[Python-ideas] Updating PEP 315: do-while loops

Terry Reedy tjreedy at udel.edu
Mon Apr 27 19:49:37 CEST 2009

A quick summary of my views:

1. When I programmed in C, I hardly if ever used do...while.  I have 
read that this is true of other C coders also.  So I see no need for a 
Python equivalent.

2. On the other hand, loop and a half use is a regular occurrence. 
Anything that does not arguably improve

while True:
   if cond: break

such as

whhile not cond:

seems pretty useless.  But even the above does not work great for 
multiple "if... break"s and not at all for "if...continue".

3. I do not see 'if cond: break' as that much of a problem.  One can 
emphasize with extra whitespace indent or comment.
   if cond:                         break #EXIT#

In any case, I see it as analogous to early return

def f():
   if cond: return

and while there are purists who object to *that*, I have not seen any 
proposals to officially support better emphasis (other than the existing 
whitespace or comment mechanisms).

In fact, it is not uncomment to use 'return expr' instead of 'ret=expr; 
break' when a loop (for or while) is the last part of a def.

4. "do ... while cond:" strikes me as ugly.  Aside from that, I do not 
like and would not use anything that puts the condition out of place, 
other than where it is executed.  I would hate to read such code and 
expect it would confuse many others.

5. We well never reach consensus.  PEP 315 might as well be withdrawn.

Terry Jan Reedy

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