如果既不是按位运算也不是 print chevron，>> 是做什么的？

Question

有没有人可以帮助弄清楚双大于号（即第 10 行中的 Drug >> float 和第 13 行中的 Drug >> int）在以下 python 脚本中的含义？

 1  from owlready2 import *
 2
 3  onto = get_ontology("http://test.org/onto.owl")
 4
 5  with onto:
 6      class Drug(Thing):
 7          def get_per_tablet_cost(self):
 8              return self.cost / self.number_of_tablets
 9
10      class has_for_cost(Drug >> float, FunctionalProperty):
11          python_name = "cost"
12
13      class has_for_number_of_tablets(Drug >> int, FunctionalProperty):
14          python_name = "number_of_tablets"
15
16  my_drug = Drug(cost = 10.0, number_of_tablets = 5)
17  print(my_drug.get_per_tablet_cost())

--- Below is the printing outcome. ---
2.0

代码复制自here并在python中运行 3.

这里的>>符号不是按位运算符。 Python's bitwise operation，比如说 x >> y，要求 y 是一个整数。很明显，这里的代码 (Drug >> float) 没有将 Drug 右移 float 位。

虽然之前问过类似的问题（见python2中的here and ), their answers mostly pointed to the so-called "print chevron"，其中>>将待打印的消息重定向到紧接着指定的类文件对象>>。它们不适用于我的问题，因为这里不是打印语句的情况。

为了追踪上面代码中的>>，我将第10行的脚本修改为class has_for_cost(Drug >> list, FunctionalProperty)（即将>> float替换为>> list）和运行代码。它引发了一个 AttributeError 如下：

AttributeError                            Traceback (most recent call last)
<ipython-input-1-1be374b27b04> in <module>()
      8             return self.cost / self.number_of_tablets
      9 
---> 10     class has_for_cost(Drug >> list, FunctionalProperty):
     11         python_name = "cost"
     12 

~/anaconda3/lib/python3.6/site-packages/owlready2/prop.py in __init__(Prop, name, bases, obj_dict)
    254 class ReasoningPropertyClass(PropertyClass):
    255   def __init__(Prop, name, bases, obj_dict):
--> 256     super().__init__(name, bases, obj_dict)
    257 
    258     if (not Prop.namespace.world is owl_world):

~/anaconda3/lib/python3.6/site-packages/owlready2/prop.py in __init__(Prop, name, bases, obj_dict)
     78 
     79       if not range is False:
---> 80         Prop.range.extend(range)
     81 
     82       if not inverse_property is False:

~/anaconda3/lib/python3.6/site-packages/owlready2/util.py in extend(self, l)
     58   def append(self, x):          old = list(self); super().append(x)         ; self._callback(self._obj, old)
     59   def insert(self, i, x):       old = list(self); super().insert(i, x)      ; self._callback(self._obj, old)
---> 60   def extend(self, l):          old = list(self); super().extend(l)         ; self._callback(self._obj, old)
     61   def remove(self, x):          old = list(self); super().remove(x)         ; self._callback(self._obj, old)
     62   def __delitem__(self, i):     old = list(self); super().__delitem__(i)    ; self._callback(self._obj, old)

~/anaconda3/lib/python3.6/site-packages/owlready2/prop.py in _range_changed(Prop, old)
    143     for x in new - old:
    144       if isinstance(x, ClassConstruct): x._set_ontology(Prop.namespace.ontology)
--> 145       x2 = _universal_datatype_2_abbrev.get(x) or x.storid
    146       Prop.namespace.ontology.add_triple(Prop.storid, rdf_range, x2)
    147 

AttributeError: type object 'list' has no attribute 'storid'

虽然有了回溯信息，我还是找不到答案。如果有人能帮助弄清楚 >> 在代码中的作用，我将不胜感激。

Answer 1

Drug >> float 调用 Drug 的 __rshift__ 方法。该方法可以为所欲为；它不一定是二进制移位操作。

阅读 the owlready2 documentation 表明 domain >> range 是创建 ObjectProperty 的奇特方式。

Answer 2

has answered the question in brief. Below is just to save time for those who don't want to download and parse the source codes of owlready2 但仍想跟踪它在实际代码中的工作原理。

Drug是在另一个对象ThingClass的基础上创建的对象，定义在/owlready2/entity.py.
Drug >> float 呼叫 Drug.__rshift__(self, float) [explained].

不是Drug而是它的基础对象ThingClass有一个__rshift__方法，在entity.py中读取：

class ThingClass(EntityClass):
    ...
    def __rshift__(Domain, Range):
        import owlready2.prop
        owlready2.prop._next_domain_range = (Domain, Range)
        if isinstance(Range, ThingClass) or isinstance(Range, ClassConstruct):
            return owlready2.prop.ObjectProperty
        else:
            return owlready2.prop.DataProperty

Drug >> float相当于__rshift__(Domain=Drug, Range=float)，其实是不是做的二进制算术运算，但 returns/creates DataProperty 或 ObjectProperty，具体取决于 Range 的类型。

如果既不是按位运算也不是 print chevron，>> 是做什么的？

What does >> do if it is neither bitwise operation nor print chevron?

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