lessThan() in subclassed QSortFilterProxyModel being called too many times and I can’t figure why

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Summary

The issue arises from excessive calls to the lessThan() method in a subclassed QSortFilterProxyModel, leading to significant performance degradation when sorting large datasets. The root cause is the interaction between manual invalidation and the proxy model’s internal sorting mechanisms, resulting in redundant comparisons.

Root Cause

  • Manual invalidation via invalidate() triggers a full resort, causing lessThan() to be called repeatedly.
  • Internal sorting logic in QSortFilterProxyModel re-evaluates comparisons for each row, even when not necessary.
  • The number of redundant calls scales with the number of rows, following the pattern: (1 + number_of_rows) * number_of_traced_calls.

Why This Happens in Real Systems

  • Dynamic sorting disabled: Setting setDynamicSortFilter(False) forces a full resort on invalidation, increasing lessThan() calls.
  • Proxy model inefficiencies: The proxy model lacks optimization for custom sorting, leading to redundant comparisons.
  • Manual invalidation: Frequent calls to invalidate() exacerbate the issue by triggering unnecessary resorts.

Real-World Impact

  • Performance degradation: Sorting 100+ rows takes 2+ seconds due to 70,000+ lessThan() calls.
  • Scalability issues: Larger datasets become impractical to sort, hindering application usability.
  • Resource consumption: Excessive computations waste CPU cycles and slow down the entire application.

Example or Code

class EventListProxyModel(QSortFilterProxyModel):
    def lessThan(self, left: QModelIndex, right: QModelIndex) -> bool:
        # Custom sorting logic here
        return super().lessThan(left, right)

How Senior Engineers Fix It

  • Optimize invalidation: Use invalidateFilter() instead of invalidate() to avoid full resorts.
  • Cache comparisons: Implement a caching mechanism in lessThan() to avoid redundant checks.
  • Custom sorting: Override sort() to implement efficient sorting logic tailored to the dataset.
  • Batch updates: Use beginResetModel() and endResetModel() for bulk updates to minimize invalidations.

Why Juniors Miss It

  • Lack of understanding: Juniors may not grasp the internal workings of QSortFilterProxyModel and its sorting mechanisms.
  • Overlooking invalidation: They might overuse invalidate() without considering its performance implications.
  • Ignoring call patterns: Juniors may fail to analyze the scaling pattern of lessThan() calls, missing the root cause.
  • Not optimizing for scale: They often focus on functionality rather than performance, leading to inefficiencies in large datasets.

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