Hi all,
It might be a risk that we create some hairy or subtle semantics, but perhaps
this idea has legs. The 'pythonic' syntax is very attractive.
# skip the silence at the start
data = (amp for amp in signal from abs(amp) > 0.05)
# drop files that are less than 600 bytes
# (already sorted into ascending order of size)
files = (file for file in filelist from os.stat(file).st_size > 600)
# Analyse bunch of log lines up to a certain timestamp
cutoff = datetime.datetime.now() - datetime.timedelta(hours=1)
checklogs = (entry for entry in log while entry.timestamp <= cutoff)
In use it feels along the same lines as else: clauses in try/finally and for
loops, both of which I like-- it seems natural in Python, keeps the meaning
clear and consistent, avoids complicated, iterative, inexpressive code.
Terry Reedy wrote:
> You're right. The syntax is ambiguous. I agree it's not a good idea,
now. :)
When while, if and from are made mutually exclusive, not mixed, on a
comprehension clause, it looks to me like the ambiguity disappears:
> x for x in xs while cond(x) if blah(x)
not legal Python-- if and while are mutually exclusive on the same
comprehension clause. On the one hand it's like writing x for x in xs if f(x) <
10 if g(x) > 100, the illegality of which no-one is complaining about. Just
write:
x for x in xs while cond(x) and cond2(x)
or
x for x in xs if cond(x) and cond2(x)
If there is a need to filter using if as well as while, then use two
comprehensions:
intermediate = [x for x in xs while cond(x)]
final = [x for x in intermediate if cond2(x)]
or the same comprehension applied to itself:
y for y in x for x in xs if cond(x) while cond2(y)
The if and the while operation *should* be kept separate-- they're different
stages of processing the data.
> > x*y for x in xs while cond(x) for y in ys if cond(y)
legal Python-- the 'if cond(y)' unambiguously refers to the inner
comprehension.
> x for x in xs if blah(x) while cond(x)
not legal Python-- if and while are mutually exclusive in the same
comprehension clause.
The objection to "while" as an abbreviation:
> e(x) for x in xs if cond(x)
>
> is an abbreviation of
>
> for x in xs:
> if cond(x)
> yield e(x)
>
> and similarly with more for and if clauses,
> whereas the analogous expansion of your proposal
>
> for x in xs:
> while cond(x):
> yield e(x)
>
> is an infinite loop and not at all what you mean.
That's because while is being used here with a different semantic to the while
loop. It actually means:
for x in xs:
if not cond(x):
break
yield e(x)
and for 'from':
emit = False
for x in xs:
if not emit:
if cond(x):
emit = True
else:
yield e(x)
Cheers,
Nick
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