Contrarily to documentation:

1. os.utime() accepts a tuple with floats passed by ns argument:
>>> os.utime(file, ns=(0.5, 0.5))
>>>

2. os.utime() does not accept explicit ns=None:
>>> os.utime(file, times=None)
>>> os.utime(file, ns=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: utime: 'ns' must be a tuple of two ints
>>>

3. os.utime() does not accept both times and ns when only times is a tuple:
>>> os.utime(file, times=None, ns=(0, 0))
>>> os.utime(file, times=(0, 0), ns=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: utime: you may specify either 'times' or 'ns' but not both
>>>

Attached is a preliminary patch to check ns != Py_None.

@serwy: can you please write a test for your patch?

_rev1 has a basic test. Is it sufficient?

@vstinner
This issue is not solved yet.
Can I finalize this issue?

The documentation issue was fixed in issue23738. The documentation no longer claims that ns can be None. If specified it must be a tuple of floats.

> The documentation issue was fixed in issue23738. The documentation no longer claims that ns can be None. If specified it must be a tuple of floats.

tuple of integers, not tuple a floats.

Python 3.9 documentation says: "If ns is specified, it must be a 2-tuple of the form (atime_ns, mtime_ns) where each member is an int expressing nanoseconds." which is correct.

Python 3.9 now emits a DeprecationWarning when passing floats:

vstinner@apu$ ./python
Python 3.9.0a0 (heads/pr/15701-dirty:a0f335c74c, Sep  6 2019, 17:38:14) 
>>> import os
>>> os.utime("x", ns=(1, 1))
>>> os.utime("x", ns=(0.1, 1))
<stdin>:1: DeprecationWarning: an integer is required (got type float).  Implicit conversion to integers using __int__ is deprecated, and may be removed in a future version of Python.

By the way, error messages are now also correct:

>>> os.utime("x", ns=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: utime: 'ns' must be a tuple of two ints

>>> os.utime("x", times=(0, 0), ns=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: utime: you may specify either 'times' or 'ns' but not both