13

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 ?

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6
  • 7
    Default parameters are only evaluated once. Duplicates exist, usually about lists or some such. Commented Jun 16, 2020 at 1:24
  • 1
    All the information of interest in stackoverflow.com/a/11416002/2864740 of stackoverflow.com/q/1132941/2864740 (note that the mutation works because the evaluation, of a new list, was only done once, not each call - same idea here, except backwards). Any answer here about the 'why' is a duplicate of the answer (first link), which includes relevant excerpt and reference. Commented Jun 16, 2020 at 1:25
  • 1
    One standard approach is then to default to None, and assign a value if the argument is None (eg. not specified). Commented Jun 16, 2020 at 1:27
  • Hi @user2864740 thanks for your reply, but now if I want to write a function that can accept 1 or 0 arg with type datetime, if age num is 1 and I can return the input datetime if 0 than return the current time(datetime.now()). how can I implement it ? Commented Jun 16, 2020 at 1:29
  • 1
    thanks @user2864740 this comment help me! Commented Jun 16, 2020 at 1:34

3 Answers 3

Reset to default
14

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.

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3 Comments

Just a advice, When comparing object identity use is over == see this or you can simply do if currentTime and that will also work fine, see my answer .
@Sarthak thaks for your reply "is" operator works for me but "if currentTime" don't
Sorry my bad it would be if not currentTime as but using is is good practice.
5

It's because when you define a function datetime.now() is evaluated only at that time and that value is stored in currentTime, So whenever you run fun1 currentTime value does not update.
you can Simply do this in TimeHelper.py

from datetime import datetime

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

Hope it helps :)

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Comments

0

When assigning the variable in the function you actually assign it only once when you import the function. To assign it dynamically you need to set the variable inside the function every time it is ran.

from datetime import datetime

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

Edit:

If you want to specifically say what time then:

from datetime import datetime

def fun1(time):
    currentTime = time
    print(currentTime)
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6 Comments

This will not allow the caller to specify a different time, which is the [only?] point of a default argument in the first place.
The edit requires a parameter, which negates the original desire to use a default value. See the question and main comments (some goals were clarified).
@user2864740 then the first aprroach is totally acceptable, you either set it or not. He did not want to have the same time each time. I gave two solutions what is the problem?
See the question and main comments.
@user2864740 I did, and I still don't get why make it more complicated that is should be?
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