[Python-Dev] capturing RETURN_VALUE (original) (raw)

Christian Tismer tismer at stackless.com
Sun Aug 8 19:04:35 CEST 2004

Dear list,

just by chance, I discovered a probably very unknown feature of the block stack machinery of Python's frames:

-- It is possible to abandon a return statement. --

My question: Is this an artifact, or can I rely on this feature to persist for a while (while while >= 1 year) ?

Example code: I have a code body which either computes and returns a value, or it raises an exception. Surrounding it, I have a while true loop, embracing a try..finally block. In case of an exception, it simply lets the exception pass. In case of a return value, it intercepts it and can do anything (in the example, it just decorates the value and returns).

def return_capture(n, do_raise=False): _have_value = False # outer while loop, just there to get cancelled while True: try: # function body, returning a value or raising if do_raise: raise ValueError, "was told to raise" # signal valid value _have_value = True retval = n*n return finally: if _have_value: break # here we end after a captured return, and can do postprocessing retval = ("here the captured return value", retval) return retval

Sample output:

return_capture(5) ('here the captured return value', 25) return_capture(5, True) Traceback (most recent call last): File "", line 1, in ? File "", line 8, in return_capture ValueError: was told to raise

Again the question: Artifact, or a reliable feature?

ciao - chris

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