Ternary operator behavior

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Category:Languages

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?


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 

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