[Python-ideas] PEP 572: Statement-Local Name Bindings

[Rob Cliffe]

On 28/02/2018 05:23, Chris Angelico wrote:
>
>> If calling `f(x)` is expensive or has side effects, the clean operation of
>> the list comprehension gets muddled. Using a short-duration name binding
>> retains the simplicity; while the extra `for` loop does achieve this, it
>> does so at the cost of dividing the expression visually, putting the named
>> part at the end of the comprehension instead of the beginning.
>>
>> Maybe add to last sentence "and of adding (at least conceptually) extra
>> steps: building a 1-element list, then extracting the first element"
> That's precisely the point that Serhiy's optimization is aiming at,
> with the intention of making "for x in [expr]" a standard idiom for
> list comp assignment. If we assume that this does become standard, it
> won't add the extra steps, but it does still push that expression out
> to the far end of the comprehension, whereas a named subexpression
> places it at first use.
I understand that creating the list could be avoided *at runtime*. My 
point was that in trying to *read and understand*
         stuff = [[y, y] for x in range(5) for y in [f(x)]]
the brain must follow the creation and unpacking of the list.  I.e. this 
is an extra cost of this particular construction.
>
>> And here's a thought: What are the semantics of
>>      a = (42 as a) # Of course a linter should point this out too
>> At first I thought this was also a laborious synonym for "a=42".  But then I
>> re-read your statement (the one I described above as crystal-clear) and
>> realised that its exact wording was even more critical than I had thought:
>>      "the new name binding will shadow the other name from the point where it
>> is evaluated until the end of the statement"
>> Note: "until the end of the statement".  NOT "until the end of the
>> expression".  The distinction matters.
>> If we take this as gospel, all this will do is create a temporary variable
>> "a", assign the value 42 to it twice, then discard it.  I.e. it effectively
>> does nothing, slowly.
>> Have I understood correctly?  Very likely you have considered this and mean
>> exactly what you say, but I am sure you will understand that I mean no
>> offence by querying it.
> Actually, that's a very good point, and I had to actually go and do
> that to confirm. You're correct that the "a =" part is also affected,
> but there may be more complicated edge cases.
I have read this thread so far - I can't say I have absorbed and 
understood it all, but I am left with a feeling that 
Expression-Local-Name-Bindings would be preferable to 
Statement-Local-Name_Bindings, so that the temporary variable wouldn't 
apply to the LHS in the above example.  I realise that this is a vague 
statement that needs far more definition, but - hand-waving for now - do 
you think it would be difficult to change the implementation accordingly?

Rob Cliffe