fix: allow expand() on clustered FlowSystem without a solution

[FBumann]

Summary

• Removed the requirement for a solution in _validate_for_expansion(), so expand() can be called on unsolved clustered FlowSystems
• Made solution expansion in expand_flow_system() conditional (if reduced_solution is not None)
• Fixes: flow_system.transform.expand().transform.clustering_data() raising ValueError when no solution exists

Test plan

• Existing clustering/expansion tests still pass
• Verify expand() works on an unsolved clustered FlowSystem
• Verify expand() still correctly expands solutions when they exist

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Expansion of flow systems no longer fails when no optimization solution exists; expansion proceeds and preserves original timesteps without raising an error.

• Tests

  • Tests updated to reflect the new behavior: expanding without a solution is supported and validated.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: a04f20f2-6bae-4d01-811e-ad5179dca6ef

📥 Commits

Reviewing files that changed from the base of the PR and between 10db018 and b0b7ec4.

📒 Files selected for processing (1)

• flixopt/transform_accessor.py

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📝 Walkthrough

Walkthrough

expand_flow_system no longer assumes a solution exists: solution-variable expansion is executed only when a reduced solution is present. Validation for expansion no longer requires a solution, only clustering. Tests updated to reflect successful expansion when no solution is available.

Changes

Cohort / File(s)	Summary
Solution Expansion Logic flixopt/transform_accessor.py	Guarded solution expansion in expand_flow_system on reduced_solution is not None; removed the explicit ValueError for missing self._fs.solution in _validate_for_expansion, leaving only clustering requirement.
Tests tests/test_clustering/test_cluster_reduce_expand.py	Renamed test_expand_without_solution_raises → test_expand_without_solution; test now calls expand() without expecting an exception and asserts expanded solution is None and timesteps match original.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• Fix: Correct expansion of segmented clustered systems #573: Modifies expansion logic in transform_accessor.py with segmented solution adjustments; closely related to control flow changes when handling solutions.
• Fix clustering expansion bugs and add ExtremeConfig validation #590: Alters expansion internals in transform_accessor.py for segmented systems and binary-event handling; relevant to expansion behavior and validation.

Poem

🐰
I hopped through clusters, soft and spry,
Found no solution—so I hopped on by.
Expansion blooms where timesteps mend,
Clusters guide the hop, not a missing end.
Carrots cheer: the flows extend! 🥕✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely describes the main change: allowing the expand() method to work on clustered FlowSystems without requiring a solution.
Description check	✅ Passed	The description covers the key changes (removing solution requirement, making expansion conditional, fixing the error), includes a test plan, and provides context. Most template sections are adequately addressed.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/expand-without-solution

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Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

flixopt/transform_accessor.py (1)

1896-1911: ⚠️ Potential issue | 🟡 Minor

Stale docstring in expand() method.

The _validate_for_expansion() changes look correct. However, the docstring for expand() (lines 1932-1934) still lists:

Raises:
    ValueError: If the FlowSystem was not created with ``cluster()``.
    ValueError: If the FlowSystem has no solution.

The second ValueError is no longer raised and should be removed to keep documentation accurate.

📝 Suggested docstring fix (outside selected range)

In the expand() method docstring around line 1932-1934, remove the stale error documentation:

 Raises:
     ValueError: If the FlowSystem was not created with ``cluster()``.
-    ValueError: If the FlowSystem has no solution.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@flixopt/transform_accessor.py` around lines 1896 - 1911, The docstring for
the expand() method still documents two ValueError cases but only one is
actually raised; update the expand() docstring to remove the stale "ValueError:
If the FlowSystem has no solution." entry so it matches the current
implementation that uses _validate_for_expansion() (which raises the
cluster-related ValueError). Locate the expand() method and remove the second
ValueError line from its Raises: section, leaving only the cluster-related
ValueError documented.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Outside diff comments:
In `@flixopt/transform_accessor.py`:
- Around line 1896-1911: The docstring for the expand() method still documents
two ValueError cases but only one is actually raised; update the expand()
docstring to remove the stale "ValueError: If the FlowSystem has no solution."
entry so it matches the current implementation that uses
_validate_for_expansion() (which raises the cluster-related ValueError). Locate
the expand() method and remove the second ValueError line from its Raises:
section, leaving only the cluster-related ValueError documented.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 31e7e5f9-a124-4be2-90ef-5f3e6f905fe5

📥 Commits

Reviewing files that changed from the base of the PR and between 15aca5d and 10db018.

📒 Files selected for processing (2)

• flixopt/transform_accessor.py
• tests/test_clustering/test_cluster_reduce_expand.py