Execution order of model_validators reversed from root_validators in v1

[sarahandromeda]

Hi all!! So we've been using pydantic at work long enough that we had to migrate over to v2 after writing all our models in v1.

We used root_validator in v1 so we swapped those out for model_validator, which we set to mode="before" as our code expected the supplied argument to be a dict. We have the validators defined in order of execution, as stated in v1. However, after the migration, the validators actually run in reverse order, with the last one defined being executed first. The execution order can be remedied by using model_validator(mode="after") but that would require some refactoring to use the model instance instead. The issue can also be remedied by reversing the order the validators are defined in, eg. defining first_validator last.

My main question, is this intended behavior?

In pydantic v1, using root_validator(pre=True) DOES execute the validators in order of their definition and that fact is stated in the docs. It seems interesting that the execution order would be totally reversed in v2 without explicitly stating that somewhere in the changelog or migration guide.

Here's a little test script that demonstrates what I described (there's one for each version, v1 and v2, so you only have to worry about running it against the right version):

Run this with pydantic 1.10.12 v1:

import typing as t
from pydantic import BaseModel, root_validator

class BasicModel(BaseModel):
    some_val_1: t.Dict = {}
    some_val_2: t.Dict = {}
    some_val_3: t.Dict = {}

    @root_validator(pre=True, skip_on_failure=True)
    @classmethod
    def validator_1(cls, values):
        print("I should run 1st")
        return values

    @root_validator(pre=True, skip_on_failure=True)
    @classmethod
    def validator_2(cls, values):
        print("I should run 2nd")
        return values

    @root_validator(pre=True, skip_on_failure=True)
    @classmethod
    def validator_3(cls, values):
        print("I should run 3rd")
        return values

BasicModel()

Run this with pydantic 2.0.3 v2:

import typing as t
from pydantic import BaseModel, model_validator

class BasicModel(BaseModel):
    some_val_1: t.Dict = {}
    some_val_2: t.Dict = {}
    some_val_3: t.Dict = {}

    @model_validator(mode="before")
    @classmethod
    def validator_1(cls, values):
        print("I should run 1st")
        return values

    @model_validator(mode="before")
    @classmethod
    def validator_2(cls, values):
        print("I should run 2nd")
        return values

    @model_validator(mode="before")
    @classmethod
    def validator_3(cls, values):
        print("I should run 3rd")
        return values

BasicModel()

[timrandg]

This issue is still occurring in v2.6.4. Runs in declared order if mode='after', but in reverse order if mode='before'.