GT4Py import error on Python 3.9.9

[tehrengruber]

I recently updated my machine to Python 3.9.9 resulting in this obscure error when importing gt4py:

Python 3.9.9 (main, Nov 20 2021, 21:30:06) 
[GCC 11.1.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import gt4py
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/tille/Development/FVM_GT4Py_storage/venv/lib/python3.9/site-packages/gt4py/__init__.py", line 45, in <module>
    from . import gtscript
  File "/home/tille/Development/FVM_GT4Py_storage/venv/lib/python3.9/site-packages/gt4py/gtscript.py", line 33, in <module>
    from gt4py.stencil_builder import StencilBuilder
  File "/home/tille/Development/FVM_GT4Py_storage/venv/lib/python3.9/site-packages/gt4py/stencil_builder.py", line 21, in <module>
    from gt4py.backend.gtc_backend.defir_to_gtir import DefIRToGTIR
  File "/home/tille/Development/FVM_GT4Py_storage/venv/lib/python3.9/site-packages/gt4py/backend/__init__.py", line 19, in <module>
    from .base import *
  File "/home/tille/Development/FVM_GT4Py_storage/venv/lib/python3.9/site-packages/gt4py/backend/base.py", line 29, in <module>
    from .module_generator import (
  File "/home/tille/Development/FVM_GT4Py_storage/venv/lib/python3.9/site-packages/gt4py/backend/module_generator.py", line 31, in <module>
    from gtc import gtir
  File "/home/tille/Development/FVM_GT4Py_storage/venv/lib/python3.9/site-packages/gtc/gtir.py", line 35, in <module>
    from eve import Node, Str, SymbolName, SymbolTableTrait, field, utils
  File "/home/tille/Development/FVM_GT4Py_storage/venv/lib/python3.9/site-packages/eve/__init__.py", line 35, in <module>
    from .concepts import (
  File "/home/tille/Development/FVM_GT4Py_storage/venv/lib/python3.9/site-packages/eve/concepts.py", line 158, in <module>
    class BaseNode(pydantic.BaseModel, metaclass=NodeMetaclass):
  File "/home/tille/Development/FVM_GT4Py_storage/venv/lib/python3.9/site-packages/eve/concepts.py", line 136, in __new__
    cls = super().__new__(mcls, name, bases, namespace, **kwargs)
  File "pydantic/main.py", line 282, in pydantic.main.ModelMetaclass.__new__
  File "pydantic/typing.py", line 287, in pydantic.typing.resolve_annotations
    For use of globalns and localns see the docstring for get_type_hints().
  File "/usr/lib/python3.9/typing.py", line 292, in _eval_type
    return t._evaluate(globalns, localns, recursive_guard)
  File "/usr/lib/python3.9/typing.py", line 553, in _evaluate
    type_ = _type_check(
  File "/usr/lib/python3.9/typing.py", line 158, in _type_check
    raise TypeError(f"{arg} is not valid as type argument")
TypeError: typing.ClassVar[typing.Dict[str, eve.concepts.ImplFieldMetadataDict]] is not valid as type argument

Python 3.8 works fine on the same machine.

[jdahm]

Indeed. This is a frustrating issue related to pydantic using internals of Python that changed in the latest versions. Duplicate of #551. A new version that fixes that is being prepared, see pydantic/pydantic#3436.