Improve error message in ListTransformer when not passing a list

[fg91]

TL;DR

Improve error message in ListTransformer in a way that would have saved us time debugging a failing workflow.

Type

Neither of the following, just adding a meaningful error message.

• 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

This workflow ...

from flytekit import task, workflow

@task
def foo() -> int:
    return 1

@task
def bar(inp: list[int]):
    print(inp)

@workflow
def wf():
    bar(inp=foo())


if __name__ == "__main__":
    wf()

... fails with:

Traceback (most recent call last):
  File ".../test.py", line 17, in <module>
    wf()
  File ".../flytekit/flytekit/core/workflow.py", line 279, in __call__
    raise exc
  File ".../flytekit/flytekit/core/workflow.py", line 276, in __call__
    return flyte_entity_call_handler(self, *args, **input_kwargs)
  File ".../flytekit/flytekit/core/promise.py", line 1051, in flyte_entity_call_handler
    result = cast(LocallyExecutable, entity).local_execute(child_ctx, **kwargs)
  File ".../flytekit/flytekit/core/workflow.py", line 298, in local_execute
    function_outputs = self.execute(**kwargs_literals)
  File ".../flytekit/flytekit/core/workflow.py", line 746, in execute
    return exception_scopes.user_entry_point(self._workflow_function)(**kwargs)
  File ".../flytekit/flytekit/exceptions/scopes.py", line 203, in user_entry_point
    raise type(exc)(f"Error encountered while executing '{fn_name}':\n  {exc}") from exc
flytekit.core.type_engine.TypeTransformerFailedError: Encountered error while executing workflow 'wf':
  Error encountered while executing 'wf':
  Failed to convert inputs of task 'test.bar':
  Error converting input 'inp' at position 0:  # <- No meaningful error comes after this colon

In this minimal example, the error is quickly apparent but one of our engineers struggled to find the issue in a large training workflow.
A hint that a list/collection is expected but not provided (as here in the respective to_literal method) would have greatly simplified the search:

flytekit.core.type_engine.TypeTransformerFailedError: Encountered error while executing workflow 'wf':
  Error encountered while executing 'wf':
  Failed to convert inputs of task 'test.bar':
  Error converting input 'inp' at position 0:
  The expected python type is 'list[int]' but the received Flyte literal value is not a collection (Flyte's representation of Python lists).

Tracking Issue

NA

Follow-up issue

NA

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (1f0d120) 51.27% compared to head (b1c0de7) 95.12%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1882       +/-   ##
===========================================
+ Coverage   51.27%   95.12%   +43.84%     
===========================================
  Files         255      131      -124     
  Lines       20283     5984    -14299     
  Branches     3357        0     -3357     
===========================================
- Hits        10400     5692     -4708     
+ Misses       9720      292     -9428     
+ Partials      163        0      -163     

see 208 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.