save original expression

src/nmreval/fit/parameter.py

@@ -116,8 +116,10 @@ class Parameter:
         self.namespace = None
         self.eval_allowed = True
         self._expr = None
+        self._expr_disp = None
 
         if isinstance(value, str):
+            self._expr_disp = value
             value = _prepare_namespace_string(value)
             self._expr = value
             self.var = False
@@ -152,7 +154,7 @@ class Parameter:
             if self._expr is None:
                 ret_string += ' (fixed)'
             else:
-                ret_string += f' (calc: {self._expr})'
+                ret_string += f' (calc: {self._expr_disp})'
 
             return ret_string
 
@@ -213,7 +215,7 @@ class Parameter:
 
     def copy(self) -> Parameter:
         if self._expr:
-            val = self._expr
+            val = self._expr_disp
         else:
             val = self._value
         para_copy = Parameter(name=self.name, value=val, var=self.var, lb=self.lb, ub=self.ub)