AttributeError: 'UUID' object has no attribute 'replace' when using backend-agnostic GUID type

This should fix it:

id = Column(GUID(as_uuid=True), ...)

from https://bitbucket.org/zzzeek/sqlalchemy/issues/3323/in-099-uuid-columns-are-broken-with:

"If you want to pass a UUID() object, the as_uuid flag must be set to True."

The pg8000 PostgreSQL database adapter is returning a uuid.UUID() object (see their type mapping documentation, and SQLAlchemy has passed that to the TypeDecorator.process_result_value() method.

The implementation given in the documentation expected a string, however, so this fails:

>>> import uuid
>>> value = uuid.uuid4()
>>> uuid.UUID(value)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mjpieters/Development/Library/buildout.python/parts/opt/lib/python2.7/uuid.py", line 133, in __init__
    hex = hex.replace('urn:', '').replace('uuid:', '')
AttributeError: 'UUID' object has no attribute 'replace'

The quick work-around is to force the value to be a string anyway:

def process_result_value(self, value, dialect):
    if value is None:
        return value
    else:
        return uuid.UUID(str(value))

or you can test for the type first:

def process_result_value(self, value, dialect):
    if value is None:
        return value
    else:
        if not isinstance(value, uuid.UUID):
            value = uuid.UUID(value)
        return value

I've submited pull request #403 to fix this in the documentation (since merged).