On 3/24/14 5:43 PM, Marko Rauhamaa wrote:
Mark H Harris<harrismh...@gmail.com>:
Yes, its about closures, totally; the most confusing aspect of
lambda in python is not only the syntax but the idea of scope and
closure (for that syntax). Everyone is confused by this initially, not
because its complicated, but because its confusing. An example:
adders= list(range(4))
for n in adders:
adders[n]=lambda a: a+n
print(adders[1](3))
6
The expected value as perceived by "normal" people is 4.
1. No, I don't think that understanding is automatically natural.
It might not seem that way for an expert, but if you Google python
lambda function closure(s) you will notice that this is pretty much the
natural way of interpreting things.
Of course the problem is that the closure grabs the *last* number in
the list which is used for each of the adder[] functions created. So, in
other words, three (3) is the number added in each of the adder
functions. But here is the rub, it is not *ever* clear to people (even
experienced python coders, me for instance) that this is how it should
work. What is needed is the explicit closure "grab" recommended by
ChrisA. But for the normal, that is just as bad (conceptually) because
while it works it strays FAR away from expected lambda constructs known
to functional programmers, and it is difficult to explain to non
functional programmers... a proverbial catch 22.
marcus
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