CODEWITHSUNDEEP
pythongoogle-app-enginedatetimegoogle-app-engine-python
pipng13579
pipng13579

Reputation: 180

Code resulting in weird datetime result on Google App Engine

I'm running a python program on Google App Engine that uses the datetime function. It's supposed to always return UTC time, but it seems to intermittently give an incorrect time. I'm not sure if there's an error with my code, or whether this is an issue on Google's side.

To get my local time (GMT +8:00), I run this function:

def SGTOffset(obj=datetime.now()):
    if isinstance(obj, datetime):
        return obj + timedelta(hours=8)
    return SGTOffset(datetime.now())

and in my main program:

today_date = commoncode.SGTOffset().date()
logging.debug('Today date: %s | Today datetime: %s' % (today_date.isoformat(), commoncode.SGTOffset().isoformat()))

In the logs, I get the following:

[25/Nov/2015:09:00:02 -0800] "GET ... etc ...
01:00:03.287     Today date: 2015-11-25 | Today datetime: 2015-11-25T15:38:20.804300

So, Google kindly formats the log datetime to my locale (GMT +8), showing that the code is run at 01:00:03.287 (26th Nov, GMT +8). Also, this is confirmed by the timestamp 25/Nov/2015:09:00:02 -0800 provided. So the code was run at 25/Nov/2015 17:00:02 UTC time.

However, my code is outputting the wrong time. The datetime that is being generated in the code 2015-11-25T15:38:20.804300 has the timezone of GMT-9:30 instead of UTC time. (Because SGToffset() adds 8 hours to datetime)

This is quite catastrophic as I use the local datetime in many areas of my program. This is also happening intermittently only, because yesterday, the same code ran and got this log:

[24/Nov/2015:09:00:00 -0800] "GET ... etc ...
01:00:02.237     Today date: 2015-11-25 | Today datetime: 2015-11-25T01:00:01.768140

Which is correct! (Google's log timestamp 01:00:02.237 matches the time generated by SGTOffset() which is 01:00:01)

Could I know what is wrong with my program, or whether this is an issue with Google App Engine?

Thank you spending time to read this question!

Upvotes: 0

Views: 434

Answers (2)

minou
minou

Reputation: 16553

I'm adding a quick answer to give you suggestions to make your code more readable:

  • obj is not a good variable name because it is not informative
  • No need for a recursive call to the function
  • Better to not use isinstance because is None gives you the needed functionality and your code will not work if some other instance type is given anyway.

Here is my suggestion:

def SGTOffset(dt=None):
    if dt is None:
        dt = datetime.now()
    return dt + timedelta(hours=8)

Or if you prefer brevity:

def SGTOffset(dt=None):
    return (dt or datetime.now()) + timedelta(hours=8)

Upvotes: 1

pipng13579
pipng13579

Reputation: 180

The problem lies with the code.

Python stores a default value of the parameter obj for the function SGTOffset() when it is first defined (when the function object is first instantiated), instead of whenever the function is called as I intuitively expected. So, the datetime value will reflect the start time of the instance in GAE.

In order to get the current time whenever I call SGTOffset() without any parameters, I should instead have used:

def SGTOffset(obj=None): # Instead of obj=datetime.now() which is converted to obj='specific datetime' when instantiated
    if isinstance(obj, datetime):
        return obj + timedelta(hours=8)
    return SGTOffset(datetime.now())

In this case, datetime.now() is called dynamically whenever it is required.

I arrived at this solution after viewing a question about old datetime values being returned.

Upvotes: 3

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