IAM policy should return an empty set of the bindings do not exist

[theacodes]

https://github.com/GoogleCloudPlatform/google-cloud-python/blob/bd9fdfba07fac63a91847628613928a569250c0f/core/google/cloud/iam.py#L71

Should return an empty set if no bindings are set for that role.

[dhermes]

Current behavior leads to KeyError, yes?

[theacodes]

ya, I'd like to be able to do:

policy = something.get_iam_policy()
policy['roles/editor'].add('whatever')
something.set_iam_policy(policy)

and it'll work regardless of if any editors had been previously set.

[dhermes]

@jonparrott ISTM like you want a collections.defaultdict(set)?

In #3325 a similar snippet was failing because policy.owners returned a frozenset. In the __getitem__ case, the raw binding will be returned, which we guarantee is a set. Is it a bad thing that the two different means of access produce different behavior?

[theacodes]

Defaultdict is pretty much what I'm after, in terms of behavior.
I'm fine with the old/deprecated methods returning a frozenset.