Environment details

• Ubuntu 18.04
• python 3.7.4
• pip 19.0.3
• google-cloud-storage version 1.29.0

Steps to reproduce

I have a bucket, with at the 'root' of the bucket 3 directories. I want to display these three directories. However, they only exist when iterating over the blobs first, without a clear reason.

Code example

from google.cloud import storage

bucket_name = 'bucket-name'
client = storage.Client()
bucket = client.get_bucket(bucket_name)
blobs = bucket.list_blobs(delimiter='/')

print('printing prefixes 1 ...')
print(blobs.prefixes)

[b for b in blobs]

print('printing prefixes 2 ...')
print(blobs.prefixes)

Output

python bucket.py 
printing prefixes 1 ...
set()
printing prefixes 2 ...
{'test-data/', 'unparsed-data/', 'raw-data/'}