Python KNN多标签散点图绘制遇InvalidIndexError，求解决方法

问题背景 #

拥有CSV数据，想要用Matplotlib绘制以纬度（latitude）和经度（longitude）为X、Y轴的多标签散点图，现有代码如下：

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt
from sklearn import datasets
from sklearn.model_selection import train_test_split
from matplotlib.colors import ListedColormap

cmap = ListedColormap(['#FF0000','#00FF00','#0000FF'])

df = pd.DataFrame(data)
X = df.drop(['Wisata','Link Gambar','Nama','Region'],axis = 1) # 仅保留纬度和经度
y = df['Wisata'] # 标签列
X_train,X_test,y_train,y_test = train_test_split(X,y,test_size = 0.2,random_state=1)

报错情况 #

执行以下绘制代码时出现错误：

plt.figure()
plt.scatter(X[:,0],X[:,1],c=y,cmap=cmap,edgecolors='k',s=20)
plt.show()

报错信息：

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
D:\Anaconda\lib\site-packages\pandas\core\indexes\base.py in get_loc(self, key, method, tolerance)
   3628             try:
-> 3629                 return self._engine.get_loc(casted_key)
   3630             except KeyError as err:

D:\Anaconda\lib\site-packages\pandas\_libs\index.pyx in pandas._libs.index.IndexEngine.get_loc()

D:\Anaconda\lib\site-packages\pandas\_libs\index.pyx in pandas._libs.index.IndexEngine.get_loc()

TypeError: '(slice(None, None, None), 0)' is an invalid key

During handling of the above exception, another exception occurred:

InvalidIndexError                         Traceback (most recent call last)
~\AppData\Local\Temp\ipykernel_12876\2344342603.py in <module>
      1 plt.figure()
----> 2 plt.scatter(X[:,0],X[:,1],c=y,cmap=cmap,edgecolors='k',s=20)
      3 plt.show()

D:\Anaconda\lib\site-packages\pandas\core\frame.py in __getitem__(self, key)
   3503             if self.columns.nlevels > 1:
   3504                 return self._getitem_multilevel(key)
-> 3505             indexer = self.columns.get_loc(key)
   3506             if is_integer(indexer):
   3507                 indexer = [indexer]

D:\Anaconda\lib\site-packages\pandas\core\indexes\base.py in get_loc(self, key, method, tolerance)
   3634                 #  InvalidIndexError. Otherwise we fall through and re-raise
   3635                 #  the TypeError.
-> 3636                 self._check_indexing_error(key)
   3637                 raise
   3638 

D:\Anaconda\lib\site-packages\pandas\core\indexes\base.py in _check_indexing_error(self, key)
   5649             # if key is not a scalar, directly raise an error (the code below
   5650             # would convert to numpy arrays and raise later any way) - GH29926
-> 5651             raise InvalidIndexError(key)
   5652 
   5653     @cache_readonly

InvalidIndexError: (slice(None, None, None), 0)

<Figure size 640x480 with 0 Axes>

错误原因 #

X是Pandas的DataFrame类型，不是numpy数组，不能使用[:,0]这种numpy数组的二维切片语法，这会触发Pandas的索引解析错误。

解决方法 #

方法1：将DataFrame转为numpy数组 #

把X转换成numpy数组后，即可使用切片索引：

plt.figure()
# 转换为numpy数组
X_np = X.to_numpy()
plt.scatter(X_np[:,0], X_np[:,1], c=y, cmap=cmap, edgecolors='k', s=20)
plt.xlabel('经度')
plt.ylabel('纬度')
plt.title('多标签散点图')
plt.show()

方法2：直接通过列名索引 #

更直观的方式是直接用列名获取经纬度数据，无需依赖列的位置（需根据实际CSV列名调整）：

plt.figure()
# 假设CSV中经度列名为longitude，纬度列名为latitude
plt.scatter(X['longitude'], X['latitude'], c=y, cmap=cmap, edgecolors='k', s=20)
plt.xlabel('经度')
plt.ylabel('纬度')
plt.title('多标签散点图')
plt.show()

补充说明 #

• 如果y是字符串类型的标签，Matplotlib的scatter函数要求c参数为数值类型，需要先将标签转为数值编码：

# 将字符串标签转为数值
y_num = pd.factorize(y)[0]
plt.scatter(X['longitude'], X['latitude'], c=y_num, cmap=cmap, edgecolors='k', s=20)

• 若要绘制训练集/测试集的散点图，只需将X替换为X_train/X_test，y替换为y_train/y_test即可。

内容的提问来源于stack exchange，提问作者Jessen Jie