list comprehension question

[J Kenneth King]

Steven D'Aprano <steve at REMOVE-THIS-cybersource.com.au> writes:

> On Thu, 07 May 2009 13:28:10 -0400, J Kenneth King wrote:
>
>> Steven D'Aprano <steven at REMOVE.THIS.cybersource.com.au> writes:
>> 
>>> On Wed, 06 May 2009 09:48:51 -0400, J Kenneth King wrote:
>>>
>>>> Emile van Sebille <emile at fenx.com> writes:
>>>> 
>>>>> On 5/5/2009 9:15 AM J Kenneth King said...
>>>>>
>>>>>> List comprehensions can make a reader of your code apprehensive
>>>>>> because it can read like a run-on sentence and thus be difficult to
>>>>>> parse. The Python documentation discourages their use and I believe
>>>>>> for good reason.
>>>>>
>>>>> Can you provide a link for this?  I'd like to see specifically what's
>>>>> being discouraged, as I'd be surprised to find routine usage frowned
>>>>> upon.
>>>>>
>>>>> Emile
>>>> 
>>>> http://docs.python.org/tutorial/datastructures.html#nested-list-
>>> comprehensions
>>>> 
>>>> 
>>>> "If you’ve got the stomach for it, list comprehensions can be nested.
>>>> They are a powerful tool but – like all powerful tools – they need to
>>>> be used carefully, if at all."
>>>
>>> How does this discourage the use of list comprehensions? At most, it
>>> warns that complicated list comps are tricky. Complicated *anything*
>>> are tricky.
>> 
>> They are tricky and need to be used carefully, *if at all*.
>> 
>> IMO this means that if there's a way to do it without a nested list
>> comprehension, then that solution should be preferred.
>
> Earlier, you claimed that list comps in general were discouraged:
>
> "List comprehensions can make a reader of your code apprehensive because 
> it can read like a run-on sentence and thus be difficult to parse. The 
> Python documentation discourages their use and I believe for good reason."

Ooops. Typo. My bad. Had it been quoted earlier it would have saved a
few posts for sure. Such is the Internet. :)

> Emile said "I'd be surprised to find routine usage frowned upon", giving 
> you the opportunity to correct his (mis)understanding. You failed to do 
> so, which I took as meaning that you agreed that routine usage of simple 
> list comps were frowned upon. Now you're talking about *nested* list 
> comps. You started off (apparently) objecting to list comps in general, 
> because they "can" make readers apprehensive. Now you seem to be saying 
> that it's only the complicated, overly-dense ones which rightly make 
> readers apprehensive which you object to.

I was rather confused by that. I use list comps all the time and said
several times that I don't object to list comps, just nested ones.

> I suppose we're making progress if we agree that the Python docs warn 
> against unnecessarily complicated nested list comps. Whether it 
> discourages as well as warns is a matter of interpretation. But there's 
> certainly no sign that list comps as a general technique are discouraged 
> just because overly-complicated list comps are tricky to read. The same 
> can be said about *any* piece of code.

Well I did my best to make my interpretation clear. If the
documentation says that nested list comps are difficult to read and
should be used rarely, if at all, then I generally consider that
"discouraging" their use. Meaning of course that in the right
situation one may be able to justify their own use of a nested comp.
However, for every day problems one wouldn't be encouraged to use them
so liberally as some tend to do.


So.. we have some sort of consensus then? This might be a rare
phenomenon on Usenet... :)

Cheers,

J.