Tiny avoid EnvField misuse (#15612)

python/sglang/srt/environ.py

@@ -7,6 +7,8 @@ from typing import Any
 
 
 class EnvField:
+    _allow_set_name = True
+
     def __init__(self, default: Any):
         self.default = default
         # NOTE: we use None to indicate whether the value is set or not
@@ -15,6 +17,7 @@ class EnvField:
         self._set_to_none = False
 
     def __set_name__(self, owner, name):
+        assert EnvField._allow_set_name, "Usage like `a = envs.A` is not allowed"
         self.name = name
 
     def parse(self, value: str) -> Any:
@@ -379,6 +382,7 @@ class Envs:
 
 
 envs = Envs()
+EnvField._allow_set_name = False
 
 
 def _print_deprecated_env(new_name: str, old_name: str):