Editing Python programming (section)

=== HOWTO get an [https://en.wikipedia.org/wiki/ISO_8601 ISO 8601] or [https://www.obj-sys.com/asn1tutorial/node14.html ASN.1 GeneralizedTime] time format with Python datetime object ===

<pre>>>> import datetime

>>> datetime.MINYEAR		  
1

>>> datetime.MAXYEAR		  
9999

>>> datetime.datetime.now()		  
datetime.datetime(2019, 3, 31, 21, 29, 47, 993877)

>>> from datetime import datetime
		  
>>> datetime.now()
		  
datetime.datetime(2019, 3, 31, 21, 30, 4, 999219)

>>> d = datetime.now()
		  
>>> d
		  
datetime.datetime(2019, 3, 31, 21, 30, 51, 362520)

>>> f = '{:%Y%m%d%H%M%S.%f}'  < -- Setup a string format to control the datetime format.
		  
>>> f.format(d)
		  
'20190331213051.362520' < -- microsecond accuracy

>>> d2 = datetime(2019, 3, 31, 21, 30, 51, 000000)  < -- Create datetime object with exactly zero microseconds to verify string format.
		  
>>> f.format(d2)
		  
'20190331213051.000000' < -- verified that datetime displays six zeros

>>> '20190331213051.000000'[:-3] < -- truncate last three digits
		  
'20190331213051.000' < -- Now we need timezone offset

>>> z = datetime.datetime.utcnow() < -- I want UTC time so I use datetime class method utcnow()
		  
>>> z
		  
datetime.datetime(2019, 4, 1, 3, 28, 53, 152240) < -- not in format I want

>>> f.format(z) < -- apply format specification used above
		  
'20190401032853.152240' < -- want to remove least 3 insignificant digits from microseconds

>>> f.format(z)[:-3]
		  
'20190401032853.152' < -- missing offset

>>> f.format(z)[:-3]+'Z' < -- simple string concatenate
		  
'20190401032853.152Z' < -- final product</pre>

<big>Summary of string formatting to get to [https://en.wikipedia.org/wiki/ISO_8601 ISO 8601] from [https://docs.python.org/3/library/datetime.html#module-datetime Python datetime object]</big>
* Format string: <code>'{:%Y%m%d%H%M%S.%f}'</code>
* Truncate last three digits from formatted string: <code>[:-3]</code>
* Concatenate 'Z' for Zulu time <code>+ 'Z'</code>

<big>Complete example (without references)</big>
<pre>>>> '{:%Y%m%d%H%M%S.%f}'.format(datetime.datetime.utcnow())[:-3] + 'Z'
							  
'20190401034018.177Z'</pre>

Same example with f-string

<pre>f'{datetime.utcnow():%Y%m%d%H%M%S.%f}'[:-3]+'Z'</pre>