This is a follow-up on [this question](https://stackoverflow.com/questions/19125091/pandas-merge-how-to-avoid-duplicating-columns?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa)

I have two dataframes that I want to merge, but I want to avoid to have duplicate columns, so I'm doing:

```
cols_to_use = df2.columns-df1.columns

```

If I print cols\_to\_use I get this:

```
 Index([col1,col2,col3...],dtype=object)

```

However, I have one column that I need it to be kept in both dfs, it is the co\_code. That's because I'm going to merge on that column.

My question is: how to add one extra column to cols\_to\_use? I need it to look like this:

```
Index([co_code,col1,col2,col3...],dtype=object)

```

I tried different synthaxes but nothing seemed to work:

```
cols_to_use = df2.columns-df1.columns+'co_code'
cols_to_use = df2.columns-df1.columns+['co_code']
cols_to_use = df2.columns-df1.columns+df2['co_code'].columns

```