Example:

vstinner@apu$ ./python
Python 3.8.0a0 (heads/master-dirty:bcd3a1a18d, Jun 23 2018, 10:31:03) 
[GCC 8.1.1 20180502 (Red Hat 8.1.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import locale
>>> locale.str(2.5)
'2.5'
>>> '{:n}'.format(2.5)
'2.5'
>>> locale.setlocale(locale.LC_ALL, '')
'fr_FR.UTF-8'
>>> locale.str(2.5)
'2,5'
>>> '{:n}'.format(2.5)
python: Objects/unicodeobject.c:474: _PyUnicode_CheckConsistency: Assertion `maxchar < 128' failed.
Aborted (core dumped)

Another example:

vstinner@apu$ ./python
Python 3.8.0a0 (heads/master-dirty:bcd3a1a18d, Jun 23 2018, 10:31:03) 
>>> import locale; locale.setlocale(locale.LC_ALL, '')
'fr_FR.UTF-8'
>>> (2.5).__format__('n')
python: Objects/unicodeobject.c:474: _PyUnicode_CheckConsistency: Assertion `maxchar < 128' failed.
Aborted (core dumped)


Result of my system Python 3.6 of Fedora 28:

vstinner@apu$ python3
Python 3.6.5 (default, Mar 29 2018, 18:20:46) 
[GCC 8.0.1 20180317 (Red Hat 8.0.1-0.19)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import locale
>>> locale.str(2.5)
'2.5'
>>> '{:n}'.format(2.5)
'2.5'
>>> locale.setlocale(locale.LC_ALL, '')
'fr_FR.UTF-8'
>>> locale.str(2.5)
'2,5'
>>> '{:n}'.format(2.5)
'°,5'
>>> '{:n}'.format(3.5)
'°,5'
>>> '{:n}'.format(33.5)
'°\x18,5'
>>> '{:n}'.format(333.5)
'°\x186,5'