PR Type

• Bug fix
• New feature
• Core Runtime change (higher bar -- see CONTRIBUTING.md)
• Docs / tooling
• Refactoring

Summary

The static graph parser silently drops the foreach= / num_parallel= keyword values on Python 3.14+, so DAGNode.foreach_param, num_parallel, and the foreach_artifact exposed by output_steps() all come back None. Node typing is unaffected, only the artifact name/value is lost. This swaps the removed ast.Str.s read for an ast.Constant fallback (the same one already used for switch conditions a few lines above).

Issue

Fixes #3236

Reproduction

Runtime: local

Commands to run: Run the flow below under Python 3.14 (where ast.Str and the .s alias are gone). On 3.12/3.13 the deprecated .s alias still resolves, so the bug stays hidden there. Save it to a file and run it with python <file>.py, or paste it into a Python 3.14 shell.

from metaflow import FlowSpec, step
from metaflow.graph import FlowGraph


class ForeachRepro(FlowSpec):
    @step
    def start(self):
        self.items = [1, 2, 3]
        self.next(self.process, foreach="items")

    @step
    def process(self):
        self.next(self.join)

    @step
    def join(self, inputs):
        self.next(self.end)

    @step
    def end(self):
        pass


g = FlowGraph(ForeachRepro)
print(g["start"].type)                                        # 'foreach'
print(g["start"].foreach_param)                               # expected 'items'
print(g.output_steps()[0]["start"].get("foreach_artifact"))   # expected 'items'

Where evidence shows up: parent console (the static graph metadata, also consumed by show, the DAG card, and Argo/Airflow compilation)

type            = 'foreach'
foreach_param   = None
foreach_artifact = None

type            = 'foreach'
foreach_param   = 'items'
foreach_artifact = 'items'

Root Cause

In DAGNode._parse, the keyword extractor read getattr(k.value, "s", None). .s was the attribute on the legacy ast.Str node. Since Python 3.8 string literals are ast.Constant with the value in .value, and the .s compatibility alias was removed in 3.14. So on 3.14 the getattr falls through to its None default and the value is dropped.

Type detection still works because it checks for the presence of the keyword name ("foreach" in keywords), not its value, which is why the type stays correct while the value goes missing. The switch-condition branch just above already had the ast.Constant fallback, so this was effectively a half-applied migration.

Why This Fix Is Correct

Restores the invariant that a string/int literal keyword in self.next(...) round-trips to foreach_param / num_parallel. The _kw_value helper keeps the ast.Str branch for older interpreters and reads ast.Constant.value everywhere else, matching the existing switch-condition handling. Nothing else in the parse path changes.

Failure Modes Considered

1. Older Python (< 3.14): ast.Str/.s still exist, and the hasattr(ast, "Str") branch preserves the old path. Verified no regression on 3.12.
2. Non-literal keyword values (e.g. foreach=some_var): the node is an ast.Name, not ast.Constant, so _kw_value returns None, same as before. No behavior change there.

Tests

• Unit tests added/updated
• Reproduction script provided
• CI passes
• If tests are impractical: explain why below and provide manual evidence above

Existing test/unit/test_graph_structure.py and test_sourceless_dag_node.py pass on 3.14 (55 passed). No new unit test added yet since the existing suite runs on the default interpreter where the bug is masked; happy to add a 3.14-gated regression test if preferred.

Non-Goals

No change to graph topology, edge resolution, matching_join, or the switch-condition path. Not touching how non-literal keyword values are handled.

AI Tool Usage

• No AI tools were used in this contribution
• AI tools were used (describe below)

Claude Code was used to reproduce the bug on Python 3.14.3, apply the fix, and verify against the existing unit tests. All generated code was reviewed, understood, and tested.