MultiDict.update() ignores empty "iterable" values while iterating

[joshfriend]

I did this to fix flask-restful/flask-restful#380 which has been annoying me for some time.

If you try to call update on a MultiDict and the mapping passed to update has an empty iterable for the value mapped to a key, that value is ignored:

>>> from werkzeug.datastructures import MultiDict
>>> x = {'empty_list': []}
>>> y = MultiDict()
>>> y.update(x)
>>> y
MultiDict([])
>>> y['empty_list'] = []
>>> y
MultiDict([('empty_list', [])])

Regular dict:

>>> x = {'empty_list': []}
>>> y = {}
>>> y.update(x)
>>> y
{'empty_list': []}

I think I read the docs pretty closely and didn't see that ignoring empty iterable values was intentional.

[untitaker]

The behavior is definetly underspecified, but I think it's the intended one. Since the value is a list, it will be interpreted as a list of values for this key, and since its empty, the key won't exist.

[untitaker]

@joshfriend any update on this? I see that the behavior is quite inconsistent, however, I'd argue that for a MultiDict (ignoring the underlying data structures), it doesn't semantically make sense to differentiate between two states where a key has no values.

[joshfriend]

@untitaker Docs updated. how's it look?

[untitaker]

Thanks!