1. class Term:
2.     def __init__(self, variable=None, const=None):
3.         self.variable = variable
4.         self.const = const
5.         self.tree = []
6.  
7.     @classmethod
8.     def const(cls, const):
9.         term = cls(None, const)
10.         return term
11.  
12.     @classmethod
13.     def var(cls, variable):
14.         term = cls(variable, None)
15.         return term
16.  
17.     def __call__(self, **kwargs):
18.         return Term.calculate(Term.exchange(self.tree, **kwargs))
19.  
20.     def __add__(self, other):
21.         term = Term()
22.         term.tree.append("+")
23.         term.tree.append(self.variable)
24.         if len(other.tree) is not 0:
25.             term.tree.append(other.tree)
26.         else:
27.             term.tree.append(other.variable)
28.         return term
29.  
30.     def __sub__(self, other):
31.         term = Term()
32.         term.tree.append("-")
33.         term.tree.append(self.variable)
34.         if len(other.tree) is not 0:
35.             term.tree.append(other.tree)
36.         else:
37.             term.tree.append(other.variable)
38.         return term
39.  
40.     def __mul__(self, other):
41.         term = Term()
42.         term.tree.append("*")
43.         term.tree.append(self.variable)
44.         if len(other.tree) is not 0:
45.             term.tree.append(other.tree)
46.         else:
47.             term.tree.append(other.variable)
48.         return term
49.  
50.     def __str__(self):
51.         return Term.to_string(self.tree)
52.  
53.     @staticmethod
54.     def exchange(tree, **kwargs):
55.         if isinstance(tree[1], str) or isinstance(tree[2], str):
56.             if isinstance(tree[1], str):
57.                 for key, value in kwargs.items():
58.                     if tree[1] == str(key):
59.                         tree[1] = value
60.             if isinstance(tree[2], str):
61.                 for key, value in kwargs.items():
62.                     if tree[2] == str(key):
63.                         tree[2] = value
64.             return (tree[0], tree[1], tree[2])
65.         elif isinstance(tree[2], tuple):
66.             return Term.exchange((tree[0], tree[1], Term.exchange(tree[2])))
67.  
68.     @staticmethod
69.     def calculate(tree):
70.         if isinstance(tree[2], int):
71.             if tree[0] == "+":
72.                 return tree[1] + tree[2]
73.             elif tree[0] == "-":
74.                 return tree[1] - tree[2]
75.             elif tree[0] == "*":
76.                 return tree[1] * tree[2]
77.         else:
78.             return Term.calculate((tree[0], tree[1], Term.calculate(tree[2])))
79.  
80.     @staticmethod
81.     def to_string(tree):
82.         if not isinstance(tree[2], list):
83.             return "({0} {1} {2})".format(tree[1], tree[0], tree[2])
84.         else:
85.             return Term.to_string((tree[0], tree[1], Term.to_string(tree[2])))
86.  
87.  
88. if __name__ == "__main__":
89.     x = Term.var("x")
90.     y = Term.var("y")
91.     z = Term.var("z")
92.     d = x + y
93.     print(d(x=1, y=2))
94.