[Python-ideas] if condition: break idiom

Josiah Carlson josiah.carlson at gmail.com
Sun Sep 21 22:24:10 CEST 2008

On Sun, Sep 21, 2008 at 12:52 PM, Arnaud Delobelle
<arnodel at googlemail.com> wrote:
> On 21 Sep 2008, at 20:24, Josiah Carlson wrote:
>> Not all statements; all *control flow* statements.  Return, yield,
>> continue, break, assert, ..., can all change program flow.  To say
>> that break and continue should be special cased is silly, as "Special
>> cases aren't special enough to break the rules".  As such, why
>> continue and break but not return or yield?  Further, the syntax is
>> *so very similar* to conditional expressions <X> if <C> vs. <X> if <C>
>> else <Y>, the lack of an else clause could be confusing to those who
>> have seen and used conditional expressions before, never mind the
>> meaning of them.
> Well my view was that break and continue are the only two statements that
> relate to loops.
> [...]

Superficially yes, but the execution of raise, return or yield can
result in the loop never completing another pass.  That a loop can
occur in an except clause even offers the ability for the no-argument
raise to make sense even there ;) .

>>> I don't agree with that: the absence of do .. while liberates the loop
>>> construct in python from its constraints and the first form above becomes
>>> more common.
>> But you were just arguing that the *lack* of do/while makes the
>> embedded if necessary.  Now you are saying that it *liberates* us from
>> the control-flow induced by do/while.  ;)  There's an argument that
>> says rather than treat the symptom (breaks in the body), treat the
>> disease (lack of a do/while).  But since the lack of a do/while isn't
>> a disease, by your own words, then the symptom is not a bug, it's a
>> feature ;)
> There is a missing link in your interpretation of my argumentation.  It is
> that I haved noticed that, as I do not have a do .. while construct at my
> disposal in Python, I do not try to shape my loops into this structure
> anymore.  I almost *never* write:
>    while True:
>        ...
>        if condition: break
> But most of the time it seems that the correct structure for a loop comes as
>    while True:
>        ...
>        if condition: break
>        ...

That also has a translation with the "first" idiom, though it's a bit
uglier (no, really):

first = 1
while first or condition:
    if not first:
    first = 0

Really though, what is pretty/elegant really varies depending on what
those ... turn into.  To be honest, I've only ever used the above once
or twice, though the standard first idiom I've used dozens of times.
Though if you really want to have fun, there's nothing like exploiting
while loops for checking error conditions. ;)

def foo(...):
    first = 1
    while first:
        first = 0
        if error_condition1:
        ... #other error conditions, processing, whatever
        #only executes if everything is ok
    #error condition

 - Josiah

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