ValueError：长度不匹配：期望轴在熊猫数据框中创建分层列时有0个元素

Question

我正在通过documentation关于Pandas中的分层索引。我试图从测试它的范例，具有层次索引创建一个空的数据框中：

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]]) 

然而，它抛出一个错误：

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 

我看不到我的代码任何问题。任何想法发生了什么？

Answer 1

问题是你有一个空的数据框有零列，你试图给它分配一个四列多索引;如果您最初创建四列的一个空的数据帧，错误将会消失：

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]]) 

或者你可以用多指标创建空的数据帧如下：

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