Description

This replaces the optional Graph.snapshot_id field with a concrete GraphSnapshot class. This explicit separation makes it much more easily to reason about (and statically verify with mypy) that a section of code that requires a snapshot (eg: all Executor logic and most io) receives a snapshot, without littering a bunch of assert graph.snapshot_id is not None. This also makes tagging a bit more clear - you are tagging (and retrieving) a specific snapshot, so having an explicit entity is more clear.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Added / updated tests and ran docs/examples/spend/demo.py.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in upstream modules