Python 3 removes name binding from outer scope

[Thomas Jollans]

On 2017-07-25 09:28, Chris Angelico wrote:
> On Tue, Jul 25, 2017 at 4:47 PM, Rustom Mody <rustompmody at gmail.com> wrote:
>> +1
>> You can call it bug or bug-promoted-to-feature :D
>>
>> I call it surprising because I dont know of any other case in python where
>> a delete is user-detectable
>> ie python's delete of objects always works quietly behind the scenes whereas
>> this adds a leakiness to the memory-management abstraction
> 
> You're conflating two things. There's nothing here that forces the
> destruction of an object; the name is simply unbound. You can confirm
> this from the disassembly in CPython:
> 
>>>> import dis
>>>> def f():
> ...  try: 1/0
> ...  except Exception as e: pass
> ...
>>>> dis.dis(f)
>   2           0 SETUP_EXCEPT            12 (to 14)
>               2 LOAD_CONST               1 (1)
>               4 LOAD_CONST               2 (0)
>               6 BINARY_TRUE_DIVIDE
>               8 POP_TOP
>              10 POP_BLOCK
>              12 JUMP_FORWARD            34 (to 48)
> 
>   3     >>   14 DUP_TOP
>              16 LOAD_GLOBAL              0 (Exception)
>              18 COMPARE_OP              10 (exception match)
>              20 POP_JUMP_IF_FALSE       46
>              22 POP_TOP
>              24 STORE_FAST               0 (e)
>              26 POP_TOP
>              28 SETUP_FINALLY            6 (to 36)
>              30 POP_BLOCK
>              32 POP_EXCEPT
>              34 LOAD_CONST               0 (None)
>         >>   36 LOAD_CONST               0 (None)
>              38 STORE_FAST               0 (e)
>              40 DELETE_FAST              0 (e)
>              42 END_FINALLY
>              44 JUMP_FORWARD             2 (to 48)
>         >>   46 END_FINALLY
>         >>   48 LOAD_CONST               0 (None)
>              50 RETURN_VALUE
>>>>
> 
> It actually does the equivalent of:
> 
> finally:
>     e = None

I wonder why it would bother to load None... (as someone not very
familiar with Python at the bytecode level)


-- Thomas

>     del e
> 
> In the normal case, this will leave the original exception loose and
> garbage-collectable, but if it's been bound to another name, it won't.
> 
> ChrisA
>