self-referencing/forward-references dataclasses are not supported

[baod-rate]

Mashumaro does not currently support self-referencing classes, the code-generation fails when it attempts to reflect on the field's type (which is a forward reference)

For example, consider the following case:

import dataclasses
from typing import Optional

@dataclasses.dataclass
class Node:
    value: str
    next: Optional['Node'] = None

@dataclasses.dataclass
class LinkedList:
    head: Optional[Node] = None

a = Node("A")
b = Node("B")
c = Node("C")

a.next = b
b.next = c

linked_list = LinkedList(head=a)

print("list", dataclasses.asdict(linked_list))
print("A", dataclasses.asdict(a))
print("B", dataclasses.asdict(b))
print("C", dataclasses.asdict(c))

This gives us the expected:

list {'head': {'value': 'A', 'next': {'value': 'B', 'next': {'value': 'C', 'next': None}}}}
A {'value': 'A', 'next': {'value': 'B', 'next': {'value': 'C', 'next': None}}}
B {'value': 'B', 'next': {'value': 'C', 'next': None}}
C {'value': 'C', 'next': None}

The equivalent classes using mashumaro:

import dataclasses
from typing import Optional

from mashumaro import DataClassJSONMixin

@dataclasses.dataclass
class Node(DataClassJSONMixin):
    value: str
    next: Optional['Node'] = None

@dataclasses.dataclass
class LinkedList(DataClassJSONMixin):
    head: Optional[Node] = None

a = Node("A")
b = Node("B")
c = Node("C")

a.next = b
b.next = c

linked_list = LinkedList(head=a)

print("list", linked_list.to_dict())
print("A", a.to_dict())
print("B", b.to_dict())
print("C", c.to_dict())

throws an error during Mashumaro's code generation:

Traceback (most recent call last):
  File "mashumaro_test.py", line 7, in <module>
    class Node(DataClassJSONMixin):
  File "mashumaro/serializer/base/dict.py", line 19, in __init_subclass__
    raise exc
  File "mashumaro/serializer/base/dict.py", line 15, in __init_subclass__
    builder.add_to_dict()
  File "mashumaro/serializer/base/metaprogramming.py", line 201, in add_to_dict
    for fname, ftype in self.fields.items():
  File "mashumaro/serializer/base/metaprogramming.py", line 78, in fields
    return self.__get_fields()
  File "mashumaro/serializer/base/metaprogramming.py", line 69, in __get_fields
    for fname, ftype in typing.get_type_hints(self.cls).items():
  File "/usr/lib/python3.9/typing.py", line 1410, in get_type_hints
    value = _eval_type(value, base_globals, localns)
  File "/usr/lib/python3.9/typing.py", line 279, in _eval_type
    ev_args = tuple(_eval_type(a, globalns, localns, recursive_guard) for a in t.__args__)
  File "/usr/lib/python3.9/typing.py", line 279, in <genexpr>
    ev_args = tuple(_eval_type(a, globalns, localns, recursive_guard) for a in t.__args__)
  File "/usr/lib/python3.9/typing.py", line 277, in _eval_type
    return t._evaluate(globalns, localns, recursive_guard)
  File "/usr/lib/python3.9/typing.py", line 533, in _evaluate
    eval(self.__forward_code__, globalns, localns),
  File "<string>", line 1, in <module>
NameError: name 'Node' is not defined

[baod-rate]

FYI, the current workaround we are using is to declare the class once first, and then redefine it. This allows mashumaro to have a stub for the class being defined during code generation:

@dataclasses.dataclass
class Node(DataClassJSONMixin):
    pass

@dataclasses.dataclass
class Node(DataClassJSONMixin):
    value: str
    next: Optional['Node'] = None

This hasn't introduced any problems in our use-cases, other than the minor issue of confusing some code analysis tools.

[Fatal1ty]

Seems like another popular library has the same issue pydantic/pydantic#2678 👀

[Fatal1ty]

@qubidt Hi. I've fixed forward references in this branch, which you can play around with for now. There are a few things I want to experiment with before I release a new version.

[Fatal1ty]

Fixed in 2.3.