Managing thirdparty package: defer.

thirdparty/defer/errors.py

@@ -0,0 +1,13 @@
+from types import FunctionType
+
+
+class DeferErrror(RuntimeError):
+    pass
+
+
+class FreeVarsError(DeferErrror):
+    def __init__(self, fn: FunctionType) -> None:
+        super().__init__(
+            "deferred function must not have free variables",
+        )
+        self.add_note("free vars: " + str(list(fn.__code__.co_freevars)))