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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"BUG: CompositeGenericTransform.contains_branch_separately only checks second component, ignoring first","articleBody":"## Bug summary\n\n`CompositeGenericTransform.contains_branch_separately()` only checks the second component (`_b`) of the composite transform, completely ignoring the first component (`_a`). This means if a composed transform is `A + B` and the target transform is a subtree of `A`, it returns `(False, False)` instead of the correct result.\n\n## Code for reproduction\n\nA composite transform `A + B` where the query transform is found in component `A`:\n\n```python\nimport matplotlib.transforms as mtransforms\nfrom matplotlib.transforms import Transform, BlendedGenericTransform\n\n# Two independent non-affine transforms\nclass NonAffine(Transform):\n    input_dims = 2\n    output_dims = 2\n    is_separable = True\n    is_affine = False\n    def transform_non_affine(self, values): return values\n\ntn1 = NonAffine()\ntn2 = NonAffine()\n\n# Blended transform: _x=tn1, _y=tn2\nblend = BlendedGenericTransform(tn1, tn2)\n\n# Composite: _a=blend, _b=tn1\ncomposite = blend + tn1\n\n# tn1 is a subtree of _a.x, but NOT of _b\nx, y = composite.contains_branch_separately(tn1)\nprint(f'x={x}, y={y}')  # Got: False, False. Expected: True, False\n```\n\n## Actual outcome\n\n`contains_branch_separately` returns `(False, False)`.\n\n## Expected outcome\n\n`contains_branch_separately` should return `(True, False)` since `tn1` is in the `_a.x` branch.\n\n## Buggy code location\n\nhttps://github.com/matplotlib/matplotlib/blob/main/lib/matplotlib/transforms.py#L2450-L2457\n\n```python\ndef contains_branch_separately(self, other_transform):\n    if self.output_dims != 2:\n        raise ValueError('contains_branch_separately only supports '\n                         'transforms with 2 output dimensions')\n    if self == other_transform:\n        return (True, True)\n    return self._b.contains_branch_separately(other_transform)  # \u003c-- only _b!\n```\n\nThe fix should check both children:\n\n```python\ndef contains_branch_separately(self, other_transform):\n    if self.output_dims != 2:\n        raise ValueError('contains_branch_separately only supports '\n                         'transforms with 2 output dimensions')\n    if self == other_transform:\n        return (True, True)\n    a_result = self._a.contains_branch_separately(other_transform)\n    b_result = self._b.contains_branch_separately(other_transform)\n    return (a_result[0] or b_result[0], a_result[1] or b_result[1])\n```\n\n## Impact\n\nThis bug affects any code that uses `contains_branch_separately` on composite transforms, including autoscaling and axis limit updates in `axes/_base.py`, `axes/_axes.py`, `collections.py`, and `contour.py`. When a composite transform's first component contains `transData`, the method would incorrectly return `False`, potentially causing autoscaling to not trigger.\n\n## Matplotlib version\n\nmain branch (all versions with this code).\n\nNote: `CompositeAffine2D` does NOT have this bug because it doesn't override `contains_branch_separately` and inherits the base `Transform` implementation which uses `contains_branch` -\u003e `_iter_break_from_left_to_right`, which correctly iterates both children.\n","author":{"url":"https://github.com/sanrishi","@type":"Person","name":"sanrishi"},"datePublished":"2026-07-22T21:27:22.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/32099/matplotlib/issues/32099"}