Running Python at a Certain Time with Datetime

Question

I would like to execute a portion of a script at 8 am each day. I have created a simplified test case that has no syntax error, but does not work properly. I think it may be because my if statement is using the time as a string, but it won't compile any other way. What am I doing wrong?

import datetime

while True:
    if datetime.datetime.now().time()  == "19:00:00.000000":
        print "it's time!"

Given that you are specific to a millisecond, it may not, as millisecond is very short time and its easy to miss it. Just stick to cron, google can lead you into the right path. — Tymoteusz Paul, Nov 10 '14 at 00:36

unutbu · Accepted Answer · 2014-11-10T03:29:56.263

3

If you are on a system with cron, then it would be better to set up a cron job. However, your problem is fixable from within Python:

First, as you noted, datetime.datetime.now().time() returns a datetime.time object, not a string:

In [89]: datetime.datetime.now().time()
Out[89]: datetime.time(19, 36, 13, 388625)

Also, although datetime.datetime.now().time() == datetime.time(19, 0) would be valid Python, the chance that you happen to execute time() at just the right moment is very slim since datetime.datetime.now() has microsecond resolution. So it would be better to test if the current time falls within some range.

However, since you only want to run the function once per day, you could instead measure the total number of seconds between now and when you want to run the function and sleep that number of seconds:

import datetime as DT
import time

while True:
    now = DT.datetime.now()
    target = DT.datetime.combine(DT.date.today(), DT.time(hour=8))
    if target < now:
        target += DT.timedelta(days=1)

    time.sleep((target-now).total_seconds())
    # do something

edited Nov 10 '14 at 03:29

answered Nov 10 '14 at 00:36

unutbu

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+1 for updating the answer with big warning about millisecond precision. – Tymoteusz Paul Nov 10 '14 at 00:47
So I only want to run the code once a day so if I test within a range, how to I ensure it only happens once? – user2242044 Nov 10 '14 at 00:47
In that case I would arrange to sleep until it was time to run the function. I've updated the code to show what I mean. – unutbu Nov 10 '14 at 01:03
This works well, thanks. Does this eat up a lot of computer cpu? – user2242044 Nov 10 '14 at 01:26
No, most of the time [it will be sleeping](http://stackoverflow.com/q/529034/190597) (or running the desired function). – unutbu Nov 10 '14 at 01:28
I added a print statement where you added the do something comment and it printed it multiple times so it's actually not quite right – user2242044 Nov 10 '14 at 03:16
Hm. I don't see an error... Perhaps you have more than one script running? – unutbu Nov 10 '14 at 03:29
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/64608/discussion-between-user2242044-and-unutbu). – user2242044 Nov 10 '14 at 06:15

score 0 · Answer 2 · answered Mar 15 '22 at 10:07

If you want to keep your code simple, you can use the below code:

import datetime
import time

while True:
    time.sleep(1)
    if datetime.datetime.now().time().strftime("%H:%M:%S")  == '19:00:00':
        print ("it's time!")

Sleep is used to generate only 1 row per 1 second (without this you will print thousands of lines. Also it is worth to convert time to string.

Running Python at a Certain Time with Datetime

2 Answers2

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