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[Python-Dev] Informal educator feedback on PEP 572 (was Re: 2018 Python Language Summit coverage, last part)

Terry Reedy tjreedy at udel.edu
Mon Jun 25 15:50:34 EDT 2018 
On 6/25/2018 8:25 AM, Paul Moore wrote:
> On 25 June 2018 at 12:44, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> Unfortunately, I think the key rationale for (b) is that if you
>> *don't* do something along those lines, then there's a different
>> strange scoping discrepancy that arises between the non-comprehension
>> forms of container displays and the comprehension forms:
> 
> I've been mostly ignoring this proposal for a while now, so I'm going
> to respond here in the context of someone with a bit of an idea of the
> underlying complexities, but otherwise coming at it as a new proposal.
> 
>>
>>      (NAME := EXPR,) # Binds a local
>>      tuple(NAME := EXPR for __ in range(1)) # Doesn't bind a local

Of course not, in local scopes where is it not executed.  But it would, 
in the nested function where the assignment *is* executed.  Ditto for 
all of the following.

>>      [NAME := EXPR] # Binds a local
>>      [NAME := EXPR for __ in range(1)] # Doesn't bind a local
>>      list(NAME := EXPR for __ in range(1)) # Doesn't bind a local
>>
>>      {NAME := EXPR} # Binds a local
>>      {NAME := EXPR for __ in range(1)} # Doesn't bind a local
>>      set(NAME := EXPR for __ in range(1)) # Doesn't bind a local
>>
>>      {NAME := EXPR : EXPR2} # Binds a local
>>      {NAME := EXPR : EXPR2 for __ in range(1)} # Doesn't bind a local
>>      set((NAME := EXPR, EXPR2) for __ in range(1)) # Doesn't bind a local
> 
> None of those "discrepancies" bother me in the slightest,

Me neither.  I pretty much agree with the rest of what Paul said.  If we 
don't want comprehensions to execute in a nested scope, then we should 
not create one.  See my response to Guido for a possible alternative.

-- 
Terry Jan Reedy
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