Fastapi: [BUG] RecursionError from response model in 0.47.1

[ysmu]

Describe the bug

FastAPI 0.47.1 will not be able to start due to a RecursionError when there is a circular reference among models. The issue seems to originate from https://github.com/tiangolo/fastapi/pull/889. This works fine in 0.46.0.

Environment

• OS: Windows
• FastAPI Version: 0.47.1
• Python version: 3.7.0

To Reproduce

from typing import Optional

from fastapi import FastAPI
from pydantic import BaseModel, Field


class Group(BaseModel):
    representative: Optional['Person'] = Field(None)


class Person(BaseModel):
    group: Optional[Group] = Field(None)


Group.update_forward_refs()


app = FastAPI()


@app.get('/group/{group_id}', response_model=Group)
def get_group(group_id):
    return []

Expected behavior

No exception

Actual output

Traceback (most recent call last):
  File "test.py", line 21, in <module>
    @app.get('/group/{group_id}', response_model=Group)
  File "D:\virtualenvs\test\lib\site-packages\fastapi\routing.py", line 494, in decorator
    callbacks=callbacks,
  File "D:\virtualenvs\test\lib\site-packages\fastapi\routing.py", line 438, in add_api_route
    callbacks=callbacks,
  File "D:\virtualenvs\test\lib\site-packages\fastapi\routing.py", line 275, in __init__
    ] = create_cloned_field(self.response_field)
  File "D:\virtualenvs\test\lib\site-packages\fastapi\utils.py", line 100, in create_cloned_field
    use_type.__fields__[f.name] = create_cloned_field(f)
  File "D:\virtualenvs\test\lib\site-packages\fastapi\utils.py", line 100, in create_cloned_field
    use_type.__fields__[f.name] = create_cloned_field(f)
  File "D:\virtualenvs\test\lib\site-packages\fastapi\utils.py", line 100, in create_cloned_field
    use_type.__fields__[f.name] = create_cloned_field(f)
  [Previous line repeated 981 more times]
  File "D:\virtualenvs\test\lib\site-packages\fastapi\utils.py", line 97, in create_cloned_field
    original_type.__name__, __config__=original_type.__config__
  File "D:\virtualenvs\test\lib\site-packages\pydantic\main.py", line 773, in create_model
    return type(model_name, (__base__,), namespace)
  File "D:\virtualenvs\test\lib\site-packages\pydantic\main.py", line 152, in __new__
    if issubclass(base, BaseModel) and base != BaseModel:
  File "D:\virtualenvs\test\lib\abc.py", line 143, in __subclasscheck__
    return _abc_subclasscheck(cls, subclass)
RecursionError: maximum recursion depth exceeded in comparison

[dmontagu]

Okay, I think this could be safely fixed by using a cache to track models that have already been cloned, and then using that to short-circuit the recursion.

I think that should still be secure since only FastAPI would be able to generate instances of the cloned types, but I'm not 100% sure quite yet.

---

That said, if we could just implement dump_as_obj in pydantic and use that to eliminate the use of cloned fields, that would solve everything.

I'll take a look at it this week; if I can't quickly throw together dump_as_obj based on my previous work, or it looks like it may take a while for it to get merged into pydantic, I'll look into using the cache-based approach described above to patch this for the short term.

---

If anyone else wants to put together a PR to fix this I would be happy to review it and offer some guidance.

[jmagnusson]

Hitting this as well. Can confirm that reverting to 0.46.0 fixes this.

[jmagnusson]

New exception for above example when using fastapi 0.48.0:

Traceback (most recent call last):
  File "/home/user/Library/Python/3.7/bin/uvicorn", line 8, in <module>
    sys.exit(main())
  File "/home/user/Library/Python/3.7/lib/python/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/user/Library/Python/3.7/lib/python/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/user/Library/Python/3.7/lib/python/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/user/Library/Python/3.7/lib/python/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/user/Library/Python/3.7/lib/python/site-packages/uvicorn/main.py", line 323, in main
    run(**kwargs)
  File "/home/user/Library/Python/3.7/lib/python/site-packages/uvicorn/main.py", line 346, in run
    server.run()
  File "/home/user/Library/Python/3.7/lib/python/site-packages/uvicorn/main.py", line 374, in run
    loop.run_until_complete(self.serve(sockets=sockets))
  File "uvloop/loop.pyx", line 1456, in uvloop.loop.Loop.run_until_complete
  File "/home/user/Library/Python/3.7/lib/python/site-packages/uvicorn/main.py", line 381, in serve
    config.load()
  File "/home/user/Library/Python/3.7/lib/python/site-packages/uvicorn/config.py", line 286, in load
    self.loaded_app = import_from_string(self.app)
  File "/home/user/Library/Python/3.7/lib/python/site-packages/uvicorn/importer.py", line 20, in import_from_string
    module = importlib.import_module(module_str)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "./fastapi-bug-894.py", line 21, in <module>
    @app.get('/group/{group_id}', response_model=Group)
  File "/usr/local/lib/python3.7/site-packages/fastapi/routing.py", line 494, in decorator
    callbacks=callbacks,
  File "/usr/local/lib/python3.7/site-packages/fastapi/routing.py", line 438, in add_api_route
    callbacks=callbacks,
  File "/usr/local/lib/python3.7/site-packages/fastapi/routing.py", line 275, in __init__
    ] = create_cloned_field(self.response_field)
  File "/usr/local/lib/python3.7/site-packages/fastapi/utils.py", line 98, in create_cloned_field
    use_type.__fields__[f.name] = create_cloned_field(f)
  File "/usr/local/lib/python3.7/site-packages/fastapi/utils.py", line 98, in create_cloned_field
    use_type.__fields__[f.name] = create_cloned_field(f)
  File "/usr/local/lib/python3.7/site-packages/fastapi/utils.py", line 98, in create_cloned_field
    use_type.__fields__[f.name] = create_cloned_field(f)
  [Previous line repeated 945 more times]
  File "/usr/local/lib/python3.7/site-packages/fastapi/utils.py", line 96, in create_cloned_field
    use_type = create_model(original_type.__name__, __base__=original_type)
  File "/home/user/Library/Python/3.7/lib/python/site-packages/pydantic/main.py", line 780, in create_model
    return type(model_name, (__base__,), namespace)
  File "/home/user/Library/Python/3.7/lib/python/site-packages/pydantic/main.py", line 153, in __new__
    fields.update(deepcopy(base.__fields__))
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 150, in deepcopy
    y = copier(x, memo)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 240, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 180, in deepcopy
    y = _reconstruct(x, memo, *rv)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 280, in _reconstruct
    state = deepcopy(state, memo)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 150, in deepcopy
    y = copier(x, memo)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 220, in _deepcopy_tuple
    y = [deepcopy(a, memo) for a in x]
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 220, in <listcomp>
    y = [deepcopy(a, memo) for a in x]
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 150, in deepcopy
    y = copier(x, memo)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 240, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 180, in deepcopy
    y = _reconstruct(x, memo, *rv)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 280, in _reconstruct
    state = deepcopy(state, memo)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 150, in deepcopy
    y = copier(x, memo)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 220, in _deepcopy_tuple
    y = [deepcopy(a, memo) for a in x]
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 220, in <listcomp>
    y = [deepcopy(a, memo) for a in x]
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 150, in deepcopy
    y = copier(x, memo)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 240, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 150, in deepcopy
    y = copier(x, memo)
  File "/usr/local/Cellar/python/3.7.6_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/copy.py", line 238, in _deepcopy_dict
    memo[id(x)] = y
RecursionError: maximum recursion depth exceeded while calling a Python object

[zamiramir]

Hitting this as well. Can confirm that reverting to 0.46.0 fixes this.

Same issue/workarround.
OS: Windows and MacOS
FastAPI Version: 0.48.0
Python version: 3.7.3

~However we don't have the issue on MacOS.~

[dmontagu]

I've been busy so haven't had a lot of time to look into this just yet.

@zamiramir that's actually very useful information -- I suspect it is related to the cythonization of pydantic.

Any chance you could run from pydantic.version import version_info; print(version_info()) in both environments (MacOS and non-MacOS), and share the output?

[jmagnusson]

I originally reproduced the issue on MacOS (both 0.47 and 0.48). Latest version of Catalina.

[dmontagu]

@jmagnusson it would be most helpful if you could share the output of the version info function as described above, which includes information about whether pydantic was compiled, etc.

[jmagnusson]

I had pydantic 1.3 installed where the command you specified did not work. However, with 1.4 I get:

python3 -c 'from pydantic.version import version_info; print(version_info())'
             pydantic version: 1.4
            pydantic compiled: False
                 install path: /usr/local/lib/python3.7/site-packages/pydantic
               python version: 3.7.6 (default, Dec 30 2019, 19:38:26)  [Clang 11.0.0 (clang-1100.0.33.16)]
                     platform: Darwin-19.3.0-x86_64-i386-64bit
     optional deps. installed: ['typing-extensions', 'email-validator']

Same issue with RecursionError: maximum recursion depth exceeded while calling a Python object after Pydantic 1.4 upgrade.

[jmagnusson]

Uninstall pydantic and installed cython first, then pydantic. Still shows pydantic compiled: False.

[zamiramir]

I've been busy so haven't had a lot of time to look into this just yet.

@zamiramir that's actually very useful information -- I suspect it is related to the cythonization of pydantic.

Any chance you could run from pydantic.version import version_info; print(version_info()) in both environments (MacOS and non-MacOS), and share the output?

Sorry just tested it again, and got the same issue on MacOS. Might have been an older version of FastAPI.

[adamteale]

I can also confirm this, reverting to 0.46.0 works.

[likeanaxon]

I've also hit this issue, confirming that downgrading to 0.46.0 works.

[jmagnusson]

Still occurs in fastapi==0.52.0.

[voegtlel]

Hi there, I've suggested another MR to fix this issue. The one of @mateuszz0000 was missing the recursive field resolution. EDIT: Coverage showed me I was wrong here and used an older version of fastapi for my initial test of the fix 🙈 Works without that part (so removed it from the commit).

Still, I think local cloning (i.e. keeping dicts of what was cloned) should be preferred before setting another hidden attribute?

[tiangolo]

Thanks for the discussion here everyone!

This was fixed by @voegtlel in https://github.com/tiangolo/fastapi/pull/1164 :rocket: :tada:

It will be available in the next release (today in a couple of hours).

I'll re-open this issue to give @ysmu a chance to confirm it's fixed and close it.

[ysmu]

Fix works for me. Thank you!

[kevlarr]

Ah thanks for fixing this, our front-end team really appreciates it ;)