Do not re-evaluate condition in conditional assignment

Question

Is there a way to do the following in a one-liner:

x = heavy_computation() if heavy_computation() > 0 else None

without heavy_computation() being called twice? Of course, one could do:

val = heavy_computation()
x = val if val > 0 else None

but I was wondering if there is a more pythonic way to achieve this in one line. This might be useful for heavy computations but also for better code readability.

it depends. If negative values aren't possible, for example, `heavy_computation() or None`. — jonrsharpe, Oct 17 '22 at 11:53

score 1 · Accepted Answer · answered Oct 17 '22 at 11:53

1

You could consider using the walrus operator (PEP 572):

x = val if (val := heavy_computation()) > 0 else None

Note that it requires a recent Python version (at least 3.8).

I'm not sure if it's much more readable though.

answered Oct 17 '22 at 11:53

jfschaefer

293
1
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Thanks! I think it's less readable than the common if else but still better than using two lines – Federico Taschin Oct 17 '22 at 11:54

Do not re-evaluate condition in conditional assignment

1 Answers1