Convert list to another form but providing same information

[Steven D'Aprano]

On Tuesday 22 March 2016 11:31, Ben Bacarisse wrote:

> Ian Kelly <ian.g.kelly at gmail.com> writes:
> 
>> On Mon, Mar 21, 2016 at 2:03 PM, Ben Bacarisse <ben.usenet at bsb.me.uk>
>> wrote:
>>> For experts here: why can't I write a lambda that has a statement in it
>>> (actually I wanted two: lambda l, i: l[i] += 1; return l)?
>>
>> https://docs.python.org/3/faq/design.html#why-can-t-lambda-expressions-
contain-statements
> 
> Thanks.  That makes it clear that it's just for syntactic and not
> semantic reasons.
> 
> However, the explanation ("because Python’s syntactic framework can't
> handle statements nested inside expressions") seemed, at first, to be
> saying you can't because you can't!  But the term "syntactic framework"
> hints that it's not really just an arbitrary choice -- that this is
> something about the way Python is parsed that make this choice
> inevitable.  Is it to do with the way that indentation has a syntactic
> role?

Don't ask me for a link, because I'm lazy, but this has been discussed like 
a bazillion times in the past, here, and on the Python-Ideas and Python-Dev 
mailing lists. No offense intended, but if you want definitive statements 
from the core developers, you can google for them :-)

But in a nutshell, there are four reasons for the limitation on Python's 
lambda statement:

- The use of indentation makes it hard to insert arbitrary statements into 
an expression without confusing the parser.

- Guido has a strict rule that Python's syntax must be parsable using a 
relatively simply parser. A LL(1) parser, if I remember correctly. So none 
of the fancy tricks and hard-to-understand syntax rules that (say) C++ uses.

- Given those limitations, nobody has been able to come up with syntax that 
has broad support.

- And besides, quite a large number of people believe that limiting lambda 
to simple, single-expression cases is a feature, not a bug.


Nobody has quite ruled out multi-statement lambda, or code blocks, but after 
20+ years of people trying to find syntax which works but isn't hated, and 
failing, I think it's probably safe to say it isn't going to happen.



-- 
Steve