如何用Matplotlib生成动画子图？ValueError报错排查

问题描述 #

我想用Python生成一个包含4个动画的图，尝试运行以下代码时出现错误：ValueError: not enough values to unpack (expected 4, got 2)

原代码：

fig, (axF, axS, axH, axHamilt) = plt.subplots(2, 2)

graphF,=axF.plot([],[])
graphS,=axS.plot([],[])
graphH,=axH.plot([],[])
graphHamilt,=axHamilt.plot([],[])
axF.set_ylim(-1.5, 1.5)
axF.set_xlim(ri, rf)
axF.grid()
axS.set_ylim(-1.5, 1.5)
axS.set_xlim(ri, rf)
axS.grid()
axH.set_ylim(-1.5, 1.5)
axH.set_xlim(ri, rf)
axH.grid()
axHamilt.set_ylim(-1.5, 1.5)
axHamilt.set_xlim(ri, rf)
axHamilt.grid()
def animate(i):
  graphF.set_data(r,Sevol[i])
  graphS.set_data(r, Fevol[i])
  graphH.set_data(r, Hevol[i])
  graphHamilt.set_data(r, Hevol[i])
  return graphF, graphS, graphH, graphHamilt
ani = FuncAnimation(fig, animate, frames=N_plots, interval=100)

plt.show()

错误原因 #

plt.subplots(2, 2)返回的子图对象是二维数组结构（形状为(2,2)），即[[ax1, ax2], [ax3, ax4]]。你直接用(axF, axS, axH, axHamilt)去解包，相当于尝试从一个包含2个元素（两行子图）的结构里取出4个变量，自然会触发"值不足"的错误。

解决方案 #

有两种简单的修复方式：

方法1：将子图数组扁平化

把二维的子图数组转成一维后再解包：

fig, axes = plt.subplots(2, 2)
axF, axS, axH, axHamilt = axes.flatten()

方法2：按二维结构解包

直接匹配子图的二维嵌套结构来解包：

fig, ((axF, axS), (axH, axHamilt)) = plt.subplots(2, 2)

修正后的完整代码示例 #

以方法2为例，修正后的代码如下：

import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation

# 提前定义示例所需变量
ri, rf = 0, 10
r = range(ri, rf)
Sevol = [[0]*len(r) for _ in range(10)]
Fevol = [[0]*len(r) for _ in range(10)]
Hevol = [[0]*len(r) for _ in range(10)]
N_plots = 10

fig, ((axF, axS), (axH, axHamilt)) = plt.subplots(2, 2)

graphF,=axF.plot([],[])
graphS,=axS.plot([],[])
graphH,=axH.plot([],[])
graphHamilt,=axHamilt.plot([],[])
axF.set_ylim(-1.5, 1.5)
axF.set_xlim(ri, rf)
axF.grid()
axS.set_ylim(-1.5, 1.5)
axS.set_xlim(ri, rf)
axS.grid()
axH.set_ylim(-1.5, 1.5)
axH.set_xlim(ri, rf)
axH.grid()
axHamilt.set_ylim(-1.5, 1.5)
axHamilt.set_xlim(ri, rf)
axHamilt.grid()
def animate(i):
  graphF.set_data(r,Sevol[i])
  graphS.set_data(r, Fevol[i])
  graphH.set_data(r, Hevol[i])
  graphHamilt.set_data(r, Hevol[i])
  return graphF, graphS, graphH, graphHamilt
ani = FuncAnimation(fig, animate, frames=N_plots, interval=100)

plt.show()

内容的提问来源于stack exchange，提问作者User8563