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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: legend no longer works with itertools.chain","articleBody":"### Bug summary\n\nCalling `plt.legend(...)` with arguments of type `\u003cclass 'itertools.chain'\u003e` no longer works, and raises `TypeError: object of type 'itertools.chain' has no len()`. This used to work - some update to the source code seems to have caused this bug.\n\n### Code for reproduction\n\n```Python\nimport matplotlib.pyplot as plt\nimport numpy as np\nimport itertools\n\ndef flip(items, ncol):\n    return itertools.chain(*[items[i::ncol] for i in range(ncol)])\n\nx = np.arange(-2*np.pi, 2*np.pi, 0.1)\nax = plt.subplot(111)\nax.plot(x, np.sin(x), label='Sine')\nax.plot(x, np.cos(x), label='Cosine')\nax.plot(x, np.arctan(x), label='Inverse tan')\n\nhandles, labels = ax.get_legend_handles_labels()\nplt.legend(flip(handles, 2), flip(labels, 2), loc=9, ncol=2)\n\nplt.grid('on')\nplt.show()\n```\n\n### Actual outcome\n\nTraceback (most recent call last):\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\n  File \"C:\\Users\\jonny\\AppData\\Local\\Packages\\PythonSoftwareFoundation.Python.3.11_qbz5n2kfra8p0\\LocalCache\\local-packages\\Python311\\site-packages\\matplotlib\\pyplot.py\", line 3628, in legend\n    return gca().legend(*args, **kwargs)\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"C:\\Users\\jonny\\AppData\\Local\\Packages\\PythonSoftwareFoundation.Python.3.11_qbz5n2kfra8p0\\LocalCache\\local-packages\\Python311\\site-packages\\matplotlib\\axes\\_axes.py\", line 337, in legend\n    self.legend_ = mlegend.Legend(self, handles, labels, **kwargs)\n                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"C:\\Users\\jonny\\AppData\\Local\\Packages\\PythonSoftwareFoundation.Python.3.11_qbz5n2kfra8p0\\LocalCache\\local-packages\\Python311\\site-packages\\matplotlib\\legend.py\", line 462, in __init__\n    if len(handles) \u003c 2:\n       ^^^^^^^^^^^^\nTypeError: object of type 'itertools.chain' has no len()\n\n### Expected outcome\n\nFor the legend to be correctly applied, as previously.\n\n### Additional information\n\n_No response_\n\n### Operating system\n\n_No response_\n\n### Matplotlib Version\n\n3.10.3\n\n### Matplotlib Backend\n\ntkagg\n\n### Python version\n\n_No response_\n\n### Jupyter version\n\n_No response_\n\n### Installation\n\npip","author":{"url":"https://github.com/jonnybolton16","@type":"Person","name":"jonnybolton16"},"datePublished":"2025-05-19T14:46:43.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/30078/matplotlib/issues/30078"}