for -- else: what was the motivation?

[avi.e.gross at gmail.com]

I just want to get clear on what may be a side issue

The suggestion is that a loop with an ELSE clause only makes sense if the
user can BREAK out without finishing, right?

My first question is whether a warning or even error makes sense if there is
no BREAK statement anywhere in the loop but there is another way to break
out such as a RETURN statement or perhaps from an error induced that is not
caught and then propagates outward?

Or is there some mechanism that would still run the ELSE clause in such
cases?

The second is to note some break statements are potentially never executed
such as: if False: break

There are actually scenarios where you can remain in a loop to the end and
not do anything rather than break, but of course in that case, the ELSE
clause running makes little sense to only run if you finish normally unless
it can use a shared Boolean variable you used to skip the rest of the loop
and use that to decide. That can be done easily enough without the ELSE, I
would think.



-----Original Message-----
From: Python-list <python-list-bounces+avi.e.gross=gmail.com at python.org> On
Behalf Of Weatherby,Gerard
Sent: Monday, October 10, 2022 2:25 PM
To: Axy <axy at declassed.art>; python-list at python.org
Subject: Re: for -- else: what was the motivation?

pylint, at least, provides a warning:

fe.py:4:0: W0120: Else clause on loop without a break statement
(useless-else-on-loop)


sum = 0
for i in range(5):
    sum += i
else:
    print("Always executes")
print(sum)


From: Python-list <python-list-bounces+gweatherby=uchc.edu at python.org> on
behalf of Axy via Python-list <python-list at python.org>
Date: Monday, October 10, 2022 at 1:10 PM
To: python-list at python.org <python-list at python.org>
Subject: Re: for -- else: what was the motivation?
*** Attention: This is an external email. Use caution responding, opening
attachments or clicking on links. ***

> On 10/10/2022 15:52, Weatherby,Gerard wrote:
>> I wonder if for/else could have been less confusing if it was 
>> referred to as for-break-else and if the else clause was only valid 
>> syntax if the for loop actually contained a break statement in the 
>> first place.
>
> Sounds reasonable. It would be something alike UnboundLocalError when 
> a local variable referenced before assignment. If they won't remove 
> "else" completely in far future, that checking really worths 
> implementing now.

Actually, I think a warning would be sufficient, as in the following quick
prototype. If someone can implement this quickly in CPython, that would be
great (last time I hacked it, it was 2.4)

Axy.

import ast

tree = ast.parse('''
# sample code
a = 0
for i in 'asd':
     a += i
     while x:
         pass
     else:
         print('wow')
     break
     print(i)
else:
     print(0)
''', mode='exec')

def check_ast(node):
     if isinstance(node, (ast.For, ast.AsyncFor, ast.While)):
         if node.orelse and have_no_break(node.body):
             print(f'Warning: the loop at line {node.lineno} has no "break"
statement,'
                   f' "else" clause at line {node.orelse[0].lineno} won\'t
run')
     else:
         for child in ast.iter_child_nodes(node):
             check_ast(child)

def have_no_break(loop_body):
     for node in loop_body:
         if isinstance(node, (ast.For, ast.AsyncFor, ast.While)):
             # nested loop
             check_ast(node)
         elif isinstance(node, ast.Break):
             return False
         elif isinstance(node, list):
             for child in ast.iter_child_nodes(node):
                 if have_no_break(child) == False:
                     return False
     return True


for node in tree.body:
     check_ast(node)
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