One-line multiple variable value assignment with an "if" condition

Question

It is possible to assign values to multiple variables.

a, b = 5, 10

I need to assign those values based on a condition and I tried,

a, b = 1, 1 if c == 1 else 5, 10

It resulted a ValueError.

ValueError: too many values to unpack

I tried with two conditions for each and it was a success.

a, b = 1 if c == 1 else 5, 1 if c == 1 else 10

But I need to achieve this using a single if condition, single line. I know this reduces the readability. But still is it possible? What am I doing wrong here?

Just out of interest, why do you need to do this?

musefan
– musefan

2015-07-16 11:33:08 +00:00
Commented Jul 16, 2015 at 11:33 — musefan
– musefan, Commented Jul 16, 2015 at 11:33

Alasdair · Accepted Answer · 2018-01-22 08:43:34Z

21

You can acheive this by putting (a, b) in parentheses.

a, b = (1, 1) if c == 1 else (5, 10)

The current code is equivalent to

a, b = 1, (1 if c == 1 else 5), 10

Which gives a value error as you are trying to unpack a 3-tuple into two variables.

edited Jan 22, 2018 at 8:43

answered Jul 16, 2015 at 11:33

Alasdair

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Padraic Cunningham · Accepted Answer · 2015-07-16 11:53:14Z

6

You could also index based using the result of c == 1:

a, b = ((5, 10), (1, 1))[c == 1]

The result of c == 1 will either be True -> 1 or False -> 0 so we will end up taking (1, 1) i.e index 1 if the condition evaluates to True or (5, 10) i.e 0 if it is False.

edited Jul 16, 2015 at 11:53

answered Jul 16, 2015 at 11:36

Padraic Cunningham

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The6thSense · Accepted Answer · 2015-07-16 11:31:47Z

2

Try this:

a, b = ((a, b) if c == 1 else (5, 10))

or this

a, b = (a, b) if c == 1 else (5, 10)

answered Jul 16, 2015 at 11:31

The6thSense

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Dimitri · Accepted Answer · 2021-05-17 08:15:05Z

1

If you are dealing with vectors, you can also use numpy.

import numpy as np
condition = np.arange(10) > 5
x = np.ones(10)
y = np.zeros(10)

(a,b) = np.where(condition, (x,y), (y,x))

print(a)
[0. 0. 0. 0. 0. 0. 1. 1. 1. 1.]
print(b) 
[1. 1. 1. 1. 1. 1. 0. 0. 0. 0.]

answered May 17, 2021 at 8:15

Dimitri

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