diff --git a/README.md b/README.md
index e0d1aa5..f7505e2 100644
--- a/README.md
+++ b/README.md
@@ -31,9 +31,10 @@ pip install lib-not-dr
 
 ```python
 import subprocess
-from lib_not_dr.nuitka import CompilerHelper
+from pathlib import Path
+from lib_not_dr.nuitka.compile import CompilerHelper
 
-compiler = CompilerHelper("main.py")
+compiler = CompilerHelper(src_file = Path("main.py"))
 
 print(compiler)
 subprocess.run(compiler.gen_subprocess_cmd())
@@ -45,9 +46,10 @@ subprocess.run(compiler.gen_subprocess_cmd())
 ```python
 import sys
 import subprocess
-from lib_not_dr.nuitka import CompilerHelper
+from pathlib import Path
+from lib_not_dr.nuitka.compile import CompilerHelper
 
-compiler = CompilerHelper("main.py", run_after_build=True)
+compiler = CompilerHelper(src_file = Path("main.py"), run_after_build=True)
 
 print(compiler)
 
diff --git a/lib_not_dr/command/descriptor.py b/lib_not_dr/command/descriptor.py
index 17c8cc6..5a479b7 100644
--- a/lib_not_dr/command/descriptor.py
+++ b/lib_not_dr/command/descriptor.py
@@ -1,7 +1,13 @@
 class CallBackDescriptor:
-    def __set_name__(self, owner, name):
+    def __init__(self, name):
         self.callback_name = name
 
     def __set__(self, instance, value):
-        if 
+        assert getattr(instance, self.callback_name) is None, f"Attribute '{self.callback_name}' has been set."
+        instance.__dict__[self.callback_name] = value
 
+    def __get__(self, instance, owner):
+        if instance is None:
+            return self
+        else:
+            return instance.__dict__[self.callback_name]
diff --git a/lib_not_dr/command/nodes.py b/lib_not_dr/command/nodes.py
index 943769a..0ed3a06 100644
--- a/lib_not_dr/command/nodes.py
+++ b/lib_not_dr/command/nodes.py
@@ -6,6 +6,9 @@
 
 import re
 from typing import Callable, List, Optional, Union
+
+from .descriptor import CallBackDescriptor
+
 try:
     from typing import Self
 except ImportError:
@@ -26,6 +29,15 @@ EMPTY_WORDS = re.compile(r"\s", re.I)
 
 
 def check_once(cls, name) -> None:
+    """
+    Check whether the attribute has been set.
+    if so, it will raise exception `AttributeError`.
+    检查属性是否已经被设置。
+    如果已经被设置，将会抛出 `AttributeError` 异常。
+    :param cls: class object
+    :param name: attribute name
+    :return: None
+    """
     if hasattr(cls, name):
         if getattr(cls, name) is not None:
             raise AttributeError(f"Attribute '{name}' has been set.")
@@ -65,6 +77,10 @@ class Flag:
 
 
 class Literal:
+    _tip = CallBackDescriptor("_tip")
+    _func = CallBackDescriptor("_func")
+    _err_callback = CallBackDescriptor("_err_callback")
+
     def __init__(self, name: str):
         self.name: str = name
         self.sub: List[Self] = []