Python ValueError：长度不匹配：在熊猫数据框中创建分层列时，预期轴有 0 个元素
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ValueError: Length mismatch: Expected axis has 0 elements while creating hierarchical columns in pandas dataframe
提问by Peaceful
I was going through the documentationabout the hierarchical indexing in Pandas. I tried testing the examples from it to create an empty dataframe with hierarchical indexing:
我正在阅读有关 Pandas 中分层索引的文档。我尝试测试它的示例以创建一个带有分层索引的空数据框：
In [5]: df = pd.DataFrame()
In [6]: df.columns = pd.MultiIndex(levels = [['first', 'second'], ['a', 'b']], labels = [[0, 0, 1, 1], [0, 1, 0, 1]])
However, it throws an error:
但是，它会引发错误：
ValueError Traceback (most recent call last)
<ipython-input-6-dd823f9b8d22> in <module>()
----> 1 df.columns = pd.MultiIndex(levels = [['first', 'second'], ['a', 'b']], labels = [[0, 0, 1, 1], [0, 1, 0, 1]])
/usr/local/lib/python3.4/dist-packages/pandas/core/generic.py in __setattr__(self, name, value)
2755 try:
2756 object.__getattribute__(self, name)
-> 2757 return object.__setattr__(self, name, value)
2758 except AttributeError:
2759 pass
pandas/src/properties.pyx in pandas.lib.AxisProperty.__set__ (pandas/lib.c:44873)()
/usr/local/lib/python3.4/dist-packages/pandas/core/generic.py in _set_axis(self, axis, labels)
446
447 def _set_axis(self, axis, labels):
--> 448 self._data.set_axis(axis, labels)
449 self._clear_item_cache()
450
/usr/local/lib/python3.4/dist-packages/pandas/core/internals.py in set_axis(self, axis, new_labels)
2800 raise ValueError('Length mismatch: Expected axis has %d elements, '
2801 'new values have %d elements' %
-> 2802 (old_len, new_len))
2803
2804 self.axes[axis] = new_labels
ValueError: Length mismatch: Expected axis has 0 elements, new values have 4 elements
I don't see any problem with my code. Any ideas what is happening?
我的代码没有任何问题。任何想法发生了什么？
回答by Psidom
The problem is that you have an empty data frame which has zero columns, and you are trying to assign a four columns multi-index to it; If you create an empty data frame of four columns initially, the error will be gone:
问题是您有一个包含零列的空数据框，并且您试图为其分配一个四列多索引；如果最初创建一个四列的空数据框，错误将消失：
df = pd.DataFrame(pd.np.empty((0, 4)))
df.columns = pd.MultiIndex(levels = [['first', 'second'], ['a', 'b']], labels = [[0, 0, 1, 1], [0, 1, 0, 1]])
Or you can create empty data frame with the multi-index as follows:
或者您可以使用多索引创建空数据框，如下所示：
multi_index = pd.MultiIndex(levels = [['first', 'second'], ['a', 'b']], labels = [[0, 0, 1, 1], [0, 1, 0, 1]])
df = pd.DataFrame(columns=multi_index)
df
# first second
# a b a b
