Empty sets and frozensets are also false.

Doc/ref/ref5.tex

@@ -1021,9 +1021,9 @@ Boolean operations have the lowest priority of all Python operations:
 
 In the context of Boolean operations, and also when expressions are
 used by control flow statements, the following values are interpreted
-as false: \code{False}, \code{None}, numeric zero of all types, empty
-sequences (strings, tuples and lists), and empty mappings (dictionaries).
-All other values are interpreted as true.
+as false: \code{False}, \code{None}, numeric zero of all types, and empty
+strings and containers (including strings, tuples, lists, dictionaries,
+sets and frozensets).  All other values are interpreted as true.
 
 The operator \keyword{not} yields \code{True} if its argument is false,
 \code{False} otherwise.