A short expression to pick from two values based on bool?

If one has two values in a list:

choices = [1,2]

and one wants to assign these values to two variables based on a bool parameter (let's call it ascending) like this:

if ascending:
  a,b = choices[1], choices[0]
else:
  a,b = choices[0], choices[1]

one can express this in one line like this:

a,b  = choices[ascending], choices[not ascending]

or

a,b = choices[::(-1)**ascending]

I like the last one better, but it's probably not extremely transparent. What are other concise but more readable ways to do the same?

If the original values are always in ascending order, perhaps this would be more expressive of your intent:

a,b = sorted(choices, reverse=ascending)

Otherwise simpler is generally better (see Kelly's answer) but I could think of ways to make it even more obscure:

For example:

a,b = (iter,reversed)[ascending](choices)

Alternatively, if one option is more likely than the other you could do the assignment based on that one and swap the variables if it's the other case:

a,b = choices
if ascending: a,b = b,a

Some ways...

b,a = choices[::ascending or -1]
a,b = choices[::-ascending or 1]
a,b = choices[::-1] if ascending else choices
a,b = choices.pop(ascending), *choices

The last one modifies the list.

Or with multiple lines but kinda clean and I suspect fastest:

if ascending:
b,a = choices
else:
a,b = choices

Benchmarks (times in nanoseconds):

 asc = False    asc = True
------------   ------------
30.2 ±  0.3    30.2 ±  0.3   if ascending:
b,a = choices
else:
a,b = choices
60.2 ±  1.3    60.8 ±  2.1   a,b = choices[ascending], choices[not ascending]
62.8 ±  0.6    63.5 ±  0.9   if ascending:
a,b = choices[1], choices[0]
else:
a,b = choices[0], choices[1]
30.3 ±  0.3   125.0 ±  3.7   a,b = choices[::-1] if ascending else choices
30.1 ±  0.1   178.2 ±  4.5   a,b = reversed(choices) if ascending else choices
124.5 ±  3.5   126.1 ±  5.8   b,a = choices[::ascending or -1]
133.6 ±  7.0   123.8 ±  2.6   a,b = choices[::-1 if ascending else 1]
143.4 ±  6.4   139.9 ±  6.1   a,b = choices[::-ascending or 1]
169.4 ± 18.5   441.2 ± 20.4   a,b = choices[::(-1)**ascending]

Code (Attempt This Online!):

from timeit import timeit
from statistics import mean, stdev

codes = '''

if ascending:
  a,b = choices[1], choices[0]
else:
  a,b = choices[0], choices[1]

a,b = choices[ascending], choices[not ascending]

a,b = choices[::(-1)**ascending]

b,a = choices[::ascending or -1]

a,b = choices[::-1 if ascending else 1]

if ascending:
  b,a = choices
else:
  a,b = choices

a,b = choices[::-ascending or 1]

a,b = choices[::-1] if ascending else choices

a,b = reversed(choices) if ascending else choices

'''.strip().split('\n\n')

choices = ['foo', 'bar']
for ascending in False, True:
  print(f'{ascending=}')
  for code in codes:
    a = b = None
    exec(code)
    print(a, b)
  print()

times = {c: ([], []) for c in codes}
def stats(c):
  false, true = (
    (mean(ts), stdev(ts))
    for ts in times[c]
    for ts in [[t*1e9 for t in sorted(ts)[:5]]]
  )
  return false[0] + true[0], false, true

for _ in range(25):
  for ascending in False, True:
    setup = f"choices = ['foo', 'bar']; {ascending = }"
    for c in codes:
      number = 10**5
      t = timeit(c, setup, number=number) / number
      times[c][ascending].append(t)

print('asc = False    asc = True')
for c in sorted(codes, key=stats):
  _, false, true = stats(c)
  # print('{:5.1f} ± {:4.1f}   {:5.1f} ± {:4.1f}  '.format(*false, *true), ' '.join(c.split()))
  print('{:5.1f} ± {:4.1f}   {:5.1f} ± {:4.1f}  '.format(*false, *true), c.strip().replace('\n', '\n' + ' '*30))