Code quality: Replace assert with ValueError in visualization.py

[ddri]

Summary

Replace assert statement with proper ValueError exception in circuit_to_latex() function.

Classification: Code Quality Improvement

Not a validated bug. Tests pass with python -O (asserts disabled). This is a defensive coding improvement, not a fix for observed failures.

Context

Part of tracking issue #1. Identified during code review. The maintainer left a # FIXME. Throw Exception comment on this line, indicating intent to fix.

Current Behavior

# visualization.py:173
assert package in ["qcircuit", "quantikz"]  # FIXME. Throw Exception

If an invalid package parameter is passed:

• Normal Python: raises AssertionError
• With python -O: silently continues, may cause confusing errors later

Validation Performed

$ python -O -m pytest quantumflow/ --tb=short
# 1332 passed — no tests fail when asserts are disabled

This means no existing test exercises this code path with invalid input.

Why Fix Anyway?

1. Maintainer flagged it with # FIXME
2. Follows pattern from upstream PRs Replace assert False with KeyError in DAGCircuit methods gecrooks/quantumflow#133, Replace assert with proper validation in channel_to_kraus() gecrooks/quantumflow#134
3. Defensive coding — better error messages for users
4. Asserts should be for internal invariants, not input validation

Technical Details

• File: quantumflow/visualization.py
• Line: 173
• Effort: ~15 minutes
• Risk: Very low

Proposed Fix

if package not in ["qcircuit", "quantikz"]:
    raise ValueError(f"Invalid LaTeX package '{package}'. Must be 'qcircuit' or 'quantikz'.")

References

• Tracking issue: Review state of QuantumFlow #1
• Similar fixes merged upstream: PRs Replace assert False with KeyError in DAGCircuit methods gecrooks/quantumflow#133, Replace assert with proper validation in channel_to_kraus() gecrooks/quantumflow#134