[Python-ideas] except expression

[Steven D'Aprano]

On Sun, Feb 16, 2014 at 06:47:20PM -0800, Ethan Furman wrote:
> On 02/15/2014 10:11 AM, Steven D'Aprano wrote:
> >
> >Certainly not! pass implies that *no return result is generated at all*,
> >[...]
> 
> Don't be silly.
> 
> def have_a_mint(some, args, here):
>     # flesh this out later
>     pass
> 
> Does anybody really think that that function will not return None?

But it isn't the *pass* that generates the "return None". It's falling 
out the bottom of the function that generates the "return None".

Do you intend for this function to return None?

def trouble_down_mine(args):
    pass
    return "NOBODY expects the Spanish Inquisition!!!"


"pass" does not mean "return from this function", as I'm sure you know. 
That's why I object to using "pass" in a form that implies "return 
None". It is a placeholder, used to satisfy the lexer/parser, and gets 
compiled out. As far as the compiler is concerned, "pass" simply 
disappears when compiled:

py> from dis import dis
py> dis("pass; spam")
  1           0 LOAD_NAME                0 (spam)
              3 POP_TOP
              4 LOAD_CONST               0 (None)
              7 RETURN_VALUE

You may notice that the compiler currently adds a (redundant?) "return 
None" to everything(?). That includes "pass", and nothing at all:

py> dis("pass")
  1           0 LOAD_CONST               0 (None)
              3 RETURN_VALUE
py> dis("")
  1           0 LOAD_CONST               0 (None)
              3 RETURN_VALUE



-- 
Steven