Note that the DeprecationWarning exception is replaced with a SyntaxError to get a more accurate error report. $ cat yield-gen.py def f(): return ((yield x) for x in range(3)) $ ./python -Wd yield-gen.py yield-gen.py:2: DeprecationWarning: 'yield' inside generator expression return ((yield x) for x in range(3)) $ ./python -We yield-gen.py File "yield-gen.py", line 2 return ((yield x) for x in range(3)) ^ SyntaxError: 'yield' inside generator expression Without this replacement the result would be: $ ./python -We yield-gen.py DeprecationWarning: 'yield' inside generator expression