[issue38444] dataclass: always generate default __init__ on __default_init__

[Shmuel H.]

New submission from Shmuel H. <shmuelhazan0 at gmail.com>:

Currently, `dataclasses.dataclass` will generate `__init__` only where the user has not defined one. 

However, sometimes, with frozen classes or dataclasses with a lot of members, redefinition of this function is not trivial,
especially if the only purpose is to change the default behaviour for only one member:
```python
from dataclasses import dataclass

@dataclass(frozen=True)
class Dataclass:
    #...big list of members
    member20: int
    
    def __init__(self, member20: str, **kwargs):
        # self.member20 = int(member20)
        object.__setattr__(self, "member20", int(member20))
        # Now we have to trivially initialize 
        # 20 other members like that :[
```
My idea is to generate the default `__init__` into `__default_init__` even, if the user has defined their own version.
That will allow them to use it like that:
 ```python
from dataclasses import dataclass

@dataclass(frozen=True)
class Dataclass:
    #...big list of members
    member20: int
    
    def __init__(self, member20: str, **kwargs):
        # Oh, that's better :)
        self.__default_init__(member20=int(member20), **kwargs)
```

Implementing that is pretty trivial (I can do that if this change will be approved). 
Please let me know what you think about that.

----------
components: Library (Lib)
messages: 354437
nosy: Shmuel H.
priority: normal
severity: normal
status: open
title: dataclass: always generate default __init__ on __default_init__
type: enhancement
versions: Python 3.7, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38444>
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