KeyError: 0当尝试访问熊猫DataFrame中的一行时

Question

我正在学习一本关于机器学习的书--学习机器学习，下面是显示MNIST图像的代码，但是当我尝试从数据集中索引单个图像时，我会得到一个keyError 0。

from sklearn.datasets import fetch_openml

mnist = fetch_openml('mnist_784', version=1)

X, y = mnist["data"], mnist["target"]

some_digit = X[0]

下面是在jupyter笔记本中运行单元格时遇到的错误。

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
File ~\anaconda3\anaconda-py\envs\data-science\lib\site-packages\pandas\core\indexes\base.py:3621, in Index.get_loc(self, key, method, tolerance)
   3620 try:
-> 3621     return self._engine.get_loc(casted_key)
   3622 except KeyError as err:

File ~\anaconda3\anaconda-py\envs\data-science\lib\site-packages\pandas\_libs\index.pyx:136, in pandas._libs.index.IndexEngine.get_loc()

File ~\anaconda3\anaconda-py\envs\data-science\lib\site-packages\pandas\_libs\index.pyx:163, in pandas._libs.index.IndexEngine.get_loc()

File pandas\_libs\hashtable_class_helper.pxi:5198, in pandas._libs.hashtable.PyObjectHashTable.get_item()

File pandas\_libs\hashtable_class_helper.pxi:5206, in pandas._libs.hashtable.PyObjectHashTable.get_item()

KeyError: 0

The above exception was the direct cause of the following exception:

KeyError                                  Traceback (most recent call last)
Input In [10], in <cell line: 3>()
      1 import matplotlib as mpl
      2 import matplotlib.pyplot as plt
----> 3 some_digit = X[0]
      4 some_digit_image = some_digit.reshape(28, 28)
      5 plt.imshow(some_digit_image, cmap = mpl.cm.binary, interpolation="nearest")

File ~\anaconda3\anaconda-py\envs\data-science\lib\site-packages\pandas\core\frame.py:3505, in DataFrame.__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]

File ~\anaconda3\anaconda-py\envs\data-science\lib\site-packages\pandas\core\indexes\base.py:3623, in Index.get_loc(self, key, method, tolerance)
   3621     return self._engine.get_loc(casted_key)
   3622 except KeyError as err:
-> 3623     raise KeyError(key) from err
   3624 except TypeError:
   3625     # If we have a listlike key, _check_indexing_error will raise
   3626     #  InvalidIndexError. Otherwise we fall through and re-raise
   3627     #  the TypeError.
   3628     self._check_indexing_error(key)

KeyError: 0

Answer 1

这将尝试访问名为0的列。

df[0]

而您的dataframe没有，因此KeyError。

若要按其索引访问dataframe的行，必须使用：

df.loc[0]

# or

df.iloc[0]

哪一个取决于dataframe的索引值，也取决于您想要完成的确切任务。它们也可用于指定列。