Bug summary

Calling plt.legend(...) with arguments of type <class 'itertools.chain'> 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.

Code for reproduction

import matplotlib.pyplot as plt
import numpy as np
import itertools

def flip(items, ncol):
    return itertools.chain(*[items[i::ncol] for i in range(ncol)])

x = np.arange(-2*np.pi, 2*np.pi, 0.1)
ax = plt.subplot(111)
ax.plot(x, np.sin(x), label='Sine')
ax.plot(x, np.cos(x), label='Cosine')
ax.plot(x, np.arctan(x), label='Inverse tan')

handles, labels = ax.get_legend_handles_labels()
plt.legend(flip(handles, 2), flip(labels, 2), loc=9, ncol=2)

plt.grid('on')
plt.show()

Actual outcome

Traceback (most recent call last):
File "", line 1, in
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
return gca().legend(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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
self.legend_ = mlegend.Legend(self, handles, labels, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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
if len(handles) < 2:
^^^^^^^^^^^^
TypeError: object of type 'itertools.chain' has no len()

Expected outcome

For the legend to be correctly applied, as previously.

Additional information

No response

Operating system

No response

Matplotlib Version

3.10.3

Matplotlib Backend

tkagg

Python version

No response

Jupyter version

No response

Installation

pip