how to test if one python module has been imported?

Question

How to test if a module has been imported in python?

for example I need the basics:

if not has_imported(\"sys\"):
   import sys

als

Answer 1

I feel the answer that has been accepted is not fully correct.

Python still has overhead when importing the same module multiple times. Python handles it without giving you an error, sure, but that doesn't mean it won't slow down your script. As you will see from the URL below, there is significant overhead when importing a module multiple times.

For example, in a situation where you may not need a certain module except under a particular condition, if that module is large or has a high overhead then there is reason to import only on condition. That does not explicitly mean you are a sloppy coder either.

https://wiki.python.org/moin/PythonSpeed/PerformanceTips#Import_Statement_Overhead