isoformat function drops microseconds part if its value is 000000

[AlexanderBolshakov]

BPO	40076
Nosy	@abalkin, @pganssle, @tirkarthi

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-03-26.21:07:35.837>
created_at = <Date 2020-03-26.14:14:48.206>
labels = ['type-bug', '3.8', '3.9', 'invalid', '3.7', 'library']
title = 'isoformat function drops microseconds part if its value is 000000'
updated_at = <Date 2020-03-26.21:11:20.750>
user = 'https://bugs.python.org/AlexanderBolshakov'

bugs.python.org fields:

activity = <Date 2020-03-26.21:11:20.750>
actor = 'p-ganssle'
assignee = 'none'
closed = True
closed_date = <Date 2020-03-26.21:07:35.837>
closer = 'Alexander Bolshakov'
components = ['Library (Lib)']
creation = <Date 2020-03-26.14:14:48.206>
creator = 'Alexander Bolshakov'
dependencies = []
files = []
hgrepos = []
issue_num = 40076
keywords = []
message_count = 3.0
messages = ['365077', '365089', '365111']
nosy_count = 5.0
nosy_names = ['belopolsky', 'SilentGhost', 'p-ganssle', 'xtreak', 'Alexander Bolshakov']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40076'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']

[AlexanderBolshakov]

isoformat function does not conform to the ISO 8601 and drops microseconds part if its value is 000000.

The issue can be reproduced using the following code snippet:

for i in range(1,10000000):
     timestamp=datetime.datetime.utcnow().isoformat()
     if len(timestamp)!=26:
         print(timestamp)

[tirkarthi]

Does timespec fulfill this use case to always return microseconds?

https://docs.python.org/3/library/datetime.html#datetime.datetime.isoformat

Return a string representing the date and time in ISO 8601 format:
YYYY-MM-DDTHH:MM:SS.ffffff, if microsecond is not 0
YYYY-MM-DDTHH:MM:SS, if microsecond is 0

The optional argument timespec specifies the number of additional components of the time to include (the default is 'auto'). It can be one of the following:

'auto': Same as 'seconds' if microsecond is 0, same as 'microseconds' otherwise.
'hours': Include the hour in the two-digit HH format.
'minutes': Include hour and minute in HH:MM format.
'seconds': Include hour, minute, and second in HH:MM:SS format.
'milliseconds': Include full time, but truncate fractional second part to milliseconds. HH:MM:SS.sss format.
'microseconds': Include full time in HH:MM:SS.ffffff format.

./python
Python 3.9.0a4+ (heads/master:6723e933c4, Mar 21 2020, 06:54:01) 
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime
>>> datetime.datetime(1, 1, 1).isoformat(timespec='auto')
'0001-01-01T00:00:00'
>>> datetime.datetime(1, 1, 1).isoformat(timespec='microseconds')
'0001-01-01T00:00:00.000000'

[pganssle]

isoformat function does not conform to the ISO 8601 and drops microseconds part if its value is 000000.

I'm not sure why you think that this does not conform to ISO 8601 - ISO 8601 is a sprawling beast of a spec and allows some crazy formats. Some examples of perfectly valid ISO 8601 strings:

--03-26
2020-W13-4T03
2020-03-26T03.5
2020-03-26T03,5
2020-03-26T03:30:40.334

There are *hundreds* of valid formats encompassed by ISO 8601.

Anyway, that's an aside. The behavior of .isoformat() is pretty clearly documented. These are the first three line of the documentation:

Return a string representing the date and time in ISO 8601 format:

  - YYYY-MM-DDTHH:MM:SS.ffffff, if microsecond is not 0
  - YYYY-MM-DDTHH:MM:SS, if microsecond is 0

I believe Karthikeyan has adequately explained how to get the behavior you want, so I am going to go ahead and close this as working as intended.