Check and assign return value of function in one line

Question

I'm assigning many properties to a dictionary, like this:

dict = {
    "prop1": round(myfunc1(), 2),
    "prop2": round(myfunc2(), 2),
}

myfunc() returns either None or a float number.

I need to do this:

result = myfunc()
if result is not None:
    result = round(result, 2)

But in one line.

The reason a normal Python one liner would not be ok is that myfunc() is a long and resource intensive function, and I definitely don't want to call it twice or implement any lazy loading inside it. This would not be ok:

result = round(myfunc(), 2) if myfunc() is not None else None

Is this doable?

This is exactly why the walrus operator (`:=`) was introduced in 3.8. Not available in previous versions though. — Daniel Roseman, Aug 28 '19 at 17:57
@d_kennetz `TypeError: type NoneType doesn't define __round__ method` — walnut, Aug 28 '19 at 17:59
You could make a helper function or lambda: `round2 = lambda x: x if x is None else round(x, 2)`. Then create the dict with `"prop1": round2(myfunc1()),....` — 001, Aug 28 '19 at 18:31
Does this answer your question? [Python - assign value and check condition in IF statment](https://stackoverflow.com/q/39739510/90527) — outis, May 02 '22 at 01:27

score 3 · Accepted Answer · answered Aug 28 '19 at 18:02

3

Ok... this is pretty horrible, and I'm not proud, but if it's gotta be one line:

result = next(None if v is None else round(v, 2) for v in (my_func(),))

Is it worth avoiding two lines over? Only you can know.

answered Aug 28 '19 at 18:02

Jon Betts

3,053
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score 3 · Answer 2 · answered Aug 28 '19 at 18:03

3

Python 3.8 (applying Walrus Operator)

result = round(f, 2) if f := myfunc() is not None else None

answered Aug 28 '19 at 18:03

Check and assign return value of function in one line

2 Answers2