dataclass and field alias

[CedricCabessa]

Hi,

Bug Report

I'm trying to write a simple wrapper around dataclass.field

To Reproduce

Here is a simple snippet that work perfectly under mypy

from dataclasses import field, dataclass

@dataclass
class C:
    some_int: int
    some_str: str = field(metadata={"doc": "foo"})
    another_int: int

c = C(42, "bla", 43)

Now in order to make doc creation easy, I want to write an helper like this

def doc(documentation: str):
    return field(metadata={"doc": documentation})

So, my snippet can be written:

@dataclass
class C:
    some_int: int
    some_str: str = doc("foo")
    another_int: int

Expected Behavior

mypy return no error like in the first snippet

Actual Behavior

error: Attributes without a default cannot follow attributes with one

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: python3.8
• Operating system and version: ubuntu 20.04

Related Info

I believe those bugs are kind of related.

#8853
#5383

There is a few users that need to alias freely assign / alias dataclass method and class.

[gvanrossum]

Fixing this will require some changes to the plugin that's used to deal with dataclass semantics. I'm not sure it is easy -- there would have to be a way to mark specific functions (like your doc()) as having similar semantics as field(). But you're free to look over the source code (start at mypy/plugins/dataclasses.py).

[hauntsaninja]

Closing, see #5383 (comment)