No need of Python code.

Create an automated action (In debug mode, menu: Settings / Technical / Automation / Automated actions) 

Model: Purchase Order.

Action to do: Create activity

Trigger: On Update.

Trigger field: Status

Before update domain: 

["|",["state","=","draft"],["state","=","sent"]]

Apply on domain: 

["|",["state","=","purchase"],["state","=","done"]]

On the tab "activity" define it as you need.