Proper autogenerate of __str__() implementation also for sqlalchemy classes?

Question

I would like to display / print my sqlalchemy classes nice and clean.

In Is there a way to auto generate a __str__() implementation in python? the answer You can it

Answer 1

This is what I use:

def todict(obj):
    """ Return the object's dict excluding private attributes, 
    sqlalchemy state and relationship attributes.
    """
    excl = ('_sa_adapter', '_sa_instance_state')
    return {k: v for k, v in vars(obj).items() if not k.startswith('_') and
            not any(hasattr(v, a) for a in excl)}

class Base:

    def __repr__(self):
        params = ', '.join(f'{k}={v}' for k, v in todict(self).items())
        return f"{self.__class__.__name__}({params})"

Base = declarative_base(cls=Base)

Any models that inherit from Base will have the default __repr__() method defined and if I need to do something different I can just override the method on that particular class.

It excludes the value of any private attributes denoted with a leading underscore, the SQLAlchemy instance state object, and any relationship attributes from the string. I exclude the relationship attributes as I most often don't want the repr to cause a relationship to lazy load, and where the relationship is bi-directional, including relationship attribs can cause infinite recursion.

The result looks like: ClassName(attr=val, ...).

--EDIT--

The todict() func that I mention above is a helper that I often call upon to construct a dict out of a SQLA object, mostly for serialisation. I was lazily using it in this context but it isn't very efficient as it's constructing a dict (in todict()) to construct a dict (in __repr__()). I've since modified the pattern to call upon a generator:

def keyvalgen(obj):
    """ Generate attr name/val pairs, filtering out SQLA attrs."""
    excl = ('_sa_adapter', '_sa_instance_state')
    for k, v in vars(obj).items():
        if not k.startswith('_') and not any(hasattr(v, a) for a in excl):
            yield k, v

Then the base Base looks like this:

class Base:

    def __repr__(self):
        params = ', '.join(f'{k}={v}' for k, v in keyvalgen(self))
        return f"{self.__class__.__name__}({params})"

The todict() func leverages off of the keyvalgen() generator as well but isn't needed to construct the repr anymore.