Python- Sympy Issue with expression equality check when evaluate=False

问题

In my project I have to use evaluate=false at the time when i am creating any Add or Mul objects. In this case I am facing a problem when I apply equality checks on these objects. The issue is because of the ordering of the arguments.

Please consider the example below:

k2=Mul(*[x,y,2],evaluate=False)
k1=Mul(*[x,2,y],evaluate=False)
print k1==k2 

The result is false as k2.args are (x,y,2) and k1.args are (x,2,y). So, while the comparison checks for tuple equality it returns false. Is there any way I can get the wanted result?

Also, if I put some operation on tuples (like reversing the order and then checking), it fails in the cases when k1 and k2 are formed from different Mul objects (like when k1.args = 2*x,y and k2.args = 2*y,x)

I can't use sorting here , as in this case Add([x+y,z],evaluate=False) and Add([x+z,y],evaluate=False) will be two different expressions. Also if i use evalaute=True, in this case Add([x+y],x]) and Add([2*x+y]) will be the same, which i don't want.

回答1:

Mul doesn't know that x and y both represent scalars. Matrix multiplication depends on the order of the arguments so in your example k1 and k2 are not necessarily equal. If you know that in your program x and y will always be scalars (or some other type of value where multiplication is commutative and associative) there may be a way to factor out the constants from each term, compare the constants and compare a sorted list of terms.

回答2:

Does the following help you?

print simplify(k1 - k2) == 0 # True
print k1 == k2 # Still False

回答3:

Found one work around.

in Add/Mul classes at the expression formation flatten all arguments using below code

   flatten_args = []
            for arg in args:
                if (arg.__class__==cls):
                    flatten_args.extend(arg.args)
                else:
                    flatten_args.append(arg)

            obj = Expr.__new__(cls, *flatten_args) 

and at the time of equality check ,i am putting one extra check sort the args list first arg_list.sort() and than compare two lists

来源：https://stackoverflow.com/questions/8079892/python-sympy-issue-with-expression-equality-check-when-evaluate-false