steve+comp.lang.python at pearwood
Aug 11, 2013, 1:26 AM
Post #3 of 4
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On Sat, 10 Aug 2013 21:41:00 -0600, Jason Friedman wrote:
> class my_class:
> def __init__(self, attr1, attr2):
> self.attr1 = attr1 #string
> self.attr2 = attr2 #string
> def __lt__(self, other):
> if self.attr1 < other.attr1:
> return True
> else:
> return self.attr2 < other.attr2
>
> I will run into problems if attr1 or attr2 is None, and they
> legitimately can be.
>
> I know I can check for attr1 or attr2 or both being None and react
> accordingly, but my real class has ten attributes and that approach will
> be long. What are my alternatives?
This is a hard question to answer, because your code snippet isn't
clearly extensible to the case where you have ten attributes. What's the
rule for combining them? If instance A has five attributes less than
those of instance B, and five attributes greater than those of instance
B, which wins?
But if I had to guess an approach, I'd start with a helper function (or
method) that compares two raw values:
def compare(a, b):
"""Return -ve for less than, 0 for equal, +ve for greater than."""
if a is None:
return 0 if b is None else -1
if b is None:
return 1
return (b < a) - (a < b)
Now, in your class, you can use this helper function to check each
attribute in turn. Assuming that if an attribute is equal, you move on to
check the next one:
class MyClass:
def _compare(self, other):
for attr in 'attr1 attr2 attr3 attr4'.split():
a, b = getattr(self, attr), getattr(other, attr)
triflag = compare(a, b)
if triflag:
return triflag
return 0
def __lt__(self, other):
if not isinstance(other, MyClass):
return NotImplemented
return self._compare(other) < 0
def __eq__(self, other):
if not isinstance(other, MyClass):
return NotImplemented
return not self._compare(other)
def __ne__(self, other):
if not isinstance(other, MyClass):
return NotImplemented
return bool(self._compare(other))
and so on. You can save a certain amount of repetition (by my count, six
lines of code) by pulling out the "if not isinstance" check into a
decorator, but since the decorator is likely to be about six lines long,
I wouldn't bother :-)
--
Steven
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