I have a proposal for an addition to the dataclasses module that I think would 
make it
easier to use private and name mangled variables. One of the benefits of the 
dataclass
decorator is that it helps lessen the amount of code when you just need a simple
constructor. A common pattern in python that isn't addressed by the current
implementation is the pattern

```python
class MyClass:

    def __init__(self, a: str, b: str):
        self._a = a
        self.__b = b
```

right now the only straightforward way of doing this with a dataclass is

```python
@dataclass
class MyClass:
    a: InitVar[str]
    b: InitVar[str]

    def __post_init__(self, a, b):
        self._a = a
        self.__b = b
```

My proposal would be to add two types to the dataclasses module, one called 
PrivateVar
 and one called MangledVar. These would add The above pattern (without the 
post_init
 ) using the syntax,

```python
@dataclass
class MyClass:
    a: PrivateVar[str]
    b: MangledVar[str]
```

I've already looked into what would need to be added to dataclasses to implement
 these and it's just a few lines of code since the class implementation would 
be very
  similar to InitVar. Is this something that other see the possibility of being 
added
   to python? (Also, I know name mangling isn't super common but I think it
 may as well be added alongside a private variable implementation))
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