Hi @Mugless Durry 

The write_protected field means: 
    To write data to a write-protected data set, you need to be a member of a group that has the “datasets:owner” action for the data set.

The weird experience you have observed could be to due someone else archiving the dataset. 

Try adding a check in your code to see if the data set exists first. 
 

ds_names = sds.name for ds in c.data_sets.list(limit=-1)]
if "world_info_myname" not in ds_names:
    raise Exception("Dataset does not exist in project.")

Let me know if anything. 

Thanks Andrian,

In both cases, YESTERDAY (working) and TODAY (not working), I checked in CDF for the data_set and confirmed that the data_set was indeed created - I checked the data_set_id in CDF.

So, I deduce from your reply:

<<To write data to a write-protected data set, you need to be a member of a group that has the “datasets:owner” action for the data set >>

that overnight I ceased to be a member of a group that has the “datasets:owner” action for ANY data_set that may be created in the future (relative to the membership change). Is that correct? 

That could be the case but seems weird.

Try creating another write_protected=True dataset and try write some mock data to it.
This should test your theory. 

Yes, another write_protected=True dataset causes the same error.

@Mugless Durry 
Since it is a learning course, I suppose the solution would be to just create the data set as write_protected=False in this case. 
 

Thanks Andrian