# Buggy code
def is_optional(annotation):
    """Returns True if the annotation is typing.Optional[SomeType]"""

    # Optionals are represented as unions

    if not is_union(annotation):
        return False

    types = annotation.__args__

    # A Union to be optional needs to have at least one None type
    return any([x == None.__class__ for x in types])  # noqa:E711


# Traceback (AttributeError)
Traceback (most recent call last):
File "/home/ignormies/.local/share/virtualenvs/gql-bf-XGX4szKA-py3.8/lib/python3.8/site-packages/graphql/type/definition.py", line 735, in fields
fields = resolve_thunk(self._fields)
File "/home/ignormies/.local/share/virtualenvs/gql-bf-XGX4szKA-py3.8/lib/python3.8/site-packages/graphql/type/definition.py", line 293, in resolve_thunk
return thunk() if callable(thunk) else thunk
File "/home/ignormies/.local/share/virtualenvs/gql-bf-XGX4szKA-py3.8/lib/python3.8/site-packages/strawberry/type.py", line 133, in <lambda>
lambda types_replacement_map=None: _get_fields(wrapped, types_replacement_map),
File "/home/ignormies/.local/share/virtualenvs/gql-bf-XGX4szKA-py3.8/lib/python3.8/site-packages/strawberry/type.py", line 76, in _get_fields
fields[field_name] = field(
File "/home/ignormies/.local/share/virtualenvs/gql-bf-XGX4szKA-py3.8/lib/python3.8/site-packages/strawberry/field.py", line 315, in field
return field(wrap)
File "/home/ignormies/.local/share/virtualenvs/gql-bf-XGX4szKA-py3.8/lib/python3.8/site-packages/strawberry/field.py", line 171, in __call__
setattr(wrap, IS_STRAWBERRY_FIELD, True)
AttributeError: 'method' object has no attribute '_is_strawberry_field'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/ignormies/.config/JetBrains/PyCharm2020.1/scratches/scratch_7.py", line 30, in <module>
schema = strawberry.Schema(query=Query)
File "/home/ignormies/.local/share/virtualenvs/gql-bf-XGX4szKA-py3.8/lib/python3.8/site-packages/strawberry/schema.py", line 25, in __init__
super().__init__(
File "/home/ignormies/.local/share/virtualenvs/gql-bf-XGX4szKA-py3.8/lib/python3.8/site-packages/graphql/type/schema.py", line 205, in __init__
collect_referenced_types(query)
File "/home/ignormies/.local/share/virtualenvs/gql-bf-XGX4szKA-py3.8/lib/python3.8/site-packages/graphql/type/schema.py", line 382, in collect_referenced_types
for field in named_type.fields.values():
File "/usr/lib/python3.8/functools.py", line 967, in __get__
val = self.func(instance)
File "/home/ignormies/.local/share/virtualenvs/gql-bf-XGX4szKA-py3.8/lib/python3.8/site-packages/graphql/type/definition.py", line 737, in fields
raise TypeError(f"{self.name} fields cannot be resolved. {error}")
TypeError: Query fields cannot be resolved. 'method' object has no attribute '_is_strawberry_field'

Process finished with exit code 1

# Fixed code
def is_optional(annotation: Type) -> bool:
    """Returns True if the annotation is Optional[SomeType]"""

    # Optionals are represented as unions

    if not is_union(annotation):
        return False

    types = annotation.__args__

    # A Union to be optional needs to have at least one None type
    return any([x == None.__class__ for x in types])  # noqa:E711