defaultdict converted to dict after __post_init__

[rshin]

Bug

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

             pydantic version: 1.5.1
            pydantic compiled: True
                 install path: <redacted>/lib/python3.7/site-packages/pydantic
               python version: 3.7.3 | packaged by conda-forge | (default, Dec  6 2019, 08:36:57)  [Clang 9.0.0 (tags/RELEASE_900/final)]
                     platform: Darwin-19.4.0-x86_64-i386-64bit
     optional deps. installed: ['typing-extensions']

If I run the following snippet:

import collections
import dataclasses
from typing import DefaultDict

from pydantic.dataclasses import dataclass


@dataclass
class X:
    f: DefaultDict[int, int] = dataclasses.field(init=False)

    def __post_init__(self):
        self.f = collections.defaultdict(int)
        self.f[2] += 1
        print(f"__post_init__: {self.f}")


x = X()
print(type(x.f))
print(x.f)

It prints the following:

__post_init__: defaultdict(<class 'int'>, {2: 1})
<class 'dict'>
{2: 1}

In other words, the defaultdict field is converted into a dict (while preserving the keys), even though the field is annotated as DefaultDict, which is surprising. Is this intended behavior?

[PrettyWood]

Hello @rshin,
You are right it comes from the default mapping validator, which always returns a dict.
@samuelcolvin A potential fix could be something like this. maybe with a try...except
Are you willing to consider it in which case I open a PR?

[samuelcolvin]

Surely better to implement a validator for DefaultDict, thus avoiding any performance impact on normal dictionaries?