pydantic: Partial update of nested model via dump -> copy -> parse doesn't work on v2

Initial Checks

• I confirm that I’m using Pydantic V2

Description

With v1, doing a partial update of a model instance from another instance could be done via

data = input_model.dict(exclude_unset=True)
updated_model = model.copy(update=data)

(this is even a recommended way in FastAPI docs)

There’s a caveat though. If those models had nested attributes/values (i.e. sub models), the resulting updated_model will have a dict as a value instead of the sub model instance.

To fix/workaround this problem in v1 one could use this trick:

updated_model = updated_model.parse_obj(updated_model)

In v2 this trick no longer works.

Is there any way to parse/validate nested values/objects like this?

Example Code

from pydantic import BaseModel


class SubModel(BaseModel):
    foo: int


class Model(BaseModel):
    bar: str
    sub: SubModel


model = Model(bar="bar", sub={"foo": 0})

# check that foo on sub is 0
assert model.sub.foo == 0


class InputModel(BaseModel):
    sub: SubModel


input_ = InputModel(sub={"foo": 1})

input_data = input_.dict(exclude_unset=True)
updated_m = model.copy(update=input_data)

# now check that foo on sub is 1
# this will raise AttributeError
try:
    assert updated_m.sub.foo == 1
except AttributeError as ex:
    assert str(ex) == "'dict' object has no attribute 'foo'"
else:
    assert False, "were expecting AttributeError"

# trick in v1 to fix this
updated_m = updated_m.parse_obj(updated_m)
assert updated_m.sub.foo == 1

With v2, the above code still generates:

AttributeError: 'dict' object has no attribute 'foo'

This is even the case if we get rid of the deprecation warnings:

--- /tmp/model_update.py	2023-09-10 13:16:14
+++ /tmp/model_update_v2.py	2023-09-10 13:16:24
@@ -22,8 +22,8 @@
 
 input_ = InputModel(sub={"foo": 1})
 
-input_data = input_.dict(exclude_unset=True)
-updated_m = model.copy(update=input_data)
+input_data = input_.model_dump(exclude_unset=True)
+updated_m = model.model_copy(update=input_data)
 
 # now check that foo on sub is 1
 # this will raise AttributeError
@@ -35,5 +35,5 @@
     assert False, "were expecting AttributeError"
 
 # trick in v1 to fix this
-updated_m = updated_m.parse_obj(updated_m)
+updated_m = updated_m.model_validate(updated_m)
 assert updated_m.sub.foo == 1

Interestingly, if the input_data had invalid data (e.g. "x" instead of 1), calling model_validate would still pass.

Python, Pydantic & OS Version

             pydantic version: 2.3.0
        pydantic-core version: 2.6.3
          pydantic-core build: profile=release pgo=false
                 install path: /Users/slafs/.pyenv/versions/3.10.9/envs/tapper-core-pydantic-v2-py310/lib/python3.10/site-packages/pydantic
               python version: 3.10.9 (main, Dec 21 2022, 11:47:15) [Clang 12.0.5 (clang-1205.0.22.9)]
                     platform: macOS-13.5.2-x86_64-i386-64bit
     optional deps. installed: ['email-validator', 'typing-extensions']