Matplotlib极坐标图添加多元素时触发StopIteration错误的原因排查

我最近在尝试用Matplotlib绘制极坐标图，还想添加一个注释箭头指向图内的某个点，但只要同时加入多个绘图元素，就会触发StopIteration错误。

我暂时没法定位到更精确的触发条件，不过发现只要满足以下任意一种情况，图就能正常显示：

• 给r换一组不同的随机数值
• 注释掉ax.set_rticks(...)这一行（自定义径向刻度的代码）
• 注释掉ax.annotate(...)（创建箭头的代码）
• 注释掉ax.scatter(an_theta, an_r)（标记箭头终点的代码）

从报错栈的信息来看，错误应该是在绘制注释箭头的环节触发的。我当前的运行环境是JupyterLab 4.2.5（使用内嵌绘图模式）、Matplotlib 3.7.1。

可复现代码 #

import numpy as np
import matplotlib.pyplot as plt

# Radius, random numbers in [0, 1)
rng = np.random.default_rng(seed=42)
r = rng.random((24,))

# 0 to 2pi
theta = np.linspace(0, 2, 24, endpoint=False) * np.pi

fig, ax = plt.subplots(subplot_kw={'projection': 'polar'})

# Data to plot
ax.plot(theta, r)
ax.scatter(theta, r, c=r, cmap='viridis')
ax.set_rticks([1, 2, 3])  # Less radial ticks
ax.grid(True)

# Where the annotation arrow should point
an_r = 0.670
an_theta = 0.820

# Visualise arrow end-point
ax.scatter(an_theta, an_r)

# Create arrow
ax.annotate('',
            xy=(an_theta, an_r),  # theta, radius
            xytext=(0, 0), 
            arrowprops=dict(facecolor='black', shrink=0),
            horizontalalignment='right'
            )

plt.show()

报错信息 #

---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/IPython/core/formatters.py:340](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/IPython/core/formatters.py#line=339), in BaseFormatter.__call__(self, obj)
    338     pass
    339 else:
--> 340     return printer(obj)
    341 # Finally look for special method names
    342 method = get_real_method(obj, self.print_method)

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/IPython/core/pylabtools.py:152](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/IPython/core/pylabtools.py#line=151), in print_figure(fig, fmt, bbox_inches, base64, **kwargs)
    149     from matplotlib.backend_bases import FigureCanvasBase
    150     FigureCanvasBase(fig)
--> 152 fig.canvas.print_figure(bytes_io, **kw)
    153 data = bytes_io.getvalue()
    154 if fmt == 'svg':

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/backend_bases.py:2342](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/backend_bases.py#line=2341), in FigureCanvasBase.print_figure(self, filename, dpi, facecolor, edgecolor, orientation, format, bbox_inches, pad_inches, bbox_extra_artists, backend, **kwargs)
   2336     renderer = _get_renderer(
   2337         self.figure,
   2338         functools.partial(
   2339             print_method, orientation=orientation)
   2340     )
   2341     with getattr(renderer, "_draw_disabled", nullcontext)():
-> 2342         self.figure.draw(renderer)
   2344 if bbox_inches:
   2345     if bbox_inches == "tight":

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py:95](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py#line=94), in _finalize_rasterization.<locals>.draw_wrapper(artist, renderer, *args, **kwargs)
     93 @wraps(draw)
     94 def draw_wrapper(artist, renderer, *args, **kwargs):
---> 95     result = draw(artist, renderer, *args, **kwargs)
     96     if renderer._rasterizing:
     97         renderer.stop_rasterizing()

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py:72](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py#line=71), in allow_rasterization.<locals>.draw_wrapper(artist, renderer)
     69     if artist.get_agg_filter() is not None:
     70         renderer.start_filter()
---> 72     return draw(artist, renderer)
     73 finally:
     74     if artist.get_agg_filter() is not None:

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/figure.py:3140](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/figure.py#line=3139), in Figure.draw(self, renderer)
   3137         # ValueError can occur when resizing a window.
   3139 self.patch.draw(renderer)
-> 3140 mimage._draw_list_compositing_images(
   3141     renderer, self, artists, self.suppressComposite)
   3143 for sfig in self.subfigs:
   3144     sfig.draw(renderer)

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/image.py:131](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/image.py#line=130), in _draw_list_compositing_images(renderer, parent, artists, suppress_composite)
    129 if not_composite or not has_images:
    130     for a in artists:
--> 131         a.draw(renderer)
    132 else:
    133     # Composite any adjacent images together
    134     image_group = []

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py:39](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py#line=38), in _prevent_rasterization.<locals>.draw_wrapper(artist, renderer, *args, **kwargs)
     36     renderer.stop_rasterizing()
     37     renderer._rasterizing = False
---> 39 return draw(artist, renderer, *args, **kwargs)

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/projections/polar.py:1037](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/projections/polar.py#line=1036), in PolarAxes.draw(self, renderer)
   1034     self.yaxis.reset_ticks()
   1035     self.yaxis.set_clip_path(self.patch)
-> 1037 super().draw(renderer)

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py:72](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py#line=71), in allow_rasterization.<locals>.draw_wrapper(artist, renderer)
     69     if artist.get_agg_filter() is not None:
     70         renderer.start_filter()
---> 72     return draw(artist, renderer)
     73 finally:
     74     if artist.get_agg_filter() is not None:

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/axes/_base.py:3064](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/axes/_base.py#line=3063), in _AxesBase.draw(self, renderer)
   3061 if artists_rasterized:
   3062     _draw_rasterized(self.figure, artists_rasterized, renderer)
-> 3064 mimage._draw_list_compositing_images(
   3065     renderer, self, artists, self.figure.suppressComposite)
   3067 renderer.close_group('axes')
   3068 self.stale = False

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/image.py:131](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/image.py#line=130), in _draw_list_compositing_images(renderer, parent, artists, suppress_composite)
    129 if not_composite or not has_images:
    130     for a in artists:
--> 131         a.draw(renderer)
    132 else:
    133     # Composite any adjacent images together
    134     image_group = []

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py:72](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py#line=71), in allow_rasterization.<locals>.draw_wrapper(artist, renderer)
     69     if artist.get_agg_filter() is not None:
     70         renderer.start_filter()
---> 72     return draw(artist, renderer)
     73 finally:
     74     if artist.get_agg_filter() is not None:

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/text.py:2032](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/text.py#line=2031), in Annotation.draw(self, renderer)
   2030     if self.arrow_patch.figure is None and self.figure is not None:
   2031         self.arrow_patch.figure = self.figure
-> 2032     self.arrow_patch.draw(renderer)
   2033 # Draw text, including FancyBboxPatch, after FancyArrowPatch.
   2034 # Otherwise, a wedge arrowstyle can land partly on top of the Bbox.
   2035 Text.draw(self, renderer)

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py:39](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/artist.py#line=38), in _prevent_rasterization.<locals>.draw_wrapper(artist, renderer, *args, **kwargs)
     36     renderer.stop_rasterizing()
     37     renderer._rasterizing = False
---> 39 return draw(artist, renderer, *args, **kwargs)

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/patches.py:4352](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/patches.py#line=4351), in FancyArrowPatch.draw(self, renderer)
   4348 # FIXME: dpi_cor is for the dpi-dependency of the linewidth.  There
   4349 # could be room for improvement.  Maybe _get_path_in_displaycoord could
   4350 # take a renderer argument, but get_path should be adapted too.
   4351 self._dpi_cor = renderer.points_to_pixels(1.)
-> 4352 path, fillable = self._get_path_in_displaycoord()
   4354 if not np.iterable(fillable):
   4355     path = [path]

File [~/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/patches.py:4327](/home/preclineu/.conda/envs/graph_svd/lib/python3.11/site-packages/matplotlib/patches.py#line=4326), in FancyArrowPatch._get_path_in_displaycoord(self)
   4325     posB = self._convert_xy_units(self._posA_posB[1])
   4326     (posA,

问题分析 #

从报错栈的调用路径可以看到，错误最终出现在FancyArrowPatch._get_path_in_displaycoord方法中，也就是绘制注释箭头的环节。结合那些能规避错误的操作来看，这应该是Matplotlib在处理极坐标下自定义径向刻度+注释箭头的组合场景时，内部迭代逻辑出现了异常，属于Matplotlib的已知bug。

备注：内容来源于stack exchange，提问作者Valkje