What is it?
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>>>
type
(NotImplemented)
<
type
'NotImplementedType'
>
|
NotImplemented is one constant in the built-in Python six namespace. Other False , True , None , Ellipsis and __debug__. And Ellipsis like, NotImplemented be re-assignment (covered). Assigned to it, and even change the attribute name, no SyntaxError. So it's not really a "true" constant. Of course, we should never change it. But for completeness:
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>>>
None
=
'hello'
...
SyntaxError: can't assign to keyword
>>> NotImplemented
NotImplemented
>>> NotImplemented
=
'do not'
>>> NotImplemented
'do not'
|
what's it for? When to use?
NotImplemented is a special value, it can be returned two yuan special methods (such __eq __ () , __lt __ () , __add __ () , __rsub __ (), etc.), indicating a type that does not perform the operations, as other types. Similarly two yuan special method, it may be treated in situ (in place) return (such __imul __ (), __ iand __ () , etc.). Also, its actual value is True:
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>>>
bool
(NotImplemented)
True
|
You may be asking yourself, "But I think when this operation is not achieved, I should produce a NotImpementedError ." We will look at some examples about why the child is not the case when two yuan special methods.
Let's look at NotImplemented usage constant, by __eq __ () for two very basic (and useless) coding classes A and B, respectively. [For this simple example, in order to avoid interference, will not achieve __ne __ () , but in general, each time to achieve __eq __ () when, __ne __ () should also be achieved unless there is a sufficient enough reason to not Realize it. ]
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# example.py
class
A(
object
):
def
__init__(
self
, value):
self
.value
=
value
def
__eq__(
self
, other):
if
isinstance
(other, A):
print
(
'Comparing an A with an A'
)
return
other.value
=
=
self
.value
if
isinstance
(other, B):
print
(
'Comparing an A with a B'
)
return
other.value
=
=
self
.value
print
(
'Could not compare A with the other class'
)
return
NotImplemented
class
B(
object
):
def
__init__(
self
, value):
self
.value
=
value
def
__eq__(
self
, other):
if
isinstance
(other, B):
print
(
'Comparing a B with another B'
)
return
other.value
=
=
self
.value
print
(
'Could not compare B with the other class'
)
return
NotImplemented
|
Now, in the interpreter:
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>>>
from
example
import
A, B
>>> a1
=
A(
1
)
>>> b1
=
B(
1
)
|
我们现在可以实验下对于 __eq__() 不同的调用，看看发生了什么。作为提醒，在Python中，a == b会调用a.__eq__(b)：
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>>> a1
=
=
a1
Comparing an A with an A
True
|
正如所望，a1等于a1（自己），使用类A中的__eq__()来进行这个比较的。比较b1和它自己也会产生类似结果：
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2
3
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>>> b1
=
=
b1
Comparing a B with another B
True
|
现在，那要是我们比较a1和b1呢？由于在A的__eq__()会检查other是不是B的一个实例，我们想要a1.__eq__(b1)去处理这个比较并返回True：
|
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2
3
|
>>> a1
=
=
b1
Comparing an A with a B
True
|
就是这样。现在，如果我们比较b1和a1（即调用b1.__eq__(a1)），我们会想要返回NotImplemented。这是因为B的__eq__()只和其他B的实例进行比较。来看看发生了什么：
|
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2
3
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|
>>> b1
=
=
a1
Could
not
compare B against the other
class
Comparing an A with a B
True
|
聪明！b1.__eq__(a1)方法返回NotImplemented，这样会导致调用A中的__eq__()方法。而且由于在A中的__eq__()定义了A和B之间的比较，所以就得到了正确的结果（True）。
这就是返回了NotImplemented的所做的。NotImplemented告诉运行时，应该让其他对象来完成某个操作。在表达b1 == a1中，b1.__eq__(a1)返回了NotImplemented，这说明Python试着用a1.__eq__(b1)。由于a1足够可以返回True，因此这个表达可以成功。如果A中的__eq__()也返回NotImplemented，那么运行时会退化到使用内置的比较行为，即比较对象的标识符（在CPython中，是对象在内存中的地址）。
注意：如果在调用b1.__eq__(a1)时抛出NotImpementedError，而不进行处理，就会中断代码的执行。而NotImplemented无法抛出，仅仅是用来进一步测试是否有其他方法可供调用。
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