os.getgroups() can be used to get the username of the user running a Python script.

#!/usr/bin/python3
import os
print(f"group IDs = {os.getgroups()}")

Which should return something like this.

group IDs = [100, 2001]

Or like this, to loop through the group IDs.

#!/usr/bin/python3
import os

group_ids = os.getgroups()

for group_id in group_ids:
  print(f"group ID = {group_id}")

Which should return something like this.

group ID = 100
group ID = 2001

Taking this a step further, grp and getgrgid and gr_name can be used to return the group name.

#!/usr/bin/python3
import grp
import os

group_ids = os.getgroups()

for group_id in group_ids:
  print(f"group ID = {group_id}")
  print(f"group name = {grp.getgrgid(group_id).gr_name}")

Which should return something like this.

group ID = 100
group name = users
group ID = 2001
group name = mygroup

Or like this, as a one liner, to create a list that contains the group names.

#!/usr/bin/python
import grp
import os

group_names = [grp.getgrgid(group_id).gr_name for group_id in os.getgroups()]