René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: LinearSegmentedColormap.from_list cannot process list with two colors","articleBody":"### Bug summary\n\nI use `LinearSegmentedColormap.from_list` to create custom colormaps. A very simple one, which just assigns two RGB values to the two end points, does not work any more after update to matplotlib 3.8.0.\n\n### Code for reproduction\n\n```python\nfrom matplotlib import colors\r\nval_col =  [(0.0, (0.95, 0.0, 0.0)), (1.0, (0.95, 0.95, 0.95))]\r\ncm = colors.LinearSegmentedColormap.from_list('test', val_col)\n```\n\n\n### Actual outcome\n\n```\r\nFile ~/packages/macports/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/matplotlib/colors.py:1067, in LinearSegmentedColormap.from_list(name, colors, N, gamma)\r\n   1063     vals = np.linspace(0, 1, len(colors))\r\n   1065 r, g, b, a = to_rgba_array(colors).T\r\n   1066 cdict = {\r\n-\u003e 1067     \"red\": np.column_stack([vals, r, r]),\r\n   1068     \"green\": np.column_stack([vals, g, g]),\r\n   1069     \"blue\": np.column_stack([vals, b, b]),\r\n   1070     \"alpha\": np.column_stack([vals, a, a]),\r\n   1071 }\r\n   1073 return LinearSegmentedColormap(name, cdict, N, gamma)\r\n\r\nFile ~/packages/macports/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/numpy/lib/shape_base.py:652, in column_stack(tup)\r\n    650         arr = array(arr, copy=False, subok=True, ndmin=2).T\r\n    651     arrays.append(arr)\r\n--\u003e 652 return _nx.concatenate(arrays, 1)\r\n\r\nValueError: all the input array dimensions except for the concatenation axis must match exactly, but along dimension 0, the array at index 0 has size 2 and the array at index 1 has size 3\r\n```\n\n### Expected outcome\n\nThis should construct a new colormap going smoothly from red to white. This used to work until I recently updated matplotlib to 3.8.0.\n\n### Additional information\n\nThe issue seems to be that the routine `to_rgba_array` takes a length-2 object and returns r, g and b that are all length 3. Specifically, the line\r\n\r\n```python\r\nr, g, b, a = to_rgba_array(colors).T\r\n```\r\n\r\ntakes `colors`, which is `((0.95, 0.0, 0.0), (0.95, 0.95, 0.95))`, and returns `r` as `[0.95 0.95 0.95]`, and so on. It's obvious that `np.column_stack([vals, r, r])` should fail in that case, because `vals` is length 2, and `r` is length 3. This used to work in an earlier version of matplotlib, so I suppose `to_rgba_array` has changed recently.\n\n### Operating system\n\nOS/X\n\n### Matplotlib Version\n\n3.8.0\n\n### Matplotlib Backend\n\nMacOSX\n\n### Python version\n\n3.11.6\n\n### Jupyter version\n\n_No response_\n\n### Installation\n\nLinux package manager","author":{"url":"https://github.com/AgilentGCMS","@type":"Person","name":"AgilentGCMS"},"datePublished":"2023-10-17T17:44:13.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/27126/matplotlib/issues/27126"}