Thanks for getting back - I’ve fixed it. For the benefit of anyone with similar problems:
I found this thread:
The user there solved the problem by logging out of apple ID and logging back in. I tried that and it didn’t help (wish I hadn’t - now resyncing all my photos 
)
However, he also diagnosed that solution by creating a new user account and trying there. This I did, and the operation worked in the new user account. I checked permissions there, and found that the app “python” was granted access to documents.
Logged back into my normal account, gave python access to documents, and it now works.
Thanks Barry for your time spent helping me.
(Can I set this to solved other than by adding the word to the title?)