blob prefixes only exist after iterating over blobs

[joostvham]

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/'}

[IlyaFaer]

@joostvham, in fact, list_blobs() returns an iterator, which from the very start have prefixes explicitly set to set(). So, it's intended to work like this. One can call this iterator lazy, as it's requesting data while iterating. In other words, when you got your blobs returned from bucket.list_blobs(delimiter='/'), you don't yet have the data. Only while iterating through blobs, you'll actually retrieve it (including prefixes).

[joostvham]

Thanks for your response. My bad, I should have read the documentation better.