python – ValueError：长度不匹配：在pandas数据帧中创建分层列时,预期的轴有0个元素

2019年8月3日 463次阅读

我正在阅读关于Pandas中的层次索引的
documentation.我尝试从中测试示例以创建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]])

但是,它会抛出一个错误：

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

我没有看到我的代码有任何问题.有什么想法发生了什么？

最佳答案 问题是你有一个空数据框,它有零列,你试图为它分配一个四列多索引;如果最初创建一个包含四列的空数据框,则错误将消失：

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]])

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
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