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