getattr and setattr on nested subobjects / chained properties?

Question

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.

• I cannot specify person.pet to setattr() because different sub-objects will be set by the same method, which parses some text and fills in the object attributes if/when a relevant key is found.

• Is there a easy/builtin way to accomplish this?

• Or perhaps I need to write a recursive function to parse the string and call getattr() multiple times until the necessary subobject is found and then call setattr() on that found subobject?

Answer 1

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

Answer 2

For an out of the box solution, you can use operator.attrgetter:

from operator import attrgetter
attrgetter(dotted_path)(obj)

Answer 3

For one parent and one child:

if __name__=='__main__':
    p = Person()

    parent, child = 'pet.name'.split('.')
    setattr(getattr(p, parent), child, 'Sparky')

    parent, child = 'residence.type'.split('.')
    setattr(getattr(p, parent), child, 'Sparky')

    print p.__dict__

This is simpler than the other answers for this particular use case.

Answer 4

unutbu's answer (https://stackoverflow.com/a/31174427/2683842) has a "bug". After getattr() fails and is replaced by default, it continues calling getattr on default.

Example: rgetattr(object(), "nothing.imag", 1) should equal 1 in my opinion, but it returns 0:

• getattr(object(), 'nothing', 1) == 1.
• getattr(1, 'imag', 1) == 0 (since 1 is real and has no complex component).

Solution

I modified rgetattr to return default at the first missing attribute:

import functools

DELIMITER = "."

def rgetattr(obj, path: str, *default):
    """
    :param obj: Object
    :param path: 'attr1.attr2.etc'
    :param default: Optional default value, at any point in the path
    :return: obj.attr1.attr2.etc
    """

    attrs = path.split(DELIMITER)
    try:
        return functools.reduce(getattr, attrs, obj)
    except AttributeError:
        if default:
            return default[0]
        raise

Answer 5

This should be a

def getNestedAttr(obj,nestedParam):
     next = obj
     for p in nestedParam.split('.'):
         next = getattr(next,p)
     return next


class Issue : pass    
issue = Issue()
issue.status = Issue()
issue.status.name = "Hello"
getattr(issue,'status.name')
'''
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Issue' object has no attribute 'status.name'
'''
getNestedAttr(issue,'status.name')

#'Hello'

simple solution

Answer 6

I made a simple version based on ubntu's answer called magicattr that also works on attrs, lists, and dicts by parsing and walking the ast.

For example, with this class:

class Person:
    settings = {
        'autosave': True,
        'style': {
            'height': 30,
            'width': 200
        },
        'themes': ['light', 'dark']
    }
    def __init__(self, name, age, friends):
        self.name = name
        self.age = age
        self.friends = friends


bob = Person(name="Bob", age=31, friends=[])
jill = Person(name="Jill", age=29, friends=[bob])
jack = Person(name="Jack", age=28, friends=[bob, jill])

You can do this

# Nothing new
assert magicattr.get(bob, 'age') == 31

# Lists
assert magicattr.get(jill, 'friends[0].name') == 'Bob'
assert magicattr.get(jack, 'friends[-1].age') == 29

# Dict lookups
assert magicattr.get(jack, 'settings["style"]["width"]') == 200

# Combination of lookups
assert magicattr.get(jack, 'settings["themes"][-2]') == 'light'
assert magicattr.get(jack, 'friends[-1].settings["themes"][1]') == 'dark'

# Setattr
magicattr.set(bob, 'settings["style"]["width"]', 400)
assert magicattr.get(bob, 'settings["style"]["width"]') == 400

# Nested objects
magicattr.set(bob, 'friends', [jack, jill])
assert magicattr.get(jack, 'friends[0].friends[0]') == jack

magicattr.set(jill, 'friends[0].age', 32)
assert bob.age == 32

It also won't let you/someone call functions or assign a value since it doesn't use eval or allow Assign/Call nodes.

with pytest.raises(ValueError) as e:
    magicattr.get(bob, 'friends = [1,1]')

# Nice try, function calls are not allowed
with pytest.raises(ValueError):
    magicattr.get(bob, 'friends.pop(0)')

Answer 7

And a easy to understand three-liner based on jimbo1qaz's answer, reduced to the very limit:

def rgetattr(obj, path, default):
    try:
        return functools.reduce(getattr, path.split(), obj)
    except AttributeError:
        return default

Usage:

>>> class O(object):
...     pass
... o = O()
... o.first = O()
... o.first.second = O()
... o.first.second.third = 42
... rgetattr(o, 'first second third', None)
42

Just keep in mind that "space" is not a typical delimiter for this use case.

Answer 8

Thanks for the accepted answer above. It was helpful. In case anyone wants to extend the use for hasattr use the code below:

def rhasattr(obj, attr):
    _nested_attrs = attr.split(".")
    _curr_obj = obj
    for _a in _nested_attrs[:-1]:
        if hasattr(_curr_obj, _a):
            _curr_obj = getattr(_curr_obj, _a)
        else:
            return False
    return hasattr(_curr_obj, _nested_attrs[-1])

Answer 9

Ok so while typing the question I had an idea of how to do this and it seems to work fine. Here is what I came up with:

def set_attribute(obj, path_string, new_value):
    parts = path_string.split('.')
    final_attribute_index = len(parts)-1
    current_attribute = obj
    i = 0
    for part in parts:
        new_attr = getattr(current_attribute, part, None)
        if current_attribute is None:
            print 'Error %s not found in %s' % (part, current_attribute)
            break
        if i == final_attribute_index:
            setattr(current_attribute, part, new_value)
        current_attribute = new_attr
        i+=1


def get_attribute(obj, path_string):
    parts = path_string.split('.')
    final_attribute_index = len(parts)-1
    current_attribute = obj
    i = 0
    for part in parts:
        new_attr = getattr(current_attribute, part, None)
        if current_attribute is None:
            print 'Error %s not found in %s' % (part, current_attribute)
            return None
        if i == final_attribute_index:
            return getattr(current_attribute, part)
        current_attribute = new_attr
        i += 1

I guess this solves my question, but I am still curious if there is a better way to do this?

I feel like this has to be something pretty common in OOP and python, so I'm surprised gatattr and setattr do not support this natively.

Answer 10

Here's something similar to ChaimG's answer, but it works with an arbitrary number of cases. However, it only supports get attributes, not setting them.

requested_attr = 'pet.name'
parent = Person()

sub_names = requested_attr.split('.')
sub = None

for sub_name in sub_names:

    try:
        sub = parent.__getattribute__(sub_name)
        parent = sub

    except AttributeError:
        raise Exception("The panel doesn't have an attribute that matches your request!")

pets_name = sub

Answer 11

I just love recursive functions

def rgetattr(obj,attr):
    _this_func = rgetattr
    sp = attr.split('.',1)
    if len(sp)==1:
        l,r = sp[0],''
    else:
        l,r = sp

    obj = getattr(obj,l)
    if r:
        obj = _this_func(obj,r)
    return obj

Answer 12

I know this post is pretty old but below code might help some one.

    def getNestedObjectValue(obj={}, attr=""):
    splittedFields = attr.split(".")
    nestedValue = ""
    previousValue = ""
    for field in splittedFields:
        previousValue = nestedValue
        nestedValue = (
            obj.get(field) if previousValue == "" else previousValue.get(field)
        )
    return nestedValue

    print(
    getNestedObjectValue(
        obj={
            "name": "ADASDASD",
            "properties": {"somefield": {"value": "zxczxcxczxcxzc"}},
        },
        attr="properties.somefield.value",
    )
)

Output

PS C:\myprograms\samples> python .\sample.py

zxczxcxczxcxzc