The following code

import sentry_sdk
from sentry_sdk import start_transaction

sentry_sdk.init(traces_sample_rate=1.0, dsn=...)
with start_transaction(op="GetDatasets", name='DUMMY') as span:
    span.set_data('foo', 'bar')
    print('foo')

will silently discard the data set in the transaction as it is only currently supported by child spans, not the transaction despite the transaction inheriting from span. One solution to fix this would be to make Transaction.set_data() raise an exception as it should not be used, but I think the best path forward would be to add actual support to extra data in Transaction.

I think this will require changes in the API thought.