dataclasses astuple and asdict crash on recursive dataclass structures #94345

zzzeek · 2022-06-27T18:17:18Z

I don't see this mentioned anywhere and it seems a bit unusual, we can't use asdict() or astuple() with dataclasses that have cycles to each other. This is something that is handled by most other stdlib features such as deepcopy, dataclasses stringify, etc.

Example below

from __future__ import annotations

import dataclasses

# dataclasses support recursive structures

@dataclasses.dataclass
class A:
    b: list[B] = dataclasses.field(default_factory=list)

@dataclasses.dataclass
class B:
    a: A

# we can make a recursive structure
a1 = A()
b1 = B(a1)
a1.b.append(b1)

# stringify supports recursion
print(a1)
print(b1)


# asdict and astuple don't however, with no way to even work around
# it (like pass in a set to track already seen objects)

# recursion overflow
dataclasses.asdict(a1)

# same
dataclasses.astuple(a1)

output:

A(b=[B(a=...)])
B(a=A(b=[...]))
Traceback (most recent call last):
  File "/home/classic/dev/sqlalchemy/test4.py", line 29, in <module>
    dataclasses.asdict(a1)
  File "/opt/python-3.10.0/lib/python3.10/dataclasses.py", line 1232, in asdict
    return _asdict_inner(obj, dict_factory)
  File "/opt/python-3.10.0/lib/python3.10/dataclasses.py", line 1239, in _asdict_inner
    value = _asdict_inner(getattr(obj, f.name), dict_factory)
  File "/opt/python-3.10.0/lib/python3.10/dataclasses.py", line 1267, in _asdict_inner
    return type(obj)(_asdict_inner(v, dict_factory) for v in obj)
  File "/opt/python-3.10.0/lib/python3.10/dataclasses.py", line 1267, in <genexpr>
    return type(obj)(_asdict_inner(v, dict_factory) for v in obj)
  File "/opt/python-3.10.0/lib/python3.10/dataclasses.py", line 1239, in _asdict_inner
    value = _asdict_inner(getattr(obj, f.name), dict_factory)
  File "/opt/python-3.10.0/lib/python3.10/dataclasses.py", line 1239, in _asdict_inner

...

  File "/opt/python-3.10.0/lib/python3.10/dataclasses.py", line 1236, in _asdict_inner
    if _is_dataclass_instance(obj):
  File "/opt/python-3.10.0/lib/python3.10/dataclasses.py", line 1201, in _is_dataclass_instance
    return hasattr(type(obj), _FIELDS)
RecursionError: maximum recursion depth exceeded while calling a Python object

there seems to be no workaround as these two methods are pretty simple and don't accept any arguments like "check_recursion", IIUC there is also no directive on field() that would change this either?

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zzzeek added the type-bug label Jun 27, 2022

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dataclasses astuple and asdict crash on recursive dataclass structures #94345

dataclasses astuple and asdict crash on recursive dataclass structures #94345

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zzzeek commented Jun 27, 2022