Author Dusty Phillips Reviewers

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If we want to place objects we define ourselves into a list and make those objects 
sortable, we have to do a bit more work. The special method 
__lt__
, which stands for 
"less than", should be defined on the class to make instances of that class comparable. 
The 
sort
method on list will access this method on each object to determine where it 
goes in the list. This method should return 
True
if our class is somehow less than the 
passed parameter, and 
False
otherwise. Here's a rather silly class that can be sorted 
based on either a string or a number:
class WeirdSortee:
def __init__(self, string, number, sort_num):
self.string = string
self.number = number
self.sort_num = sort_num
def __lt__(self, object):
if self.sort_num:
return self.number < object.number
return self.string < object.string
def __repr__(self):
return"{}:{}".format(self.string, self.number)
The 
__repr__
method makes it easy to see the two values when we print a list. 
The 
__lt__
method's implementation compares the object to another instance of 
the same class (or any duck typed object that has 
string
, 
number
, and 
sort_num
attributes; it will fail if those attributes are missing). The following output illustrates 
this class in action, when it comes to sorting:
>>> a = WeirdSortee('a', 4, True)
>>> b = WeirdSortee('b', 3, True)
>>> c = WeirdSortee('c', 2, True)
>>> d = WeirdSortee('d', 1, True)
>>> l = [a,b,c,d]
>>> l
[a:4, b:3, c:2, d:1]
>>> l.sort()
>>> l
[d:1, c:2, b:3, a:4]
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Chapter 6