dict):
> lists = {}
> for key in dict:
> value = dict[key]
> if not value in lists:
> lists[value] = [key]
> else:
> lists[value].append(key)
> return lists
>
> def invert_list_to_lists(list):
> lists = {}
> for key in range(len(list)):
> value = list[key]
> if not value in lists:
> lists[value] = [key]
> else:
> lists[value].append(key)
> return lists
>
> They are the same except for the expression in "for key in ...". Can they
> be combined into one function? How can I determine if the argument is
> like a list (with numeric indices that are not stored in the list) or a
> dict
> (with arbitrary keys that are stored)? I said "object" in the subject,
> but I want to support Python primitive types, class instances, extension
> module types (array, dictproxy, dbm, gdbm, etc.), and any future types.
>
> I've thought about looking for keys(), looking for the special method
> names that allow you to override indexing behavior, and looking at the
> class or
> type of the object. I could be wrong, but I don't think any of those
> strategies will work with all arguments.
Instead of the (unreliable) introspection approach you could let the client
code decide:
>>> def invert(pairs):
... result = {}
... for k, v in pairs:
... if v in result:
... result[v].append(k)
... else:
... result[v] = [k]
... return result
...
>>> invert(dict(a=1, b=2, c=1).iteritems())
{1: ['a', 'c'], 2: ['b']}
>>> invert(enumerate([1,1,2,3]))
{1: [0, 1], 2: [2], 3: [3]}
Peter
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