Python datetime.now() as a default function parameter return same value in different time

Question

Now I got some problem that I can't explain and fix.
This is my first python module

TimeHelper.py

from datetime import datetime

def fun1(currentTime = datetime.now()):
    print(currentTime)

and another is

Main.py

from TimeHelper import fun1
import time

fun1()
time.sleep(5)
fun1()

When I run the Main.py, the out put is
2020-06-16 09:17:52.316714
2020-06-16 09:17:52.316714
My problem is why the time will be same in the result ? Is there any restrict when passing datetime.now() in to default parameter ?

Answer 1

I think I find the answer. Thanks for @user2864740
So I change my TimeHelper.py to this

from datetime import datetime

def fun1(currentTime = None):
    if currentTime is None:
        currentTime = datetime.now()
    print(currentTime)

and anything work in my expectation.