Inconsistency between datetime.now() and datetime.fromtimestamp(time.time(), None)

[Miksus]

BPO	44831
Nosy	@rhettinger, @pfmoore, @abalkin, @vstinner, @larryhastings, @tjguk, @stevendaprano, @zware, @zooba, @pganssle, @Miksus

^{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 = None
created_at = <Date 2021-08-04.17:49:51.110>
labels = ['type-bug', 'library', 'OS-windows', '3.11']
title = 'Inconsistency between datetime.now() and datetime.fromtimestamp(time.time(), None)'
updated_at = <Date 2021-10-04.08:57:15.356>
user = 'https://github.com/Miksus'

bugs.python.org fields:

activity = <Date 2021-10-04.08:57:15.356>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'Windows']
creation = <Date 2021-08-04.17:49:51.110>
creator = 'Miksus'
dependencies = []
files = []
hgrepos = []
issue_num = 44831
keywords = []
message_count = 10.0
messages = ['398919', '398922', '399068', '399070', '399090', '399096', '399097', '403063', '403124', '403125']
nosy_count = 11.0
nosy_names = ['rhettinger', 'paul.moore', 'belopolsky', 'vstinner', 'larry', 'tim.golden', 'steven.daprano', 'zach.ware', 'steve.dower', 'p-ganssle', 'Miksus']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44831'
versions = ['Python 3.11']

Linked PRs

• gh-88994: Change datetime.now to half-even rounding for consistency with datetime.fromtimestamp. #134258

[Miksus]

I am trying to measure time twice and the second measurement gives a time that is 1 microsecond before the first measurement about half of the time.

My experiment in short:
---------------------------------------------------

import time, datetime
start = time.time()
end = datetime.datetime.now()

start = datetime.datetime.fromtimestamp(start, None)
assert end >= start # fails about half the time.

---

The problem is somewhat interesting. This does not fail:
---------------------------------------------------

import time, datetime
start = time.time()
end = time.time()

start = datetime.datetime.fromtimestamp(start, None)
end = datetime.datetime.fromtimestamp(end, None)
assert end >= start

---

And neither does this:
---------------------------------------------------

import datetime
start = datetime.datetime.now()
end = datetime.datetime.now()
assert end >= start

---

And it seems datetime.datetime.now() works the same way as to how I handled the "start" time in my first experiment:
https://github.com/python/cpython/blob/3.6/Lib/datetime.py#L1514
and therefore the issue seems to be under the hood.

I have tested this on two Windows 10 machines (Python 3.6 & 3.8) in which cases this occurred. This did not happen on Raspberry Pi OS using Python 3.7.

In short:

• The time module imported in datetime.datetime.now() seems to measure time slightly differently than the time module imported by a Python user.
• This seems to be Windows specific.

My actual application has some code in between the measurements suffering from the same problem thus this is not an issue affecting only toy examples.

[Miksus]

I accidentially posted Python 3.6 link to the declaration of datetime.datetime.now() but this has been unchanged:

cpython/Lib/datetime.py

Line 1696 in 3d2b4c6

def now(cls, tz=None):

The actual piece of code as of now:
...
import time as _time
...
@classmethod
def now(cls, tz=None):
"Construct a datetime from time.time() and optional time zone info."
t = _time.time()
return cls.fromtimestamp(t, tz)

[stevendaprano]

Confirmed on Windows with Python 3.9.6.

[stevendaprano]

I've replicated this under Linux as well.

def test():
    start = time.time()
    end = datetime.datetime.now()
    start = datetime.datetime.fromtimestamp(start, None)
    assert end >= start

Then run it in a loop:

>>> for i in range(10000000):
...     test()
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 5, in test
AssertionError
>>> i
22

So while it is not as frequent as on Windows, it does occur on Linux as well.

[abalkin]

Can someone try to replicate this while disabling the C acceleration:

import sys
sys.modules[‘_datetime’] = None

(Before any other imports.)

If anything, this is likely to be a problem with the C implementation.