Yield after the return in Python function.

[Avi Gross]

	Terry: ... '__missing__' is new since I learned Python ...

With so many new dunder variables added, I am wondering when some dunderhead
comes up with:

	__mifflin__

The documented use paper is:

https://theoffice.fandom.com/wiki/Dunder_Mifflin_Paper_Company



-----Original Message-----
From: Python-list <python-list-bounces+avigross=verizon.net at python.org> On
Behalf Of Terry Reedy
Sent: Monday, April 5, 2021 3:01 PM
To: python-list at python.org
Subject: Re: Yield after the return in Python function.

On 4/5/2021 1:53 PM, Chris Angelico wrote:
> On Tue, Apr 6, 2021 at 3:46 AM Terry Reedy <tjreedy at udel.edu> wrote:
>> *While 'a and not a' == False in logic, in Python it might raise 
>> NameError.  But that would still mean that it is never True, making 
>> 'yield 0' still unreachable.

When I wrote that, I knew I might be missing something else.

> And even just the lookup can have side effects, if your code is 
> pathologically stupid.

Or pathologically clever.

>>>> class Wat(dict):
> ...     def __missing__(self, key):
> ...             global count
> ...             count -= 1
> ...             return count

'__missing__' is new since I learned Python.  I barely took note of its
addition and have never used it.  Thanks for the example of what it can do.
One could also make it randomly return True or False.

>>>> count = 2
>>>> eval("print(a and not a)", Wat(print=print))
> True
> 
> So Python can't afford to treat this as dead code.

This gets to the point that logic and math are usually atemporal or at least
static (as in a frozen snapshot), while computing is dynamic.  In algebra,
the canon is that all instances of a variable are replaced by the same
value.

Python *could* do the same for expresssions: load 'a' (in this case) once
into a register or stack slot and use that value consistently throughout the
expression.  Replacing the eval with the following exec has the same effect.

exec("tem=a; print(tem and not tem)", Wat(print=print)) # print False

In this example, one could disable the binding with __setitem__ (resulting
in printing 0), but python code cannot disable internal register or stack
assignments.

--
Terry Jan Reedy

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