Re: Conditional iteration

Dear Duncan,

Points taken. Its just a workaround for a specific case, I know. Maybe I
just love the elegance of

new_list = [x for x in some_list if some_cond]

and like to see it extended...

I got I nice tip on generators however which would allow me to something
similar without consuming too much additional resources.

Kind regards,

Arjan




Duncan Booth wrote:

at <at@xxxxxxx> wrote:

By the way,

I think by approving

a = b if condition else c

used to avloind

if condition:
a = b
else:
a = c

Neither of those is much of an improvement over the other, and in fact if
b or c are complex expressions I would definitely favour the longhand
form. The benefit of having a conditional expression though is that in
some situations it can make the code clearer by allowing you to avoid
creating a name at all. e.g. if 'a' was then used as a parameter in a
function call:

d = fn(b if condition else c)

and a has disappeared entirely.


which is dealing with same psychological problem, Guido also
recognizes some need...

Is it redundant according to your criteria, yes I would say:

a = {True: a, False: c}[condition]

or

a = [c, a][condition]

would yield exactly the same even in one sentence....

You do realise, I hope, that neither of these last two gives the same
results as the inline 'if'?

x = 0
print 3/x if x != 0 else -1
-1
print {True: 3/x, False: -1}[x != 0]

Traceback (most recent call last):
File "<pyshell#2>", line 1, in <module>
print {True: 3/x, False: -1}[x != 0]
ZeroDivisionError: integer division or modulo by zero
print [-1, 3/x][x != 0]

Traceback (most recent call last):
File "<pyshell#3>", line 1, in <module>
print [-1, 3/x][x != 0]
ZeroDivisionError: integer division or modulo by zero


and before you suggest the good old standby and/or technique, that fails
for other values:

print x != 0 and 3/x or -1
-1
x=5
print x != 0 and 3/x or -1
-1


You need to use the messy:

print (x != 0 and (3/x,) or (-1,))[0]
0

to get exactly the same effect as the inline if with loss of both
readability and performance.

.

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