[Python-ideas] Control Flow - Never Executed Loop Body

[Sven R. Kunze]

Hi Python-ideas,

this is a follow up from an thread of Python-List: 
https://mail.python.org/pipermail/python-list/2016-March/705205.html


What do you think about the following proposal?


*Abstract*
Control structures such as "for", "while" and "try" feature some various 
additional clauses being executed under certain conditions. This 
proposal adds another extra clause to "for" and "while" in order to 
provide an easy way to discover whether the body of a loop as never been 
executed at all.

*Motivation**
*Coming from the Web application development, we periodically can see a 
benefit of a hypothetical "empty" clause of Python's for loop like this:

for item in my_iterator:
      # do per item
empty:
      # do something else

The same goes for "while" although there the choice of the name "empty" 
might be questionable. "empty" refers to the involved iterable which is 
not really existing in case of "while". However, there is not need of 
introducing inconsistencies among the loop constructs as both feature 
the "else" clause as well. Better suggestions are welcome; also see the 
keyword section.

*Keyword*
Keywords under consideration have been in (an attempt of dispassionate) 
order of preference:
1) empty -> most obvious, most people responded with a solution to solve 
the intended problem
2) else -> already taken but to some would be the preferred one
3) or -> as alternative if a new keyword is too much
4) except -> as alternative if a new keyword is too much
5) instead -> less obvious and new keyword

*Use-Cases*
1) As humans mostly have a hard time to discover the difference between 
*nothing**as error* and *nothing**as really empty*, GUI/Web interfaces 
display an explicit notice in case of empty lists to signify the correct 
execution of a process/action/search or the other case.

2) Single way and no boilerplate of handling empty loops. As the 
discussion on Python-List showed are there quite some number of 
different ways of achieving this kind of functionality.

3) Less errors in development and maintenance. Some of the proposed 
solutions would not work in some circumstances (either during 
development or depending on the type of the items) which can hide errors.

*Implementation*
To be done.

*Maintainer*
Me.

*Issues**
*People I talked to suggested "else" as an alternative to "empty" 
because "empty is not quite clear". Unfortunately, "else" is already 
taken and is itself not quite clear. "then" has been proposed as an 
alternative for "else" in an attempt for proper understanding of "else". 
If that would be an accepted proposal, it would make room for "else" to 
be used for the usage of the "empty keyword proposed here.

*Related Work**
*This proposal is partly inspired by template engines like Django or 
jinja2 which both already provide such facility.

Django:
{% for item in my_iterator %}
...
{% empty %}
...
{% endfor %}

jinja2:
{% for item in my_iterator %}
...
{% else %}
...
{% endfor %}

Best,
Sven
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