assign a dataframe to part of another dataframe, warning A value is trying to be set on a copy of a slice from a DataFrame
#### A small, complete example of the issue
when i assign a dataframe to part of another dataframe, a warning occured :
D:\Program Files\Anaconda3\lib\site-packages\pandas\core\indexing.py:465: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.

my code like this:
df.iloc[:, ::2] = df.iloc[:,::2].fillna(method="ffill", axis=1)
this worked ,but there is a warning , so i change the code like this:
price = df.iloc[:,::2].copy()
price.fillna(method="ffill", axis=1, inplace=True)
price.fillna(method="bfill", axis=1, inplace=True)
df.loc[:, price.columns.tolist()] = price
still work, and still warning.

i'm wonder in the http://pandas-docs.github.io/pandas-docs-travis/indexing.html, there is example:
 df.loc[:,['B', 'A']] = df[['A', 'B']]

this code is much the same like my code, but there is no warning at all. 

i doubt that in my code ,my df is a large dataframe, so maybe somthing i don't know in the low level process of pandas .
```python
# Your code here

```

#### Expected Output

#### Output of ``pd.show_versions()``

<details>
# Paste the output here pd.show_versions() here

</details>
