Add profile_name option to sessions

[danielgtaylor]

This change makes it possible to create a session using a profile name, which
was previously only possible via an environment variable. It allows you to
create clients and resources from any number of sessions using any number of
profiles.

Given that we have the following ~/.aws/credentials:

[default]
aws_access_key_id = DEFAULT
aws_secret_access_key = SECRET1

[dev]
aws_access_key_id = DEV
aws_secret_access_key = SECRET2

[prod]
aws_access_key_id = PROD
aws_secret_access_key = SECRET3

You can do the following:

import boto3.session

dev = boto3.session.Session(profile_name='dev')
prod = boto3.session.Session(profile_name='prod')

for session in [dev, prod]:
    print('Profile: ' + session.profile)
    s3 = session.resource('s3')
    for bucket in s3.buckets.all():
        print(bucket.name)
    print('')

It is also possible to setup the default session with a profile:

import boto3

boto3.setup_default_session(profile_name='dev')

s3 = boto3.resource('s3')

And of course you can still use the environment variable:

$ BOTO_DEFAULT_PROFILE=dev ipython
>>> import boto3
>>> s3dev = boto3.resource('s3')

Once a session is created, the profile is immutable. The profile_name
property is provided for convenience and just surfaces the underlying
Botocore session's profile property.

Why not provide a profile name to the client and resource methods?
Currently there is no easy way to clone a session, and the behavior of
creating a new blank session (from one which may have customizations)
leads to possibly unintended side-effects like clients without the
proper event handlers registered. This would be an additive change
should we want to revisit in the future. At this point, the change
in this commit provides a way for Boto 3 users to use multiple profiles
without resorting to creating their own Botocore sessions.

I'm definitely open to suggestions and counter-arguments.

cc @jamesls @kyleknap

[danielgtaylor]

I would strongly prefer that this always return a string, but it could be either 'default' or a blank string. It makes code like this simpler:

if session.profile.startswith('prod'):
    # Do something here!
    pass

[jamesls]

I think this is probably ok at this layer. I believe the reason it's like that in botocore is because parts of the code need to distinguish between "was explicitly set to default by the user" and "try the default profile if one hasn't been set". The former case is an error, the latter case just means move on to the next credential provider.

[danielgtaylor]

This is one possible implementation of #21.

[jamesls]

I think this seems reasonable. Looks good to me,

[kyleknap]

Looks good 🚢

[danielgtaylor]

Merged in 8da893b

[coveralls]

Changes Unknown when pulling 2a10ea6 on session-profile into * on develop*.

[ream88]

For everybody coming from Google: it is now called AWS_PROFILE.

$ AWS_PROFILE=dev ipython
>>> import boto3
>>> s3dev = boto3.resource('s3')

[quiver]

kudos go to @Haihappen

[GBR-613]

It does not work for me now (Boto3 v1.11.9, Python v3.8.1), at least with S3

The profile I specify is listed in boto3.Session().available_profiles
AWS CLI does work with the profile.
Who could verify this functionality is not simply broken in the last versions?
Thanks in advance!