[Python] service account does not have storage.objects.get access for Google Cloud Storage

I have created a service account in Google Cloud Console and selected role Storage / Storage Admin (i.e. full control of GCS resources).

gcloud projects get-iam-policy my_project seems to indicate that the role was actually selected:

- members:
- serviceAccount:my_sa@my_project.iam.gserviceaccount.com
role: roles/storage.admin
- members:
- serviceAccount:my_sa@my_project.iam.gserviceaccount.com
role: roles/storage.objectAdmin
- members:
- serviceAccount:my_sa@my_project.iam.gserviceaccount.com
role: roles/storage.objectCreator


And documentation clearly indicates that role roles/storage.admin comprises permissions storage.objects.* (as well as storage.buckets.*).

But when I try using that service account in conjunction with the Google Cloud Storage Client Library for Python, I receive this error message:

my_sa@my_project.iam.gserviceaccount.com does not have storage.objects.get access to my_project/my_bucket.
​

So why would the selected role not be sufficient in this context?

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