fix: add types to DatasetReference constructor

[kserruys]

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Fixes#1598 🦕

[chalmerlowe]

LGTM

[chalmerlowe]

Welp:

We appear to be faced with a mismatch. Doing a run of mypy against the code produces this error:

google/cloud/bigquery/dataset.py:187: error: Argument 1 to "DatasetReference" has incompatible type "Optional[str]"; expected "str"

Feel free to dig into this and see what produces that incompatible type on line 187 and what we might need to do to resolve this issue.

tests failed

[chalmerlowe]

@kserruys let's run kokoro and see what kinda results we get from the tests.

[chalmerlowe]

@kserruys

The two types of prerelease dependency test failures are a known issue that I need to tackle and we can safely ignore them in this case.

The Kokoro test failure relates to the overall test coverage.
It looks like enough has changed in that portion of code by adding a new conditional branch that the Python library coverage feels our tests are not comprehensive enough.

Are you comfortable diving into the test suite to see what needs to be added to the tests to provide coverage for the new conditional?

[kserruys]

Here else: covers the same cases as elif len(parts) > 2:

Parameter dataset_id is a string, because of that list parts will always have at least 1 item.
By using else we can reassure pytest-cov that everything is ok.

[kserruys]

@kserruys

The two types of prerelease dependency test failures are a known issue that I need to tackle and we can safely ignore them in this case.

The Kokoro test failure relates to the overall test coverage. It looks like enough has changed in that portion of code by adding a new conditional branch that the Python library coverage feels our tests are not comprehensive enough.

Are you comfortable diving into the test suite to see what needs to be added to the tests to provide coverage for the new conditional?

@chalmerlowe

Sure :). I just took a while to find some time to do this.
Please have a look at the updated code. Coverage on my local machine return 100% now.

Regards

[tswast]

Thanks @kserruys so much for the contribution and for your patience on this!