python 初始认识重载操作符的利器：functools.total_ordering
啊，最近被问到重载操作符，想要比较两个对象大小，之前理解不深刻呢，对__cmp__()和__gt__()和__lt__()，有了点疑问：有了前者，还要后面两个干嘛？
1、
先做个实验__cmp__() 和 __gt__() 冲不冲突
class Door(object):
def __init__(self):
self.value = 0
def __cmp__(self, other):
print '====my cmp===='
if self.value > other.value:
return 1
if self.value < other.value:
return -1
return 0
def __gt__(self, other):
print '====my gt===='
return self.value > other.value
def __lt__(self, other):
print '====my lt===='
return self.value < other.value
a = Door()
b = Door()
a.value = 20
b.value = 20
print a == b
print a > b
print a < b====my cmp====
True
====my gt====
False
====my lt====
Falsclass Door(object):
def __init__(self):
self.value = 0
def __cmp__(self, other):
print '====my cmp===='
if self.value > other.value:
return 1
if self.value < other.value:
return -1
return 0
a = Door()
b = Door()
a.value = 20
b.value = 20
print a == b
print a > b
print a < b====my cmp====
True
====my cmp====
False
====my cmp====
False
果然是比较的时候，优先使用__gt__(), __lt__(), __eq__()，如果找不到则使用__cmp__()。
Ps: 至于这个使用__cmp__()还是__gl__()的控制逻辑并没有找到相关文档。有大神看到这里求指导呀~
可惜python3 没有cmp()函数啦，我们要尽可能少用cmp()，否则还能来一波：
def __cmp__(self, other):
assert isinstance(other, A) # assumption for this example return
return cmp((self.name, self.age, self.other), (other.name, other.age, other.other))2、
当然在Stack Overflow上，找到更优解: https://stackoverflow.com/questions/1061283/lt-instead-of-cmp
最牛逼的还是得用functools.total_ordering()：
def total_ordering(cls):
"""Class decorator that fills in missing ordering methods"""
convert = {
'__lt__': [('__gt__', lambda self, other: not (self < other or self == other)),
('__le__', lambda self, other: self < other or self == other),
('__ge__', lambda self, other: not self < other)],
'__le__': [('__ge__', lambda self, other: not self <= other or self == other),
('__lt__', lambda self, other: self <= other and not self == other),
('__gt__', lambda self, other: not self <= other)],
'__gt__': [('__lt__', lambda self, other: not (self > other or self == other)),
('__ge__', lambda self, other: self > other or self == other),
('__le__', lambda self, other: not self > other)],
'__ge__': [('__le__', lambda self, other: (not self >= other) or self == other),
('__gt__', lambda self, other: self >= other and not self == other),
('__lt__', lambda self, other: not self >= other)]
}
roots = set(dir(cls)) & set(convert)
if not roots:
raise ValueError('must define at least one ordering operation: < > <= >=')
root = max(roots) # prefer __lt__ to __le__ to __gt__ to __ge__
for opname, opfunc in convert[root]:
if opname not in roots:
opfunc.__name__ = opname
opfunc.__doc__ = getattr(int, opname).__doc__
setattr(cls, opname, opfunc)
return cls
这个函数简直碉堡。
这是一个类的装饰器，内部逻辑大概是如果是实现了__lt__()，那么程序会自动帮你实现了__gt__(), __ge__()等等的N多函数。
不过注意观察，就用__lt__()这一个分支来说，
'__lt__': [('__gt__', lambda self, other: not (self < other or self == other)),
('__le__', lambda self, other: self < other or self == other),
('__ge__', lambda self, other: not self < other)],The operators "<", ">", "==", ">=", "<=", and "!=" compare the values
of two objects. The objects need not have the same type. If both are
numbers, they are converted to a common type. Otherwise, objects of
different types *always* compare unequal, and are ordered consistently
but arbitrarily. You can control comparison behavior of objects of
non-built-in types by defining a "__cmp__" method or rich comparison
methods like "__gt__", described in section Special method names.3、
用functools.total_ordering来搞一波示例代码
from functools import total_ordering
@total_ordering
class Door(object):
def __init__(self):
self.value = 0
self.first_name = ''
self.last_name = ''
def __eq__(self, other):
print '=== my eq==='
return (self.first_name, self.last_name) == (other.first_name, other.last_name)
def __gt__(self, other):
print '=== my total_ordering==='
return (self.first_name, self.last_name) > (other.first_name, other.last_name)
a = Door()
b = Door()
a.first_name = 'ouyang'
a.last_name = 'guoge'
b.first_name = 'aaaa'
b.last_name = 'bbbb'
print a == b
print a > b
print a < b
print a <= b
print a >= b结果：
=== my eq===
False
=== my total_ordering===
True
=== my total_ordering===
False
=== my total_ordering===
False
=== my total_ordering===
True
以上