x %doSomething% y
对于任何x和任何y都很容易做到(见下面的代码),除非x是str.
是否有任何方法(例如添加特殊方法或引发特定错误)导致旧样式字符串格式化失败(类似于1％doSomthing如何通过TypeError失败)并恢复到doSomething对象中定义的__rmod__方法？
class BinaryMessage(object):
def __init__(self,fn):
self._fn = fn
def __rmod__(self,LHS):
return BinaryMessagePartial(self._fn,LHS)
class BinaryMessagePartial(object):
def __init__(self,fn,LHS):
self._fn = fn
self._LHS = LHS
def __mod__(self,RHS):
return self._fn(self._LHS,RHS)
def _doSomething(a,b):
return a + b
doSomething = BinaryMessage(_doSomething)
result = 5 %doSomething% 6
assert result == 11
解决方法
不幸的是,这在Python中目前是不可能的.行为在评估循环中是硬编码的：
TARGET(BINARY_MODULO) {
PyObject *divisor = POP();
PyObject *dividend = TOP();
PyObject *res = PyUnicode_CheckExact(dividend) ?
PyUnicode_Format(dividend,divisor) :
PyNumber_Remainder(dividend,divisor);
(从Python 3.5 source code开始,PyUnicode是Python str类型).
这是不幸的,因为对于其他所有类型,您可以通过使用右侧操作数的子类来阻止LHS .__ mod__方法;从documentation：
Note: If the right operand’s type is a subclass of the left operand’s type and that subclass provides the reflected method for the operation,this method will be called before the left operand’s non-reflected method. This behavior allows subclasses to override their ancestors’ operations.
这将是唯一的选项,str％other永远不会返回NotImplemented,所有RHS类型都被接受(实际的str.__mod__ method只接受RHS的str对象,但在这种情况下不调用).
我认为这是Python中的一个错误,提交为issue #28598.