Question

我尝试了以下语句：

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

不了解其原因。如果我有索引，那为什么不添加列呢？

Answer 1

我认为问题是首先没有创建列DataFrame，因此如果要分配列名称，则会失败。

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

如果要在仅具有索引的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

Answer 2

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

熊猫在分配索引Python3.5.0后未添加列

2 个答案: