Author Dusty Phillips Reviewers

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 171 
]
>>> for i in l:
... i.sort_num = False
...
>>> l.sort()
>>> l
[a:4, b:3, c:2, d:1]
The first time 
we call 
sort
, it sorts by numbers because 
sort_num
is 
True
on all the 
objects being compared. The second time, it sorts by letters. The 
__lt__
method 
is the only one we need to implement to enable sorting. Technically, however, if it 
is implemented, the class should normally also implement the similar 
__gt__
, 
__
eq__
, 
__ne__
, 
__ge__
, and 
__le__
methods so that all of the 
<
, 
>
, 
==
, 
!=
, 
>=
, and 
<=
operators also work properly. You can get this for free by implementing 
__lt__
and 
__eq__
, and then applying the 
@total_ordering
class decorator to supply the rest:
from functools import total_ordering
@total_ordering
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)
def __eq__(self, object):
return all((
self.string == object.string,
self.number == object.number,
self.sort_num == object.number
))
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---

Python Data Structures
[
 172 
]
This is useful if we want to be able to use operators on our objects. However, if all we 
want to do is customize our sort orders, even this is overkill. For such a use case, the 
sort
method can take an optional 
key
argument. This argument is a function that 
can translate each object in a list into an object that can somehow be compared. For 
example, we can use 
str.lower
as the key argument to perform a case-insensitive 
sort on a list of strings: