Add get_user_credentials function to get your user credentials.

[tswast]

This function is intended to be used in interactive code examples, such
as from a Jupyter notebook or when developing scripts locally. It uses
the same client ID as the gcloud auth authentication-default login
command-line tool, so should work for most Google APIs.

Tested manually with the BigQuery API:

import os

import google_auth_oauthlib
from google.cloud import bigquery

credentials = google_auth_oauthlib.get_user_credentials(
    ["https://www.googleapis.com/auth/cloud-platform"]
)

client = bigquery.Client(
    credentials=credentials,
    project=os.environ["PROJECT_ID"],
)
df = client.query(
    """
    SELECT
      CONCAT(
        'https://stackoverflow.com/questions/',
        CAST(id as STRING)) as url,
      view_count
    FROM `bigquery-public-data.stackoverflow.posts_questions`
    WHERE tags like '%google-bigquery%'
    ORDER BY view_count DESC
    LIMIT 10
    """
).to_dataframe()
print(df)

Closes #39

[tswast]

Some screenshots of this in action:

[busunkim96]

LGTM, but I think it would be best to have a few more people set eyes on it.

[busunkim96]

I see more interest in this in googleapis/google-auth-library-python#271, have you heard from anyone on this?

[theacodes]

I *really* don't think we should perpetuate the use of that shared client ID. I would chat with Hong before going forward here.

…

On Tue, Jun 4, 2019 at 9:34 AM Bu Sun Kim ***@***.***> wrote: I see more interest in this in googleapis/google-auth-library-python#271 <googleapis/google-auth-library-python#271>, have you heard from anyone on this? — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#40?email_source=notifications&email_token=AAB5I44Q2OTUWWPZTYUFBDTPY2KRZA5CNFSM4HI24HF2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGODW5ETCA#issuecomment-498747784>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAB5I4Y2P3OIPFJ5HOZKZ6DPY2KRZANCNFSM4HI24HFQ> .

[tswast]

What if we make client ID and client secret required?

[theacodes]

That's fine.

…

On Tue, Jun 4, 2019 at 9:52 AM Tim Swast ***@***.***> wrote: What if we make client ID and client secret required? — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#40?email_source=notifications&email_token=AAB5I43DU7HW6TWR5IQAW4LPY2MU5A5CNFSM4HI24HF2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGODW5GHGY#issuecomment-498754459>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAB5I4253QCXHRFPT2YMVK3PY2MU5ANCNFSM4HI24HFQ> .

[tswast]

@busunkim96 This PR is ready for another review.

Once this is merged, is autorelease configured for this repo?

[busunkim96]

It'll be a manual release. (looks like this repo still needs to be moved off of Travis, in fact 😦)

Would you mind adding an example of how to use this in the comments like we have for flow.py?

example for flow

[tswast]

Would you mind adding an example of how to use this in the comments like we have for flow.py?

Great idea. Done! I've confirmed that it renders well in the docs.