No constants

[cicdw]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

This PR completely avoids the auto-generation and execution of Python constants which are upstream of a given task. Instead, these constants are tracked on the Flow object directly, and artificial edges are created at runtime to manage them as upstream dependencies.

Why is this PR important?

The autogeneration of constants is perhaps one of the more confusing aspects of Prefect's functional API for newcomers. In addition, Cloud users do not find Constant[str] type tasks to be useful or informative, and ultimately these just eat up real estate on all screens. The current PR completely avoids this situation unless a user explicitly opts into it via wrapping their objects in prefect.tasks.core.constants.Constant()

Labeled as WIP as I still need to:

• add more tests
• update documentation
• handle Cloud retries / failures

but I wanted some initial feedback on the implementation here.

[codecov]

Codecov Report

Merging #1730 into master will decrease coverage by <.01%.
The diff coverage is 95.83%.

[joshmeek]

What's the key noticeable difference that one would see when this is merged? Am I thinking about it right in that we won't see all the Constant task run logs anymore unless we declare Constants in our flow?

[cicdw]

@joshmeek yup that's one major difference; another is that

with Flow("blah") as flow:
    add(2, 4)

will result in a Flow with a single task. This will be very apparent in flow.visualize() and in the UI views.

[cicdw]

@jlowin / @joshmeek this is ready for review: note that I am choosing to not implement a serializer for ConstantResultHandler at this time because the planned cadence of our releases could cause issues for people who want to start working off 0.7.2 once it's released later this week.

[joshmeek]

Looks good from my standpoint

gotta move