beam: fix safe_eval unary minus precedence vs **

wizard-work/beam/safe_eval.py

@@ -141,10 +141,10 @@ class _Parser:
         return value
 
     def term(self):
-        value = self.power()
+        value = self.unary()
         while self.peek()[0] == _OP and self.peek()[1] in ("*", "/", "//", "%"):
             op = self.advance()[1]
-            rhs = self.power()
+            rhs = self.unary()
             if op == "*":
                 value = value * rhs
             elif op == "/":
@@ -161,20 +161,20 @@ class _Parser:
                 value = value % rhs
         return value
 
-    def power(self):
-        base = self.unary()
-        if self.peek()[0] == _OP and self.peek()[1] == "**":
-            self.advance()
-            exp = self.power()  # right-assoc
-            return base ** exp
-        return base
-
     def unary(self):
         if self.peek()[0] == _OP and self.peek()[1] in ("+", "-"):
             op = self.advance()[1]
-            val = self.unary()
+            val = self.power()  # unary lower precedence than ** (python semantics)
             return -val if op == "-" else val
-        return self.primary()
+        return self.power()
+
+    def power(self):
+        base = self.primary()
+        if self.peek()[0] == _OP and self.peek()[1] == "**":
+            self.advance()
+            exp = self.unary()  # right-assoc, exponent may be signed
+            return base ** exp
+        return base
 
     def primary(self):
         t = self.peek()

wizard-work/beam/test_safe_eval.py

@@ -4,6 +4,8 @@ cases = [
     ("2+3*4", 14),
     ("(2+3)*4", 20),
     ("2**3**2", 512),  # right assoc
+    ("-2**2", -4),  # unary lower than **
+    ("2**-2", 0.25),  # signed exponent
     ("10//3", 3),
     ("10%3", 1),
     ("-5+2", -3),