[Solved] Python one-liner if-else statement

Question

I'm trying to write a one-liner if-else statement

a = 1
b = 10
c = 20
c = 9 if a > b else c -= 1

My expectation is c will become 19, instead it returns SyntaxError: invalid syntax, highlighting character '-'.

If changing to...
(c = 9) if a>b else (c -= 1)
It also returns invalid syntax. Maybe you meant '==' or ':=' instead of '='?, highlighting '(c' on the left.

However below code works.
c = 9 if a > b else 100

Why it is not possible increase / decrease the existing c in one-liner?

Answer 1

Wong Hei Ming ,

since assignments can not be used in ternary conditional expressions, we can use in this case:

a = 1
b = 10
c = 20
c = 9 if a > b else c - 1
print(c) # result => 19

Answer 2

how about
a = 1
b = 10
c = 20
#c = 9 if a > b else c -= 1
c -= 11 if a>b else 1
print(c)

Answer 3

You can use the walrus operator to make an assignment in the same line where you use it.

c = 9 if a > b else (c := 20) - 1

But I find it utterly pointless because the value 20 is never actually used or remembered any longer, anyway.

Other oneliner options:

d = 9 if a > b else 19

e = [19, 9][a > b]

Answer 4

"One line" if statements in python are called ternary operator and it differs from traditional if,else blocks.

https://www.geeksforgeeks.org/ternary-operator-in-python/

https://www.dataquest.io/blog/python-ternary-operator/

Answer 5

you have to use += to increase or -= to decrease python variables

Answer 6

Bob_Li
Do you mean...?
c -= 1 if a < b else 9
Putting shorthand on the left side and reverse the condition?

Answer 7

yes, that is a another way to write it.
But I thought you wanted c=9 for the else condition? So it should be
c -= 1 if a<b else 11
because 20-11=9

Answer 8

Bob_Li
I'm practicing class, getter and setter. And I just make up the example above to aviod my real code.
The setter logic is very simple and think one-liner can save space.
Once it is done I may post it somewhere.
But unfortunately it won't run in playground because it is highly interactive.

Answer 9

Mokarom Hasan everyone already knows that.
The question was how to write it in one line.

Answer 10

Tibor Santa
that last one is an innovative way of using index 0 or 1. False, True resolves to 0 and 1.
It is the shortest if else statement I have ever seen.
It also works for tuples

c = (19,9)[a>b]

or doing if else without writing if-else:

opt = input()
print(('you picked 1', 'something else')[opt!='1'])

Answer 11

Wong Hei Ming
taking all the above suggestions and going crazy with it.
You can actually do a chaining if-else-if. But this is just for fun. Codes like these are unreadable.

a, b, c = 1, 10, 20
p_all = lambda:print(f'a={a} b={b} c={c}')
print('initial state:')
p_all()

#ordering of conditions is tricky because of short circuiting.
mut_c = lambda x: 47 if a==b else 100 if b==100 else 9 if a>b else x-1 if a<b else x

print('\nc after aplying mutation function:')
c=mut_c(c)
p_all()

print('\ntests:')

a = 11
c=mut_c(c)
p_all()

a = 10
c=mut_c(c)
p_all()

b = 100
c=mut_c(c)
p_all()

a = 100
c=mut_c(c)
p_all()

b = 5
c=mut_c(c)
p_all()

Answer 12

a = 1
b = 10
c = 20
if a > b :
c = 9
else :
c -= 1
print(c)

Answer 13

hello