How to add a suffix (or prefix) to each column name?

You can use a list comprehension:

df.columns = [str(col) + '_x' for col in df.columns]

There are also built-in methods like .add_suffix() and .add_prefix() as mentioned in another answer.

The following is the nicest way to add suffix in my opinion.

df = df.add_suffix('_some_suffix')

As it is a function that is called on DataFrame and returns DataFrame - you can use it in chain of the calls.