Python: How do I mock datetime.utcnow()?

Question

I have the below:

from datetime import datetime

def get_report_month_key():
    month_for_report = datetime.utcnow()
    return month_for_report.strftime(\"         


        

Answer 1

What also works when patching built-in Python modules turns out to be complicated (as it is with datetime, see e.g. https://solidgeargroup.com/mocking-the-time or https://nedbatchelder.com/blog/201209/mocking_datetimetoday.html or https://gist.github.com/rbarrois/5430921) is wrapping the function in a custom one which then can be easily patched.

So, instead of calling datetime.datetime.utcnow(), you use a function like

import datetime


def get_utc_now():
    return datetime.datetime.utcnow()

Then, patching this one is as simple as

import datetime

# use whatever datetime you need here    
fixed_now = datetime.datetime(2017, 8, 21, 13, 42, 20)
with patch('your_module_name.get_utc_now', return_value=fixed_now):
    # call the code using get_utc_now() here
    pass

Using the patch decorator instead of the context manager would work similarly.