Environment

• Python 3.4.3
• Ubuntu 15.04

Bang or Non-Bang

In Python, we can merge 2 list in 2 way. extend method and + operator. But they effect in different way.

extend method

• Extends the list.
• Returns None.

+ operator

• Creates a new list from 2 list.
• Returns a new merged list.
• The original lists are not changed.

Thus, extend method is a bang method, you can fall into a bag spiral if you use it by mistake.