On 11/07/2020 06:22, Олег Комлев wrote:
ELSE-clause in FOR and WHILE has unclear syntax. I suggest new clause instead:

if COND:
  ...
[elif COND:
  ...]
[else:
  ...]

This IF-clause like must be immediately after FOR- or WHILE-cycle (only comment allowed between). It looks like a regular IF, but COND is special.
COND may be "break", "pass" or "finally". "if break:" - if used break-operator to exit cycle. "if pass:" - cycle executed 0 times. "if finally:" - cycle executed 0 or more times ("pass-case"  is included in "finally-case"). For compatibility only "else:" means "if finally:".
It's compatible enhancement. No new keyword. There can be no combination "break", "pass" or "finally" after "if"/"elif:" in current version.
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This idea was suggested as enhancement issue (https://bugs.python.org/issue41272).
Eric V. Smith (eric.smith) advised me to put this on the python-ideas mailing list (https://bugs.python.org/issue41272#msg373486).
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Just to spell it out: If I understand you correctly, you are proposing that after a 'for'/'while' loop
    'if break:' has its intuitive meaning: "if the loop was terminated with a 'break'".
    'if finally:' means "if the loop completed (possibly with zero iterations) without a 'break'"
        (equivalent to 'else' at present).
    'if pass:' means "if there were zero iterations of the loop (obviously, without a 'break')"
    (I assume that a partial iteration terminated by 'continue' would count as an iteration)
plus 'elif' when you want to handle more than one of the above.  So
    if COND1:
        ...
    elif COND2:
        ...
would be legal, but
    if COND1:
        ...
    if COND2:
        ...
would not.

Something to consider: Would it be legal to mix these CONDs with other conditions in the 'elif' chain?  E.g.
    if break:
        ...
    elif x > 0:
        ...
    elif finally:
        ...

I would be the first to agree that the use of 'else' after 'for'/'while' is one of Python's more obscure features (I understand even veterans trip up over it).
I like the general idea and I like 'if break:', but IMO the meanings of the other two are not obvious.
I would suggest 'if not break:' rather than 'if finally:'.  (And PEP 8 could recommend it rather than 'else:'. :-))

My gut feeling (backed by no evidence) is that dealing with the case of zero iterations is not needed frequently enough to cater for it.
And I don't know if there is a good way to spell it (which might be another reason not to do it).
Plus it's something which could be added later.

Best wishes
Rob Cliffe