pandas.Series.shift  
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df1 = pd.DataFrame({'A':[1,2,3],'B':[1,2,3]})
df2 = pd.DataFrame(df1)
df2.B = df2.B.shift(-1)
print('id-df1 :',id(df1),'id-df2:',id(df2))
print('df1:\n',df1)
print('df2:\n',df2)
```
#### Problem description
![pdchage](https://user-images.githubusercontent.com/31531236/45286006-37cf2e00-b517-11e8-9cf5-90eea196e6ac.png)

I shift the df2  -1 , but df1 changed also. can somone help me?
I donnot know  why the df1  is changed too.

oh, I use df2 = df1.copy() solve this, but is pd.DataFrame() not better than copy()？