time.get_clock_info reports "adjustable=False" for implementation="CLOCK_MONOTONIC"

[FlorianKrause]

BPO	38394
Nosy	@vstinner

^{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 2019-10-07.14:01:54.249>
labels = ['3.7', 'type-bug', 'library']
title = 'time.get_clock_info reports "adjustable=False" for implementation="CLOCK_MONOTONIC"'
updated_at = <Date 2019-10-07.14:11:43.618>
user = 'https://bugs.python.org/FlorianKrause'

bugs.python.org fields:

activity = <Date 2019-10-07.14:11:43.618>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-10-07.14:01:54.249>
creator = 'Florian Krause'
dependencies = []
files = []
hgrepos = []
issue_num = 38394
keywords = []
message_count = 2.0
messages = ['354097', '354099']
nosy_count = 2.0
nosy_names = ['vstinner', 'Florian Krause']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38394'
versions = ['Python 3.7']

Linked PRs

• gh-82575: Adjust time.get_clock_info *adjustable* attribute doc #135920
• [3.14] gh-82575: Adjust time.get_clock_info *adjustable* attribute doc (GH-135920) #140097
• [3.13] gh-82575: Adjust time.get_clock_info *adjustable* attribute doc (GH-135920) #140098

[FlorianKrause]

clock_gettime(CLOCK_MONOTONIC) is adjustable via NTP, as described here: https://linux.die.net/man/2/clock_gettime

(and also mentioned in PEP-418).

Yet, time.get_clock_info reports it as "adjustable=False" (this even seems to be hardcoded for all clocks on Linux).

[vstinner]

Well, that's more a documentation issue than a bug.

"adjustable=False" in the context of Python means that the clock cannot jump a day forward or one day backwards.

For example, on my Fedora 30, I cannot set CLOCK_MONOTONIC clock as root:

$ sudo python3
>>> import time
>>> time.clock_settime(time.CLOCK_MONOTONIC, time.clock_gettime(time.CLOCK_MONOTONIC))
OSError: [Errno 22] Invalid argument

In the Linux kernel, NTP adjusts CLOCK_MONOTONIC so it really measures time in *seconds*, rather something faster or slower than one second :-)

Python time.monotonic() documentation clearly states that it uses the unit of one second:
https://docs.python.org/dev/library/time.html#time.monotonic
"Return the value (in fractional seconds) of a monotonic clock, i.e. a clock that cannot go backwards. (...)"

CLOCK_MONOTONIC_RAW must not be used for time.monotonic() because it doesn't use an unit of 1 second.

Feel free to propose a documentation enhancement :-)

https://docs.python.org/dev/library/time.html#time.get_clock_info