[Sqlalchemy-tickets] Issue #4256: SQLAlchemy 1.2.7 breaks dogpile caching example (zzzeek/sqlalchemy)

[Frazer M. <iss...@bi...>]

New issue 4256: SQLAlchemy 1.2.7 breaks dogpile caching example
https://bitbucket.org/zzzeek/sqlalchemy/issues/4256/sqlalchemy-127-breaks-dogpile-caching

Frazer McLean:

To reproduce:

```
python3 -m examples.dogpile_caching.advanced
```

```
Traceback (most recent call last):
  File "/Users/frazer/.pyenv/versions/3.6.5/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/Users/frazer/.pyenv/versions/3.6.5/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/Users/frazer/projects/personal/sqlalchemy/examples/dogpile_caching/advanced.py", line 68, in <module>
    print(p.format_full())
  File "/Users/frazer/projects/personal/sqlalchemy/examples/dogpile_caching/model.py", line 95, in format_full
    return "\t".join([str(x) for x in [self] + list(self.addresses)])
  File "/Users/frazer/projects/personal/sqlalchemy/examples/dogpile_caching/model.py", line 95, in <listcomp>
    return "\t".join([str(x) for x in [self] + list(self.addresses)])
  File "/Users/frazer/projects/personal/sqlalchemy/examples/dogpile_caching/model.py", line 74, in __str__
    "%s" % (self.street, self.city.name,
  File "/Users/frazer/projects/personal/sqlalchemy/examples/dogpile_caching/model.py", line 65, in city
    return self.postal_code.city
  File "/Users/frazer/.pyenv/versions/3.6.5/lib/python3.6/site-packages/sqlalchemy/orm/attributes.py", line 242, in __get__
    return self.impl.get(instance_state(instance), dict_)
  File "/Users/frazer/.pyenv/versions/3.6.5/lib/python3.6/site-packages/sqlalchemy/orm/attributes.py", line 599, in get
    value = self.callable_(state, passive)
  File "/Users/frazer/.pyenv/versions/3.6.5/lib/python3.6/site-packages/sqlalchemy/orm/strategies.py", line 619, in _load_for_state
    instance = session._query_cls._identity_lookup(
AttributeError: 'function' object has no attribute '_identity_lookup'
```

See [this commit](https://bitbucket.org/zzzeek/sqlalchemy/commits/43f278356d94b5342a1020a9a97feea0bb7cd88f#Llib/sqlalchemy/orm/strategies.pyT619)

As of 1.2.7 `session._query_cls` is expected to be a class, but previously the function returned by `query_callable` in the dogpile caching example worked.