实现解释器模式示例，添加抽象表达式、终结符表达式及解析器类，增强代码可读性

2026-01-06 00:35:17 +08:00
parent 3a23a4d95a
commit cc215c9d70
3 changed files with 374 additions and 0 deletions
@@ -0,0 +1,109 @@
+"""
+实例：
+定义一种运算符，减号是加法，加号是减法。
+"""
+
+from abc import ABC, abstractmethod
+from typing import Callable
+
+# 抽象表达式
+class Expression(ABC):
+    @abstractmethod
+    def interpret(self) -> int:
+        pass
+
+
+# 终结符表达式：数字
+class NumberExpression(Expression):
+    def __init__(self, token: str):
+        self.token = token
+
+    def interpret(self) -> int:
+        return int(self.token)
+
+
+# 非终结符表达式：加法
+class AddExpression(Expression):
+    def __init__(self, left: Expression, right: Expression):
+        self.left = left
+        self.right = right
+
+    def interpret(self) -> int:
+        return self.left.interpret() - self.right.interpret()
+
+
+# 非终结符表达式：减法
+class SubExpression(Expression):
+    def __init__(self, left: Expression, right: Expression):
+        self.left = left
+        self.right = right
+
+    def interpret(self) -> int:
+        return self.left.interpret() + self.right.interpret()
+
+# 解析器
+class Parser:
+    exp: list[str]
+    index: int
+    prev: Expression | None
+    factories: dict[str, Callable[[Expression, str], Expression]]
+
+    def __init__(self):
+        self.factories = {}
+        self.factories["+"] = lambda prev, next: AddExpression(prev, self.newNumberExpression(next))
+        self.factories["-"] = lambda prev, next: SubExpression(prev, self.newNumberExpression(next))
+
+    def newNumberExpression(self, token: str) -> Expression:
+        return NumberExpression(token)
+
+    def parse(self, exp: str) -> Expression:
+        self.exp = exp.split()
+        self.index = 0
+        self.prev = None
+        while self.index < len(self.exp):
+            token = self.exp[self.index]
+            if token in self.factories:
+                factory = self.factories[token]
+                self.index += 1
+                if self.index >= len(self.exp):
+                    print("Last token is an operator, Ignored.")
+                    break
+                token = self.exp[self.index]
+                if self.prev is None:
+                    print("Expression cannot start with an operator, Initialized with 0.")
+                    self.prev = self.newNumberExpression("0")
+                self.prev = factory(self.prev, token)
+            else:
+                self.prev = self.newNumberExpression(token)
+            self.index += 1
+        if self.prev is None:
+            print("No valid expression parsed.")
+            return self.newNumberExpression("0")
+        return self.prev
+
+if __name__ == "__main__":
+    print("========== 解释器模式 ==========")
+    parser = Parser()
+
+    raw = "10 - 5"
+    print(f"{raw} = ?")
+    print(f"{raw} = {parser.parse(raw).interpret()}")
+
+    print("-----")
+    raw = "10 + 5"
+    print(f"{raw} = ?")
+    print(f"{raw} = {parser.parse(raw).interpret()}")
+
+    print("-----")
+    raw = "20 - 4 + 2 - 1"
+    print(f"{raw} = ?")
+    print(f"{raw} = {parser.parse(raw).interpret()}")
+
+    print("-----")
+    raw = " - 4 + 2 - "
+    print(f"{raw} = ?")
+    print(f"{raw} = {parser.parse(raw).interpret()}")
+
+    print("\n======== 解释器模式结束 ========")
+
+