[Sqlalchemy-tickets] Issue #3848: returning None (or anything) from @declared_attr means it won't be called further, though this usage may be intuitive on AbstractConcreteBase (zzzeek/sqlalchemy)

[Michael B. <iss...@bi...>]

New issue 3848: returning None (or anything) from @declared_attr means it won't be called further, though this usage may be intuitive on AbstractConcreteBase
https://bitbucket.org/zzzeek/sqlalchemy/issues/3848/returning-none-or-anything-from

Michael Bayer:

this is a conceptual one that might not be possible to fix.  With AbstractConcreteBase, we're both "abstract" and "mapped", so the scheme below is intuitive.  But because it returns None, the logic in _extract_mappable_attributes ("# using @declared_attr for some object that isn't Column/MapperProperty; remove from the dict_ and place the evaluated value onto the class.") blows it away.   

need to re-identify the rationale for the "remove the attribute" logic, determine if the "might return None" style is likely only with AbstractConcreteBase or in a more general sense, then identify a potential warning, or even behavioral change, to make this more intuitive.   A special symbol like "don't map" might serve as a more explicit replacement for None.


```
#!python

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.declarative import declared_attr
from sqlalchemy.ext.declarative import AbstractConcreteBase

Base = declarative_base()


class FooBase(AbstractConcreteBase, Base):
    @declared_attr
    def foo(cls):
        print "FOO! ", cls.__name__
        if cls.__name__ != 'A':
            #return Column(String)  # gets discarded, but affects A
            return None # causes attr to not be called again
        else:
            return Column(Integer)

class A(FooBase):
    __tablename__ = 'a'
    id = Column(Integer, primary_key=True)



e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)


```

output:


```
#!python

CREATE TABLE a (
	id INTEGER NOT NULL, 
	PRIMARY KEY (id)
)

```

1.1 for the moment but this might be a more long term thing.