On Fri, Feb 14, 2014 at 9:04 AM, Chris Withers <ch...@simplistix.co.uk> wrote:
> Hi All,
>
> Sending this to python-dev as I'm wondering if this was considered when the
> choice to have objects of different types raise a TypeError when ordered...
>
> So, the concrete I case I have is implementing stable ordering for the
> python Range objects that psycopg2 uses. These have 3 attributes that can
> either be None or, for sake of argument, a numeric value.
>
> To implement __lt__ in Python 2, I could do:
>
> def __lt__(self, other):
> if not isinstance(other, Range):
> return True
> return ((self._lower, self._upper, self._bounds) <
> (other._lower, other._upper, other._bounds))
>
> Because None < 1 raises a TypeError, in Python 3 I have to do:
>
> def __lt__(self, other):
> if not isinstance(other, Range):
> return NotImplemented
> for attr in '_lower', '_upper', '_bounds':
> self_value = getattr(self, attr)
> other_value = getattr(other, attr)
> if self_value == other_value:
> pass
> elif self_value is None:
> return True
> elif other_value is None:
> return False
> else:
> return self_value < other_value
> return False
>
> Am I missing something? How can I get this method down to a sane size?
It was considered. It's not obvious where you want "None" to appear in
your ordering, so you will have to implement this by yourself. I can't
come up with anything obviously shorter.
Or, well, this, but it's not exactly pretty:
def __lt__(self, other):
if not isinstance(other, Range):
return NotImplemented
ninf = float('-inf') # So None is lower than anything else.
return ((self._lower if self._lower is not None else ninf,
self._upper if self._upper is not None else ninf,
self._bounds if self._bounds is not None else ninf) < (
other._lower if other._lower is not None else ninf,
other._upper if other._upper is not None else ninf,
other._bounds if other._bounds is not None else ninf))
Or maybe:
def __lt__(self, other):
if not isinstance(other, Range):
return NotImplemented
s = (self._lower, self._upper, self._bounds)
if None is s:
return True
o = (other._lower, other._upper, other._bounds)
if None in o:
return False
return s < o
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