Validator of multiple fields does not behave consistently standalone and in another list

[Jakku Sakura]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.8.2
            pydantic compiled: True
                 install path: /usr/local/lib/python3.9/site-packages/pydantic
               python version: 3.9.1 (default, Jan  8 2021, 17:17:43)  [Clang 12.0.0 (clang-1200.0.32.28)]
                     platform: macOS-12.1-x86_64-i386-64bit
     optional deps. installed: ['typing-extensions']
...

from pydantic import *
from typing import List
class Foo(BaseModel):
    a: str
    b: str

    @validator("a", "b")
    @classmethod
    def validate(cls, x) -> str:
        assert isinstance(x, str)
        return x

class Bar(BaseModel):
    foos: List[Foo]

Foo(a="a", b="b")
Foo.parse_obj({
    'a': 'a',
    'b': 'b'

})
Bar.parse_obj({
    'foos': [{
        'a': 'a',
        'b': 'b'
    }]
})

python3 test.py
Traceback (most recent call last):
  File "/Users/jack/Dev/test_python/test.py", line 22, in <module>
    Bar.parse_obj({
  File "pydantic/main.py", line 578, in pydantic.main.BaseModel.parse_obj
  File "pydantic/main.py", line 406, in pydantic.main.BaseModel.__init__
pydantic.error_wrappers.ValidationError: 1 validation error for Bar
foos -> 0
   (type=assertion_error)

[Adrian Garcia Badaracco]

I get the following on both v1.10 and v2:

from typing import List

from pydantic import BaseModel, validator


class Foo(BaseModel):
    a: str
    b: str

    @validator("a", "b")
    @classmethod
    def val_a_b(cls, x: str) -> str:
        assert isinstance(x, str)
        return x


class Bar(BaseModel):
    foos: List[Foo]


print(Foo(a="a", b="b"))
"""
a='a' b='b'
"""
print(Foo.parse_obj({'a': 'a', 'b': 'b'}))
"""
a='a' b='b'
"""
print(Bar.parse_obj({'foos': [{'a': 'a', 'b': 'b'}]}))
"""
foos=[Foo(a='a', b='b')]
"""

So it seems like this is fixed.