deterministic graph execution

[XzzX]

pyiron_workflow/pyiron_workflow/channels.py

Lines 494 to 509 in 9e776ad

	def fetch(self) -> None:
	"""
	Sets :attr:`value` to the first value among connections (i.e. the most recent)
	that is something other than `NOT_DATA`; if no such value exists (e.g. because
	there are no connections or because all the connected output channels have
	`NOT_DATA` as their value), :attr:`value` remains unchanged.
	I.e., the connection with the highest priority for updating input data is the
	0th connection; build graphs accordingly.
	Raises:
	RuntimeError: If the owner is :attr:`running`.
	"""
	for out in self.connections:
	if out.value is not NOT_DATA:
	self.value = out.value
	break

Is this a feature? This makes graph execution non deterministic, does it?

[liamhuber]

It is a feature, and for the current implementation of the "while" flow of information it's a critical one. We decided at the meeting that it would be sensible to switch to only allowing each input to have at most one connection (still arbitrary connections for output, of course).

I agree it muddies graph execution. I guess other stuff does too, like the If node, but I'm fully on-board with making it more deterministic here. After formulating the new while loop (#501), and restricting the number of input connections (#502), the only remaining bit to close this issue is to always fetch the upstream data even if you currently have a data-like value and upstream doesn't. AFAIK the only place this "feature" is leveraged is in the while-loop stuff, so once #501 is done, I don't anticipate the further changes to close this having any impact on the test suite or anybody's applications.