CODEWITHSUNDEEP
pythonternary-operator
Yassine Faris
Yassine Faris

Reputation: 991

Ternary operator behavior

I recently started using ternary operator but I encountered a case where I needed to use multiple ternary operator in the same line, but they didn't seem to work as I expected.

Can someone please give a explanation why those line give different result.

x = 1 if True else 2 + 3 if False else 4  # x = 1, I expected 5
x = (1 if True else 2) + (3 if False else 4)  # x = 5

If I add parentheses I get the expected result, but I don't understand what the parentheses change.

And if I rotated the addition, without the parentheses, I get the correct value.

3 if False else 4 + 1 if True else 2  # x = 5

However, I get wrong result if the second ternary operator is False:

3 if False else 4 + 1 if False else 2  # x = 5 # x = 2 ???

Is it because you shouldn't multiple ternary operator in the same line, or is their an other reason?

Upvotes: 4

Views: 253

Answers (3)

Dan
Dan

Reputation: 45741

Think of it as applying parentheses around each of the three inputs:

x = (1) if (True) else ((2 + 3) if (False) else (4))

Here you can clearly see that x will equal 1 and that your 2 + 3... won't be run.

When you 'rotated' it:

(3) if (False) else ((4 + 1) if (True) else (2))

So now the 3 doesn't run because the condition is False so you get the 4 +... term instead

and in your last case

(3) if (False) else ((4 + 1) if (False) else (2))

it returns 2 because the (4+1) is the true condition for your second condition (which is False)

If you're still confused, write it out as a regular if statement:

x = (1) if (True) else ((2 + 3) if (False) else (4))

becomes

if True:
    x = 1
else:
    if False:
        x = 2 + 3
    else:
        x = 4

Upvotes: 5

ducminh
ducminh

Reputation: 1352

The reason is operator precedence. Conditional expressions have the lowest-but-one precedence, only lambda expression is lower. Therefore, the expression

1 if True else 2 + 3 if False else 4

is evaluated as

1 if True else ((2 + 3) if False else 4)

which returns 1.

Upvotes: 6

klutt
klutt

Reputation: 31296

x = 1 if True else 2 + 3 if False else 4

is the same as

x = (1) if (True) else (2 + 3 if False else 4)

(Unless you want to play around with different constructs just for fun, I really don't recommend writing code like this.)

Upvotes: 4

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