Fix ability to pass None to task with Optional kwarg, add test

[fg91]

TL;DR

from flytekit import task, workflow, Workflow
from dataclasses import dataclass
from dataclasses_json import dataclass_json
from typing import Optional

@dataclass_json
@dataclass
class Config:
    lr: float = 0.1

@task
def func(cfg: Optional[Config] = None) -> None:
    print(cfg is None)

wf = Workflow(name="foo")
wf.add_entity(func, cfg=None)

wf()

This code snippet works up to flytekit version 1.5.0. It does not work in 1.6.0 (change introduced in this commit), failing with this error:

╭──────────────────────────────────────────────────── Traceback (most recent call last) ─────────────────────────────────────────────────────╮
│ /home/username/miniconda3/envs/debug/lib/python3.9/site-packages/flytekit/core/promise.py:1002 in create_and_link_node                   │
│                                                                                                                                            │
│ ❱ 1002 │   │   │   │   binding_from_python_std(                                                                                            │
│                                                                                                                                            │
│ /home/username/miniconda3/envs/debug/lib/python3.9/site-packages/flytekit/core/promise.py:690 in binding_from_python_std                 │
│                                                                                                                                            │
│ ❱  690 │   binding_data = binding_data_from_python_std(ctx, expected_literal_type, t_value, t_v                                            │
│                                                                                                                                            │
│ /home/username/miniconda3/envs/debug/lib/python3.9/site-packages/flytekit/core/promise.py:632 in binding_data_from_python_std            │
│                                                                                                                                            │
│ ❱  632 │   │   raise AssertionError(                                                                                                       │
╰────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
AssertionError: Failed to bind data None with literal type [<FlyteLiteral simple: STRUCT metadata { fields { key: "definitions" value { 
struct_value { fields { key: "ConfigSchema" value { struct_value { fields { key: "type" value { string_value: "object" } } fields { key: 
"properties" value { struct_value { fields { key: "lr" value { struct_value { fields { key: "type" value { string_value: "number" } } fields {
key: "title" value { string_value: "lr" } } fields { key: "format" value { string_value: "float" } } fields { key: "default" value { 
number_value: 0.1 } } } } } } } } fields { key: "additionalProperties" value { bool_value: false } } } } } } } } fields { key: "$schema" value
{ string_value: "http://json-schema.org/draft-07/schema#" } } fields { key: "$ref" value { string_value: "#/definitions/ConfigSchema" } } } 
structure { tag: "Object-Dataclass-Transformer" }>, <FlyteLiteral simple: NONE structure { tag: "none" }>].

The above exception was the direct cause of the following exception:

╭──────────────────────────────────────────────────── Traceback (most recent call last) ─────────────────────────────────────────────────────╮
│ /home/username/torch-experiments/none.py:18 in <module>                                                                                  │
│                                                                                                                                            │
│ ❱ 18 wf.add_entity(func, cfg=None)                                                                                                         │
│                                                                                                                                            │
│ /home/username/miniconda3/envs/debug/lib/python3.9/site-packages/flytekit/core/workflow.py:551 in add_entity                             │
│                                                                                                                                            │
│ ❱ 551 │   │   │   n = create_node(entity=entity, **kwargs)                                                                                 │
│                                                                                                                                            │
│ /home/username/miniconda3/envs/debug/lib/python3.9/site-packages/flytekit/core/node_creation.py:93 in create_node                        │
│                                                                                                                                            │
│ ❱  93 │   │   outputs = entity(**kwargs)                                                                                                   │
│                                                                                                                                            │
│ /home/username/miniconda3/envs/debug/lib/python3.9/site-packages/flytekit/core/base_task.py:304 in __call__                              │
│                                                                                                                                            │
│ ❱ 304 │   │   return flyte_entity_call_handler(self, *args, **kwargs)  # type: ignore                                                      │
│                                                                                                                                            │
│ /home/username/miniconda3/envs/debug/lib/python3.9/site-packages/flytekit/core/promise.py:1092 in flyte_entity_call_handler              │
│                                                                                                                                            │
│ ❱ 1092 │   │   return create_and_link_node(ctx, entity=entity, **kwargs)                                                                   │
│                                                                                                                                            │
│ /home/username/miniconda3/envs/debug/lib/python3.9/site-packages/flytekit/core/promise.py:1012 in create_and_link_node                   │
│                                                                                                                                            │
│ ❱ 1012 │   │   │   raise AssertionError(f"Failed to Bind variable {k} for function {entity.name                                            │
╰────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
AssertionError: Failed to Bind variable cfg for function func.

I assume the elif case should not be entered if the value is None. Is this correct @pingsutw ?

Type

• Bug Fix
• Feature
• Plugin

Are all requirements met?

• Code completed
• Smoke tested
• Unit tests added
• Code documentation added
• Any pending items have an associated Issue

Complete description

How did you fix the bug, make the feature etc. Link to any design docs etc

Tracking Issue

https://github.com/flyteorg/flyte/issues/

Follow-up issue

NA
OR
https://github.com/flyteorg/flyte/issues/

[codecov]

Codecov Report

Merging #1657 (90160ee) into master (eafcc82) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1657      +/-   ##
==========================================
+ Coverage   71.01%   71.03%   +0.01%     
==========================================
  Files         336      336              
  Lines       30756    30765       +9     
  Branches     5572     5573       +1     
==========================================
+ Hits        21842    21853      +11     
+ Misses       8368     8367       -1     
+ Partials      546      545       -1     

Impacted Files	Coverage Δ
flytekit/core/promise.py	54.72% <100.00%> (ø)
tests/flytekit/unit/core/test_promise.py	90.81% <100.00%> (+0.92%)	⬆️

... and 1 file with indirect coverage changes

[pingsutw]

Good catch, thank you @fg91