问_add_ vs _radd_：表达式是如何计算的？

EN

class A1:
    def __init__ (self):pass
    def __add__(self, other):
        if isinstance(other, A2):return 111
        if isinstance(other, A1):return 222
        return 333
    def __radd__(self, other):
        if isinstance(other, A1):return 444
        return 555

class A2(A1):
    def __init__(self) : pass
    def __radd__(self, other):
        if isinstance(other, A1):return 666
        return 777

a1 = A1()
a2 = A2()
print(a1 + a1, a2 + a2, a1 + a2, a2 + a1)

222 111 666 222