When we need add a column to a DataFrame often we write:

```
df['newcol'] = 123

```

This changes (mutates) the original df object, which is not always desired.

What would be a fast and idiomatic way to do it? Here's one option, but it is about 10 slower than the above assignment.

```
df2 = concat([df, DataFrame(123, index=df.index, columns=['newcol'])], axis=1)

```