Python’s default arguments are evaluated once when the function is defined, not each time the function is called. Consider for example

% cat test_defaults.py 
import datetime
import time

def test_defaults(prefix, now=datetime.datetime.now()):
    print(prefix, now)

test_defaults('one')
time.sleep(1)

test_defaults('two')
time.sleep(1)

test_defaults('three', datetime.datetime.now())
time.sleep(1)

test_defaults('four')

which outputs:

% python3 test_defaults.py
one 2021-08-14 20:13:27.830082
two 2021-08-14 20:13:27.830082
three 2021-08-14 20:13:29.832481
four 2021-08-14 20:13:27.830082

Notice that 'one', 'two' and 'four' have the same time. It changes only when it is explicitly overridden.

• https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments - It is worth reading this article once end-to-end. It explains the same concept in a better way with a different example. It also goes over two more common python gotchas. Well written with high information density.
• https://www.mail-archive.com/python-list@python.org/msg443694.html - I got the initial version of this script from here.