Python update object from dictionary

Question

Is there a built-in function/operator I could use to unpack values from a dictionary and assign it into instance variables?

This is what I intend to do:

c = MyClass()
c.foo = 123
c.bar = 123

# c.foo == 123 and c.bar == 123


d = {'bar': 456}
c.update(d)

# c.foo == 123 and c.bar == 456

Something akin to dictionary update() which load values from another dictionary but for plain object/class instance?

score 51 · Accepted Answer · edited Apr 03 '19 at 17:45

51

there is also another way of doing it by looping through the items in d. this doesn't have the same assuption that they will get stored in c.__dict__ which isn't always true.

d = {'bar': 456}
for key,value in d.items():
    setattr(c,key,value)

or you could write a update method as part of MyClass so that c.update(d) works like you expected it to.

def update(self,newdata):
    for key,value in newdata.items():
        setattr(self,key,value)

check out the help for setattr

setattr(...)
    setattr(object, name, value)
    Set a named attribute on an object; setattr(x, 'y', v) is equivalent to
    ''x.y = v''.

edited Apr 03 '19 at 17:45

ViaTech

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answered Jan 02 '09 at 21:02

Jehiah

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I already know about setattr... Can you explain more on why some items would not be in __dict__ ? I actually ended up still using this kind of loop+setattr because __dict__ somehow wasn't available. – chakrit Jan 03 '09 at 07:39
7

sure, the main difference is between class attributes and instance attributes. Only the latter are in `__dict__`. so if you have class Test: a = 1 t = Test() the object `t` has an empty `__dict__` because a is a class attribute t.b = 2 now it has a `__dict__` of `{'b':2}` – Jehiah Jan 07 '09 at 22:51
and where are class attributes stored? in `Test.__dict__` I suppose? (it would be ok) – n611x007 Apr 26 '13 at 15:07

score 36 · Answer 2 · answered Jan 01 '09 at 21:14

36

Have you tried

f.__dict__.update( b )

?

answered Jan 01 '09 at 21:14

S.Lott

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http://www.python.org/doc/2.5.2/ref/types.html or http://docs.python.org/reference/datamodel.html#the-standard-type-hierarchy. The section titled "classes" is pretty explicit on precisely how this works. – S.Lott Jan 01 '09 at 21:23
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This will silently misbehave when a class attribute isn't stored in the instance's `__dict__`. E.g., properties and class attributes. In general, Jehiah's answer is safer. – Jon-Eric Feb 09 '12 at 16:19
1

setattr is the most preferable way to go here. Using internal object.__dict__ is not recommended. – qubird Nov 24 '17 at 14:12
this misbehaved for me exactly as @Jon-Eric described – riemann_lebesgue Jun 27 '22 at 18:12

score 9 · Answer 3 · edited May 23 '17 at 12:18

9

Also, maybe it would be good style to have a wrapper around the dict's update method:

def update(self, b):
    self.__dict__.update(b)

PS: Sorry for not commenting at @S.Lott 's post but I don't have the rep yet.

edited May 23 '17 at 12:18

Community

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answered Jan 01 '09 at 21:21

hyperboreean

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Duplicate of S.Lott's answer. – pppery Jun 03 '16 at 12:53

score 1 · Answer 4 · answered Mar 28 '19 at 07:45

You can try doing:

def to_object(my_object, my_dict):
    for key, value in my_dict.items():
        attr = getattr(my_object, key)
        if hasattr(attr, '__dict__'):
            to_object(attr, value)
        else:
            setattr(my_object, key, value)

obj = MyObject()
data = {'a': 1, 'b': 2}
to_object(obj, data)

Python update object from dictionary

4 Answers4