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
>>>