replacing `else` with `then` in `for` and `try`

[Steve D'Aprano]

On Thu, 2 Nov 2017 08:12 am, Alexey Muranov wrote:

> Hello,
> 
> what do you think about the idea of replacing "`else`" with "`then`" in
> the contexts of `for` and `try`?


Yes, this, exactly!!!

(For while and for loops, but not try -- see below.)

I have argued this for many years. The current choice of "else" is painfully
misleading, and it causes people (including myself) to wrongly guess that
the "else" block runs only if the for/while block doesn't run at all:


# This is wrong!
for x in sequence:
    ...
else:
    print("sequence is empty")


The actually semantics of "else" is that the block is UNCONDITIONALLY run
after the for/while loop completes, unless you jump out of the loop using
return, raise or break. That makes it a "then" block, not "else".


> It seems clear that it should be rather "then" than "else."  Compare
> also "try ... then ... finally" with "try ... else ... finally".

I disagree about the try block though. The semantics of the try block are:

try:
   A
except:
   B
else:
   C
finally:
   D

(1) code block A is attempted;

(2) IF an exception occurs, jump to code block B;

(3) otherwise (else), no exception occurs, so jump to code block C;

(4) finally run code block D on your way out, regardless of which blocks are
executed and how you exit them.


So I think "else" is correct here. The else block only gets called if there is
no exception.


> Currently, with "else", it is almost impossible to guess the meaning
> without looking into the documentation.

It is worse than that: it is easy to guess the WRONG meaning, namely that the
else block runs when the for/while loop doesn't execute at all (the for-loop
sequence is empty, or the while-loop condition is initially false).


> Off course, it should not be changed in Python 3, maybe in Python 4 or
> 5, but in Python 3 `then` could be an alias of `else` in these contexts.

Unfortunately, this is almost certainly not going to happen. It would require
adding a new keyword, and unless Guido changes his mind, he doesn't think
this change is worthwhile.

If I were the BDFL, this would be the first change I make :-)



-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.