It seems quite unpythonic to force users to write such constructs when dealing with the oh-so-useful conditional operations in DynamoDM:

try:
    thread_item.save(Thread.forum_name.exists())
except PutError as e:
    if isinstance(e.cause, ClientError):
        code = e.cause.response['Error'].get('Code')
        print(code == "ConditionalCheckFailedException")

PynamoDB already issues it own exceptions - why not perform this if and raise a ConditionalCheckFailed? why perpetuate the bad API for boto3 (in this regard) into this otherwise very pythonic library?

Make this work:

try:
    thread_item.save(Thread.forum_name.exists())
except ConditionalCheckFailed:
    pass