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....@python.org> On
Behalf Of Terry Reedy
Sent: Monday, April 5, 2021 3:01 PM
To: python-list@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 <tjre...@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
--
https://mail.python.org/mailman/listinfo/python-list
--
https://mail.python.org/mailman/listinfo/python-list
