empty clause of for loops

[Peter Otten]

Sven R. Kunze wrote:

> On 16.03.2016 11:47, Peter Otten wrote:
>>
>> What would you expect?
> 
> A keyword filling the missing functionality? Some Python magic, I
> haven't seen before. ;-)
> 
>>
>>>>> class Empty(Exception): pass
>> ...
>>>>> def check_empty(items):
>> ...     items = iter(items)
>> ...     try:
>> ...         yield next(items)
>> ...     except StopIteration:
>> ...         raise Empty
>> ...     yield from items
>> ...
>>>>> try:
>> ...    for item in check_empty("abc"): print(item)
>> ... except Empty: print("oops")
>> ...
>> a
>> b
>> c
>>>>> try:
>> ...    for item in check_empty(""): print(item)
>> ... except Empty: print("oops")
>> ...
>> oops
> 
> He will be highly delighted so see such a simplistic solution. ;-)
> 
>> I'm kidding, of course. Keep it simple and use a flag like you would in
>> any other language:
>>
>> empty = True:
>> for item in items:
>>      empty = False
>>      ...
>> if empty:
>>      ...
>>
> 
> He likes this approach. Thanks. :-)
> 
> 
> Although, I for one would like a keyword. I remember having this issue
> myself, and found that the "empty" variable approach is more like a
> pattern. As usual, patterns are workarounds for features that a language
> misses.

I'd put that the other way round: syntactical support for every pattern 
would make for a rather unwieldy language. You have to choose carefully, and 
this requirement could easily be fulfilled by a function, first in your 
personal toolbox, then in a public libary, then in the stdlib. 

If you don't like exceptions implement (or find) something like

items = peek(items)
if items.has_more():
   # at least one item
   for item in items:
       ...
else:
   # empty

Only if such a function is used a lot or cannot be conceived without severe 
shortcumings adding to the syntax should be considered. The (hypothetical) 
question you should answer: which current feature would you throw out to 
make room for your cool new addition?