On 9/9/2021 5:36 PM, dn via Python-list wrote:
Why does Python not have a repeat-until loop construct?
1. It is not needed. You covered that.
2. It is rare useful. For loops are common. While loops are occasional
(nearly an order of magnitude less common than for loops. Fractional
loop constructs are rare. ("loop-and-a-half" is a mislabel since at not
even one loop is guaranteed.) "do-while" or "repeat-until is even rarer
since fractional-loop include this as a special case.
3. Adding 'until' as a keyword *now*, rather than in 1.0 or at least
several versions ago, has cost so far judged to outweigh the small
benefit. The PEP parser makes contextual keywords much more easily
possible but there is a cost to having a context dependent grammar.
Consider this 3.10.0 snippet:
>>> match, case = 1, 1
>>> match match:
... case case:
... print('matched')
...
...
matched
>>> match case:
... case match:
... print('matched')
...
...
matched
To me, having a word sometimes be a keyword and sometime not make code
harder to read. In IDLE, it is a bit easier as the keyword uses of
'match' and 'case' above are correctly highlighted as keywords, and the
non-keywords uses not highlighted. But this is harder that for
full-time keywords with sane code that works in an re-based highlighter.
Underscore, not used above, but also a new contextual keyword, is even
harder. Three of us could not get all the cases we tested correct and I
suspect doing so without running the PEG parser may be impossible.
Since highlighting is redone with each keystroke, I suspect doing the
latter would add a noticeable and unacceptable lag between keystrokes
and display.
--
Terry Jan Reedy
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