Pandas 未在分配索引后添加列 Python3.5.0

Question

我试过以下说法：

import pandas as pd
df = pd.DataFrame(index=[i for i in range(5)])
df.columns = ["res"+str(i) for i in range(5)]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python35\lib\site-packages\pandas\core\generic.py", line 5080, in __setattr__
    return object.__setattr__(self, name, value)
  File "pandas\_libs\properties.pyx", line 69, in pandas._libs.properties.AxisProperty.__set__
  File "C:\Python35\lib\site-packages\pandas\core\generic.py", line 638, in _set_axis
    self._data.set_axis(axis, labels)
  File "C:\Python35\lib\site-packages\pandas\core\internals\managers.py", line 155, in set_axis
    'values have {new} elements'.format(old=old_len, new=new_len))
ValueError: Length mismatch: Expected axis has 0 elements, new values have 5 elements

不明白这是什么原因。如果我有索引，那么为什么没有添加该列？

Answer 1

import pandas as pd
df = pd.DataFrame(index= range(5))
for i in range(5):
  df['res'+str(i)]= '0'
print (df)

  res0 res1 res2 res3 res4
0    0    0    0    0    0
1    0    0    0    0    0
2    0    0    0    0    0
3    0    0    0    0    0
4    0    0    0    0    0

Answer 2

我认为问题首先是没有创建列DataFrame，所以如果要分配列名它失败了。

解决方案是将参数 columns 添加到 DataFrame 构造函数，索引应简化 - 仅分配 range：

df = pd.DataFrame(index= range(5), columns=["res"+str(i) for i in range(5)])
print (df)
  res0 res1 res2 res3 res4
0  NaN  NaN  NaN  NaN  NaN
1  NaN  NaN  NaN  NaN  NaN
2  NaN  NaN  NaN  NaN  NaN
3  NaN  NaN  NaN  NaN  NaN
4  NaN  NaN  NaN  NaN  NaN

如果想在只有索引的 DataFrame 之后分配可以使用DataFrame.reindex:

df = pd.DataFrame(index=range(5))
df = df.reindex(["res"+str(i) for i in range(5)], axis=1)
print (df)
   res0  res1  res2  res3  res4
0   NaN   NaN   NaN   NaN   NaN
1   NaN   NaN   NaN   NaN   NaN
2   NaN   NaN   NaN   NaN   NaN
3   NaN   NaN   NaN   NaN   NaN
4   NaN   NaN   NaN   NaN   NaN