def eval(self)return self left eval(self right eval( def inorder(self)return "(self left inorder(self right inorder(") class plusnodedef __init__(selfleftright)self left left self right right def eval(self)return self left eval(self right eval( def inorder(self)return "(self left inorder(self right inorder(") class numnodedef __init__(selfnum)self num num def eval(self)return self num def inorder(self)return str(self num the inorder methods in sect provide for an inorder traversal because each binary operator is added to the string in between the two operands to do postorder traversal of the tree we would write postorder method that would add each binary operator to the string after postorder traversing the two operands note that because of the way postorder traversal is writtenparentheses are never needed in postfix expressions one other traversal is possiblecalled preorder traversal in preorder traversaleach binary operator is added to the string before its two operands given the infix expression ( the prefix equivalent is againbecause of the way prefix expression is writtenparentheses are never needed in prefix expressions parsing prefix expressions abstract syntax trees are almost never constructed by hand they are often built automatically by an interpreter or compiler when python program is executed the python interpreter scans it and builds an abstract syntax tree of the program this part of the python interpreter is called parser parser is programor part of programthat reads file and automatically builds an abstract syntax tree of the expression ( source program)and reports syntax error if the program or expression is not properly formed the exact details of how this is accomplished this copy belongs to 'acha