Fix "flip" of arguments in relational expressions #30
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This fixes "flip" of arguments in relational expressions, mentioned in the issue sympy/sympy#7951. The docs says [1] (see also [2]) about relational ops: There are no swapped-argument versions of these methods (to be used when the left argument does not support the operation but the right argument does); rather, __lt__() and __gt__() are each other’s reflection, __le__() and __ge__() are each other’s reflection, and __eq__() and __ne__() are their own reflection. If the operands are of different types, and right operand’s type is a direct or indirect subclass of the left operand’s type, the reflected method of the right operand has priority, otherwise the left operand’s method has priority. So, if B is a subclass of A, we can't avoid calling the "reflected" method. For example, A() < B() will be silently transformed to B() > A(). Only know workaround per se - avoid subclassing of "public" classes. For example, old class hierarchy for symbols is forbidden: Symbol / \ Dummy Wild In this patch, common base class for symbols was implemented. References ========== .. [1] https://docs.python.org/3.5/reference/datamodel.html#object.__lt__ .. [2] https://bugs.python.org/issue22052 .. [3] sympy/sympy#7951
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Fix "flip" of arguments in relational expressions
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see sympy/sympy#7951 and https://bugs.python.org/issue22052