def inside if/else ternary expression

Question

Is there a way to convert this:

if counts:
    def a(l):
        return a_with_counts(l)
else:
    def a(l):
        return a_without_counts(l)

into a ternary expression?

I tried something like this

def a(l):
    return a_with_counts(l) if counts else a_without_counts(l)

but I do not want the if counts to be evaluated each time I call a(l), I want to do it once in the beginning of my method and then evaluate directly the assigned function each time I call a(l). Is this possible?

Thank you!

why not add `counts` as a parameter to the function `def a(l, counts=True):` etc. — Chris_Rands, Oct 26 '18 at 13:23
@B001ᛦ, because, for a given input, I will have to run that method many times and `counts` remains constant. — mkserge, Oct 26 '18 at 21:29
@Chris_Rands I don't want to evaluate `counts` each time. For a given input I want to do it once, and then use the assigned function throughout :) — mkserge, Oct 26 '18 at 21:31

jhansen · Accepted Answer · 2018-10-26T13:33:32.037

5

You could achieve that by defining a closure as follows:

def gen_a(counts):
    return a_with_counts if counts else a_without_counts

a = gen_a(counts)

This is equivalent to writing

a = a_with_counts if counts else a_without_counts

if you only intend to call this once.

edited Oct 26 '18 at 13:33

answered Oct 26 '18 at 13:23

jhansen

1,096
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Aha! Thank you, this is exactly what I need. I have to admit this wouldn't have crossed my mind! – mkserge Oct 26 '18 at 21:23

score 3 · Answer 2 · answered Oct 26 '18 at 13:22

3

with a ternary in a lambda ?

a = lambda counts:a_with_counts if counts else a_without_counts

then

a(True)   # or False

will create a_with_counts (resp a_without_counts) function that you can call.

answered Oct 26 '18 at 13:22

Jean-François Fabre

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you don't need a lambda to do this ... `a = (a_with_counts if counts else a_without_counts)(*args)` – Skandix Oct 26 '18 at 13:29
1

Named lambdas :S – roganjosh Oct 26 '18 at 13:29
3

Do note the Python [PEP 8 style guide](https://www.python.org/dev/peps/pep-0008/#programming-recommendations]) *Always use a def statement instead of an assignment statement that binds a lambda expression directly to an identifier.* – Parfait Oct 26 '18 at 13:29
@Parfait interesting. lambda are designed to remain anonymous. The sole interest here is to one-line define them. But jhansen answer (that I upvoted BTW) does that with a simple function. – Jean-François Fabre Oct 26 '18 at 13:32
1

@Skandix OP doesn't want to evaluate `counts` every time – Jean-François Fabre Oct 26 '18 at 13:33
1

damn I feel guilty about the `lambda` now because of you :) – Jean-François Fabre Oct 26 '18 at 13:35
I wasn't aware that you weren't aware they go against PEP8, thought you'd get my reference :) I'm not sure exactly what situation it avoids other than obscurity – roganjosh Oct 26 '18 at 13:36
well, I have some dark areas :) PEP talks about confusing tracebacks... thanks for your comments. duly noted. _must not use lambda, must not use lambda_ – Jean-François Fabre Oct 26 '18 at 13:37
@Jean-FrançoisFabre: ok, leave it with `a_gen = a_with_counts if counts else a_without_counts`, without the call. still no lambda needed (meanwhile this is jhansen's answer) – Skandix Oct 26 '18 at 13:39
1

It has been well mentioned on StackOverflow: see [here](https://stackoverflow.com/a/25010243/1422451), [here](https://stackoverflow.com/a/45790434/1422451), and [here](https://stackoverflow.com/a/50091199/1422451) – Parfait Oct 26 '18 at 13:41
damn I have read only 85% of the posts. Guess those ones went below the radar. – Jean-François Fabre Oct 26 '18 at 13:50
@Jean-FrançoisFabre, thanks I tried that, but ran into the same PEP warning :) Although it did not cross my mind that I can assign a name to the output of a(True) and use that afterwards :) – mkserge Oct 26 '18 at 21:34

def inside if/else ternary expression

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