diff --git a/python/triton/tools/build_extern.py b/python/triton/tools/build_extern.py
index 551def69d..f4141c31f 100644
--- a/python/triton/tools/build_extern.py
+++ b/python/triton/tools/build_extern.py
@@ -1,10 +1,24 @@
 import argparse
 import subprocess
 from abc import ABC, abstractmethod
+from typing import Dict, List, Optional
 
 
 class Symbol:
-    def __init__(self, name: str, op_name: str, ret_type: str, arg_names: list, arg_types: list) -> None:
+    _name: str
+    _op_name: str
+    _ret_type: str
+    _arg_names: List[str]
+    _arg_types: List[str]
+
+    def __init__(
+        self,
+        name: str,
+        op_name: str,
+        ret_type: str,
+        arg_names: List[str],
+        arg_types: List[str],
+    ) -> None:
         '''
         A symbol is a function declaration.
 
@@ -17,31 +31,31 @@ class Symbol:
         self._name = name
         self._op_name = op_name
         self._ret_type = ret_type
-        self._arg_names = arg_names
-        self._arg_types = arg_types
+        self._arg_names = list(arg_names)
+        self._arg_types = list(arg_types)
 
     @property
-    def name(self):
+    def name(self) -> str:
         return self._name
 
     @property
-    def op_name(self):
+    def op_name(self) -> str:
         return self._op_name
 
     @property
-    def ret_type(self):
+    def ret_type(self) -> str:
         return self._ret_type
 
     @property
-    def arg_names(self):
+    def arg_names(self) -> List[str]:
         return self._arg_names
 
     @property
-    def arg_types(self):
+    def arg_types(self) -> List[str]:
         return self._arg_types
 
 
-def convert_type(type_str):
+def convert_type(type_str) -> Optional[str]:
     if type_str == "i32":
         return "int32"
     elif type_str == "u32":
@@ -59,7 +73,7 @@ def convert_type(type_str):
         return None
 
 
-def to_unsigned(type_str):
+def to_unsigned(type_str) -> str:
     if type_str == "int32":
         return "uint32"
     elif type_str == "int64":
@@ -69,7 +83,19 @@ def to_unsigned(type_str):
 
 
 class ExternLibrary(ABC):
-    def __init__(self, name: str, path: str, format: bool = True, grouping: bool = True) -> None:
+    _name: str
+    _path: str
+    _symbols: Dict[str, Symbol]
+    _format: bool
+    _grouping: bool
+
+    def __init__(
+        self,
+        name: str,
+        path: str,
+        format: bool = True,
+        grouping: bool = True,
+    ) -> None:
         '''
         Abstract class for extern library.
 
@@ -80,34 +106,34 @@ class ExternLibrary(ABC):
         self._name = name
         self._path = path
         self._symbols = {}
-        self._format = True
+        self._format = format
         self._grouping = grouping
 
     @property
-    def name(self):
+    def name(self) -> str:
         return self._name
 
     @property
-    def path(self):
+    def path(self) -> str:
         return self._path
 
     @property
-    def symbols(self):
+    def symbols(self) -> Dict[str, Symbol]:
         return self._symbols
 
     @property
-    def grouping(self):
+    def grouping(self) -> bool:
         return self._grouping
 
     @abstractmethod
-    def parse_symbols(self, input_file):
+    def parse_symbols(self, input_file) -> None:
         pass
 
     @abstractmethod
     def _output_stubs(self) -> str:
         pass
 
-    def generate_stub_file(self, output_dir):
+    def generate_stub_file(self, output_dir) -> None:
         file_str = self._output_stubs()
         if file_str is None or len(file_str) == 0:
             raise Exception("file_str is empty")
@@ -123,6 +149,8 @@ class ExternLibrary(ABC):
 
 
 class Libdevice(ExternLibrary):
+    _symbol_groups: Dict[str, List[Symbol]]
+
     def __init__(self, path) -> None:
         '''
         Constructor for Libdevice.
@@ -132,7 +160,7 @@ class Libdevice(ExternLibrary):
         super().__init__("libdevice", path)
         self._symbol_groups = {}
 
-    def _extract_symbol(self, line):
+    def _extract_symbol(self, line) -> Optional[Symbol]:
         # Extract symbols from line in the following format:
         # "define [internal] <ret_type> @<name>(<arg_types>,)"
         entries = line.split("@")
@@ -174,7 +202,7 @@ class Libdevice(ExternLibrary):
                 arg_types[i] = to_unsigned(arg_type)
         return Symbol(func_name, op_name, ret_type, arg_names, arg_types)
 
-    def _group_symbols(self):
+    def _group_symbols(self) -> None:
         symbol_set = {}
         for symbol in self._symbols.values():
             op_name = symbol.op_name
@@ -244,7 +272,7 @@ class Libdevice(ExternLibrary):
             else:
                 self._symbol_groups[op_name] = [symbol]
 
-    def parse_symbols(self, input_file):
+    def parse_symbols(self, input_file) -> None:
         if len(self.symbols) > 0:
             return
         output = subprocess.check_output(["grep", "define", input_file]).decode().splitlines()
@@ -256,7 +284,7 @@ class Libdevice(ExternLibrary):
 
         self._group_symbols()
 
-    def _output_stubs(self):
+    def _output_stubs(self) -> str:
         # Generate python functions in the following format:
         # @extern.extern
         # def <op_name>(<args>, _builder=None):
@@ -297,7 +325,10 @@ class Libdevice(ExternLibrary):
 
 
 class LLVMDisassembler:
-    def __init__(self, path):
+    _path: str
+    _ll_file: str
+
+    def __init__(self, path) -> None:
         '''
         Invoke llvm-dis to disassemble the given file.
 
@@ -306,23 +337,28 @@ class LLVMDisassembler:
         self._path = path
         self._ll_file = "/tmp/extern_lib.ll"
 
-    def disasm(self, lib_path):
+    def disasm(self, lib_path: str) -> None:
         subprocess.Popen([self._path, lib_path, "-o", self.ll_file],
                          stdout=subprocess.PIPE).communicate()
 
     @property
-    def ll_file(self):
+    def ll_file(self) -> str:
         return self._ll_file
 
     @property
-    def path(self):
+    def path(self) -> str:
         return self._path
 
 
 extern_libs = ["libdevice"]
 
 
-def build(llvm_dis_path, lib_path, lib_name, output_dir):
+def build(
+    llvm_dis_path: str,
+    lib_path: str,
+    lib_name: str,
+    output_dir: str,
+) -> None:
     '''
       Interface function to build the library file.