feat(graph): add --dependencies-only and --dependents-only flags to graph subset

[dhellmann]

Summary

• Add --dependencies-only to graph subset: restricts output to the target package and its transitive dependencies, excluding any packages that depend on the target.
• Add --dependents-only to graph subset: restricts output to the target package and its transitive dependents, excluding the target's own dependencies.
• Using both flags together is an error.

Implementation notes

When --dependencies-only is active, ROOT is not reachable via the normal upward traversal, so:

• ROOT is always added to the included node set to keep the graph rooted.
• ROOT's original edges are suppressed during the edge-copy pass (to avoid spurious edges from other top-level packages that happen to be transitive deps of the target).
• An artificial ROOT → target edge is added so the subset serializes correctly.

When --dependents-only is active, the upward traversal naturally reaches ROOT, so no special handling is needed.

Test plan

• test_graph_subset_dependencies_only — target and its deps are included; dependents are not.
• test_graph_subset_dependencies_only_no_dependents — confirms dependents absent vs full subset.
• test_graph_subset_dependents_only — target and its dependents are included; target's deps are not.
• test_graph_subset_dependents_only_and_dependencies_only_are_exclusive — combining both flags exits non-zero with a clear error message.

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📥 Commits

Reviewing files that changed from the base of the PR and between 955c0d0 and fca6661.

📒 Files selected for processing (2)

• src/fromager/commands/graph.py
• tests/test_commands_graph.py

🚧 Files skipped from review as they are similar to previous changes (2)

• tests/test_commands_graph.py
• src/fromager/commands/graph.py

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📝 Walkthrough

Walkthrough

The graph subset command adds mutually exclusive --dependencies-only and --dependents-only filters. Subset extraction adjusts traversal accordingly, always retains ROOT, and handles ROOT edges for dependency-only output. The subset graph builder can skip ROOT-originating edges, and CLI integration tests cover both modes and invalid combined usage.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly names the main change: new dependencies-only and dependents-only flags for graph subset.
Description check	✅ Passed	The description matches the implementation and tests, including the two flags, exclusivity, and ROOT handling.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[dhellmann]

@rd4398 @LalatenduMohanty I had a need for this feature this morning to prep some data for an internal conversation.

[rd4398]

This looks good!

[mergify]

Tick the box to add this pull request to the merge queue (same as @mergifyio queue).

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[dhellmann]

This looks good!

What's the process now for adding things to the merge queue? Do I click the button myself, for example?

[rd4398]

@mergify rebase

[mergify]

rebase

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