assignment expressions with conditional expression

Question

The following snippet

def expensive_function(x):
    return x

x = 10.5
(int(y) if y.is_integer() else y := expensive_function(x))

raises

SyntaxError: cannot use assignment expressions with conditional expression

Can assignment expressions not be used in this way?

If not, what is wrong with my assumption: I was under the impression the idea is to pre-assign a dummy value to an expensive operation in a single expression.

To clarify the idea is asking if assignment operations can be used to simplify by assigning expensive_function(x) to a dummy variable

def expensive_function(x):
    return x

x = 10.5
(int(expensive_function(x))
 if expensive_function(x).is_integer()
 else expensive_function(x))

Could you not just do `int(y) if y.is_integer() else expensive_function(x)`? — Sayse, Oct 16 '20 at 14:14
this is a minimal example - the code structure can be changed but I am focusing on the usage of the assignment operator explicitly with conditional expressions — Alexander McFarlane, Oct 16 '20 at 14:16
In your minimal sample, the role of `y` is not clear. When are you assigning value to `y`, before invoking its `is_integer()` method? — fountainhead, Oct 16 '20 at 14:21
that's the issue - I wasn't aware that the statement was assigned in order from left to right. I thought that assignments were made up-front — Alexander McFarlane, Oct 16 '20 at 14:23
Your edit made the question even more confusing, I'd assumed that `y` was a different variable predefined other than `x`, now its unclear what you're trying to save yourself from doing — Sayse, Oct 16 '20 at 14:24
Alexander: While the snippet reproduces the error, `y` is undefined, which makes working on a solution more difficult. It appears to be the name of a `float` value, but I shouldn't need to guess. Being imprecise like that is likely at least part of the reason why there have been so many answers to your questio — and it wastes everyone's time. — martineau, Oct 16 '20 at 15:16

keepAlive · Accepted Answer · 2020-10-26T10:09:48.130

2

What about

z = int(y) if (y := expensive_function(x)).is_integer() else y

?

Actually, in a if cond else b, there are two conditional expressions: the a- and the b-members. But the middle member, i.e. the cond one is not conditional: it is always evaluated, explaining why using an assigment operator there raises no error.

A prior-to-3.8 approach (i.e. with no Walrus Operator) can be

z = (
    lambda y: int(y) if y.is_integer() else y
)(
    expensive_function(x)
)

edited Oct 26 '20 at 10:09

answered Oct 16 '20 at 14:24

keepAlive

4,938
4
20
36

This doesn't answer the question of "Can assignment expressions not be used in this way?" – Sayse Oct 16 '20 at 14:26
@Sayse does it answer the question now ? :) – keepAlive Oct 16 '20 at 14:46
1

that seems exactly what I'm asking for... I will check – Alexander McFarlane Oct 16 '20 at 14:50
@keepAlive - I still don't understand what the question is exactly – Sayse Oct 16 '20 at 14:51
1

Thanks, this addresses both the issue of the assignment not being evaluated up-front as well as showing how to use assignments to avoid expensive function calls in a loop – Alexander McFarlane Oct 16 '20 at 15:52

score 1 · Answer 2 · answered Oct 16 '20 at 14:22

1

int(y) if y.is_integer() else y := expensive_function(x)

is equivalent to

def foo(x):
  if y.is_integer():
     return int(y)
  else:
     y = expensive_function(x)
     return y
foo(x)

Now you can see where the problem is. y isn't defined!

answered Oct 16 '20 at 14:22

Eeshaan

1,158
3
16

thanks - I previously thought assignment was done first; then the rest of the expression – Alexander McFarlane Oct 16 '20 at 14:49
great! although I was wondering why you would want to convert to int if it's already an int?! – Eeshaan Oct 16 '20 at 15:00
perhaps you were looking for `if not y.is_integer()`? – Eeshaan Oct 16 '20 at 15:03
Maybe because `is_integer` does the same kind of job as `isdigit`, which checks implicite types of strings, e.g. `int('2') if '2'.isdigit() else '2'`. – keepAlive Oct 16 '20 at 15:07
@keepAlive oh yeah, that could be! – Eeshaan Oct 16 '20 at 15:11
@AlexanderMcFarlane would be great, if you can upvote / accept one our answers. If none of the answers are helpful, kindly tell us what exactly you're looking for – Eeshaan Oct 16 '20 at 15:15
both are answer two parts of the question tbh - sorry I had to run into meetings. Yours pointed out that misnomer that the assignment wasn't evaluated upfront, whereas https://stackoverflow.com/a/64391064/4013571 shows how to correctly assign it - as a result I upvoted them both, many thanks for your help – Alexander McFarlane Oct 16 '20 at 15:50

assignment expressions with conditional expression

2 Answers2