René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: Axes.grouped_bar() with non-string orientation (e.g., NumPy array) raises ambiguous truth-value error instead of clean ValueError","articleBody":"### Bug summary\n\nPassing a non-string value (like a NumPy array) to orientation in Axes.grouped_bar() triggers a misleading “ambiguous truth value” error from _api.check_in_list. The function should instead raise a clear ValueError stating the value is not valid.\n\n\n### Code for reproduction\n\n```Python\nimport numpy as np\nimport matplotlib.pyplot as plt\n\nfig, ax = plt.subplots()\nax.grouped_bar([[1, 2, 3], [3, 2, 1]], orientation=np.array([1, 2, 3]))\n```\n\n### Actual outcome\n\nAxes.grouped_bar() fails inside _api.check_in_list() because NumPy arrays cannot be evaluated in a boolean context.\nInstead of raising a clear validation error, it produces a misleading NumPy truth-value error, causing the test to fail.\n\n\u003cimg width=\"1600\" height=\"1172\" alt=\"Image\" src=\"https://github.com/user-attachments/assets/0a24242f-ae50-45ba-a15e-af44768f4333\" /\u003e\n\n### Expected outcome\n\nAfter applying the fix, invalid inputs (including NumPy arrays) are handled gracefully, and all tests expecting a clear ValueError now pass.\n\n\n\n### Additional information\n\n\n1. Conditions under which this bug happens:\n\nThe bug occurs whenever the orientation parameter passed to Axes.grouped_bar() is not a string, such as:\norientation = np.array([1, 2, 3])  # NumPy array  \norientation = 1                    # integer  \norientation = None                 # NoneType\n\nThese types trigger an internal truth-value evaluation inside _api.check_in_list, which expects a scalar comparable value (like a string).\n\n2. Edge cases affected:\n\nNumPy arrays (np.array([...]))\nNon-string types (integers, floats, None, lists, etc.)\nAny custom object that doesn’t implement __eq__ safely with strings\n\n3. Behavior in earlier versions:\n\nThis issue has likely existed since the introduction of Axes.grouped_bar() (Matplotlib 3.11, provisional API).\nOther similar Matplotlib APIs (e.g., barh, stem, etc.) already validate string-type enums, so they are not affected.\n\n4. Root cause (why this happens):\n\nThe function directly calls:\n\n_api.check_in_list([\"vertical\", \"horizontal\"], orientation=orientation)\n\nWhen orientation is a NumPy array, the expression val not in values inside check_in_list() performs elementwise comparison, returning an array of booleans.\n\nPython then attempts to interpret that array as a single truth value, which triggers NumPy’s error:\nValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all().\n\n5. Proposed fix (confirmed working):\n\nAdd an early type guard before calling _api.check_in_list():\n\nif not isinstance(orientation, str):\n    raise ValueError(f\"{orientation!r} is not a valid value for orientation\")\n_api.check_in_list([\"vertical\", \"horizontal\"], orientation=orientation)\n\n-- This ensures non-string inputs are rejected immediately and consistently,\n-- prevents the ambiguous truth-value error, and\n-- aligns grouped_bar() with Matplotlib’s standard API validation behavior.\n\n### Operating system\n\nMacOS\n\n### Matplotlib Version\n\n3.11.0.dev1446+g319295e28.d20251030\n\n### Matplotlib Backend\n\nmacosx\n\n### Python version\n\nPython 3.13.7\n\n### Jupyter version\n\n_No response_\n\n### Installation\n\npip","author":{"url":"https://github.com/ilakkmanoharan","@type":"Person","name":"ilakkmanoharan"},"datePublished":"2025-10-30T20:25:35.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":7},"url":"https://github.com/30706/matplotlib/issues/30706"}