Python mutable default arguments: why your list keeps growing (tested examples)

<p>If you spend enough time writing backend Python code, you will eventually ship a "ghost bug." </p> <p>You know the type. A web server where User B somehow sees a cached item

<p>Here is a bug that is easy to ship. A function takes a dict of default settings and hands each new user a copy. Someone adds a tag to one user. Every future user now has that