Doc string

Author: huonw

mypy/checker.py

@@ -5732,6 +5732,16 @@ def get_expr_name() -> str:
             self.fail(message_registry.TYPE_ALWAYS_TRUE.format(format_expr_type()), expr)
 
     def check_for_optional_non_truthy_type(self, t: Type, expr: Expression) -> None:
+        """Check if a type involves both None and types that aren't always true, catching
+        suspicious cases of falsy values being lumped together with None.
+
+        Used in checks like::
+
+            if x: # <---
+
+            not x  # <---
+
+        """
         t = get_proper_type(t)
         if not isinstance(t, UnionType):
             return  # not a Optional or Union at all