Postponed evaluation `from __future__ import annotations` failed with DataClassTransformer

[mickjermsurawong-stripe]

Problem: Task taking input of dataclasses object with List[T] will fail deserialization when the object definition is in postponed evaluation mode. The error is ValueError: Only generic univariate typing.List[T] type is supported. failing code

Repro We can reproduce the error in a simpler setup similar to the existing type engine test

# Postponed evaluation
from __future__ import annotations

# Class definition
from dataclasses import dataclass
from dataclasses_json import dataclass_json
from typing import List

@dataclass_json
@dataclass
class MyDataClass:
    foo_list: List[str]

# Serialization similar to tests 
from flytekit.core.context_manager import FlyteContext
from flytekit.core.type_engine import DataclassTransformer

ctx = FlyteContext.current_context()
tf = DataclassTransformer()
lt = tf.get_literal_type(MyDataClass)
o = MyDataClass(["a", "b"])
lv = tf.to_literal(ctx, o, MyDataClass, lt)
ot = tf.to_python_value(ctx, lv=lv, expected_python_type=MyDataClass)

Cause: When postponed evaluation is enabled, the type from dataclass fields is a raw string "List[str]", and not typing. The current code trying to get subtype T in List[T] thus fails.

import dataclasses
f = dataclasses.fields(MyDataClass)[0]
f.type, type(f.type) # ('List[str]', str)

If from __future__ import annotations is removed before the dataclass definition, deserialization to_python_value above works without error. This is because the field type is of typing as expected by the code.

import dataclasses
f = dataclasses.fields(MyDataClass)[0]
f.type, type(f.type) # (typing.List[str], typing._GenericAlias)

Proposal: either make documentation clearer, or handle the type mapping of raw string in this postponed evaluation mode.

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[Kota-Karthik]

@samhita-alla I am interested in doing this issue So kindly assign me this issue Thank You : )

[samhita-alla]

@Kota-Karthik, are you working on creating a PR for this issue?

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