[Python-Dev] python 3 niggle: None < 1 raises TypeError

[M.-A. Lemburg]

On 17.02.2014 13:12, Nick Coghlan wrote:
> On 17 Feb 2014 21:15, "M.-A. Lemburg" <mal at egenix.com> wrote:
>>
>> On 15.02.2014 07:03, Stephen J. Turnbull wrote:
>>> M.-A. Lemburg writes:
>>>
>>>  > IMO, it was a mistake to have None return a TypeError in
>>>  > comparisons, since it makes many typical data operations
>>>  > fail, e.g.
>>>
>>> I don't understand this statement.  The theory is that they *should*
>>> fail.
>>>
>>> The example of sort is a good one.  Sometimes you want missing values
>>> to be collected at the beginning of a list, sometimes at the end.
>>> Sometimes you want them treated as top elements, sometimes as bottom.
>>> And sometimes it is a real error for missing values to be present.
>>> Not to mention that sometimes the programmer simply hasn't thought
>>> about the appropriate policy.  I don't think Python should silently
>>> impose a policy in that case, especially given that the programmer may
>>> have experience with any of the above treatments in other contexts.
>>
>> None is special in Python and has always (and intentionally) sorted
>> before any other object. In data processing and elsewhere in Python
>> programming, it's used to signal: no value available.
> 
> This is the first I've ever heard of that sorting behaviour being an
> intentional feature, rather than just an artefact of Python 2 allowing
> arbitrarily ordered comparisons between different types. Can you point me
> to the relevant entry in the Python 2 language reference?

This is not documented anywhere in the language spec, AFAIK. It
is documented in the code (Python 2.7; Object/object.c):

default_3way_compare(PyObject *v, PyObject *w)
...
    /* None is smaller than anything */
    if (v == Py_None)
        return -1;
    if (w == Py_None)
        return 1;

Note that it's not important whether None is smaller or greater
than any other object. The important aspect is that it's sorting
order is consistent and doesn't raise a TypeError when doing an
ordered comparison with other objects.

-- 
Marc-Andre Lemburg
eGenix.com

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