feat: support autoconversion of Entity to Key for purposes of delete & delete_multi

[crwilcox]

Fixes #122

Can discuss if we should

• have a warning
  or
• modify documented type of delete and delete_multi to mention they can take Entity

❯ python tests/system/test_delete.py
Deleting 20 entities: 5121006972698624, 5150910414061568, 5629847317512192, 5635426882682880, 5644748102565888, 5660593176444928, 5670520389369856, 5680769993277440, 5693029306335232, 5697954694299648, 5701878616686592, 5708050082037760, 5709769612460032, 5744781917421568, 5746397093560320, 5746853299617792, 5754048175144960, 5756417252261888, 5762132310228992, 6251525647106048
/Users/crwilcox/workspace/python-datastore/google/cloud/datastore/client.py:682: UserWarning: A list of :class:`google.cloud.datastore.key.Key` is expected. Extracting Key from :class:`google.cloud.datastore.entity.Entity`.
  warnings.warn(message, UserWarning)

[wescpy]

LGTM

[wescpy]

• [L683] This works but may result in occasional snarky comments. I would not, however, rewrite it as key = getattr(key, 'key') cuz that's worse.
• [L646] Would any changes be req'd in this part of the docstring?

[crwilcox]

may result in occasional snarky comments.

@wescpy I don't quite understand. Could you elaborate.

As for docstr, that is part of the open conversation. We can either

1. make this a stealth change to help smooth migration. As written with the warning, I wouldn't want to add to docs
2. say this is actually supported, in which case, no warning should happen, but we should specify in types you can hand an entity as well.

I would be fine to call this supported, but started from the 'smoothing' over as lightest touch.

[wescpy]

Snarky meaning some people may criticize naming choice in key = key.key while it's perfectly valid IMO. Anyway, it was an aside and a mostly irrelevant comment... pls ignore. For the other one, I'm leaning #2. I'd also like to hear from members of the community as well, esp. those who work w/Datastore regularly. In particular, I'd like to know whether this is something they find useful.

[tseaver]

Needs unit tests for this feature.

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[crwilcox]

@tseaver sort of waiting until we agree if to go with full support or sneaky support. Do you have an opinion? I am not terrified about adding type coercion myself, and this will make migration from NDB a bit smoother.

[tseaver]

@crwilcox I would actually go with full support (no warning, update the docstring).

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feedback addressed. LGTM elsewhere ;)

[dmahugh]

LGTM

[engelke]

LGTM