pythonpython-2.7recursionattributessetattr

getattr and setattr on nested subobjects / chained properties?


I have an object (Person) that has multiple subobjects (Pet, Residence) as properties. I want to be able to dynamically set the properties of these subobjects like so:

class Person(object):
    def __init__(self):
        self.pet = Pet()
        self.residence = Residence()

class Pet(object):
    def __init__(self,name='Fido',species='Dog'):
        self.name = name
        self.species = species

class Residence(object):
    def __init__(self,type='House',sqft=None):
        self.type = type
        self.sqft=sqft


if __name__=='__main__':
    p=Person()
    setattr(p,'pet.name','Sparky')
    setattr(p,'residence.type','Apartment')
    print p.__dict__

Currently I get the wrong output: {'pet': <__main__.Pet object at 0x10c5ec050>, 'residence': <__main__.Residence object at 0x10c5ec0d0>, 'pet.name': 'Sparky', 'residence.type': 'Apartment'}

As you can see, instead of setting the name attribute on the Pet subobject of the Person, a new attribute pet.name is created on the Person.


Solution

  • You could use functools.reduce:

    import functools
    
    def rsetattr(obj, attr, val):
        pre, _, post = attr.rpartition('.')
        return setattr(rgetattr(obj, pre) if pre else obj, post, val)
    
    # using wonder's beautiful simplification: https://stackoverflow.com/questions/31174295/getattr-and-setattr-on-nested-objects/31174427?noredirect=1#comment86638618_31174427
    
    def rgetattr(obj, attr, *args):
        def _getattr(obj, attr):
            return getattr(obj, attr, *args)
        return functools.reduce(_getattr, [obj] + attr.split('.'))
    

    rgetattr and rsetattr are drop-in replacements for getattr and setattr, which can also handle dotted attr strings.


    import functools
    
    class Person(object):
        def __init__(self):
            self.pet = Pet()
            self.residence = Residence()
    
    class Pet(object):
        def __init__(self,name='Fido',species='Dog'):
            self.name = name
            self.species = species
    
    class Residence(object):
        def __init__(self,type='House',sqft=None):
            self.type = type
            self.sqft=sqft
    
    def rsetattr(obj, attr, val):
        pre, _, post = attr.rpartition('.')
        return setattr(rgetattr(obj, pre) if pre else obj, post, val)
    
    def rgetattr(obj, attr, *args):
        def _getattr(obj, attr):
            return getattr(obj, attr, *args)
        return functools.reduce(_getattr, [obj] + attr.split('.'))
    

    if __name__=='__main__':
        p = Person()
        print(rgetattr(p, 'pet.favorite.color', 'calico'))
        # 'calico'
    
        try:
            # Without a default argument, `rgetattr`, like `getattr`, raises
            # AttributeError when the dotted attribute is missing
            print(rgetattr(p, 'pet.favorite.color'))
        except AttributeError as err:
            print(err)
            # 'Pet' object has no attribute 'favorite'
    
        rsetattr(p, 'pet.name', 'Sparky')
        rsetattr(p, 'residence.type', 'Apartment')
        print(p.__dict__)
        print(p.pet.name)
        # Sparky
        print(p.residence.type)
        # Apartment