Ternary conditional operator in Python

Python Programming

Question or problem about Python programming:

If Python does not have a ternary conditional operator, is it possible to simulate one using other language constructs?

How to solve the problem:

Solution 1:

Yes, it was added in version 2.5. The expression syntax is:

a if condition else b

First condition is evaluated, then exactly one of either a or b is evaluated and returned based on the Boolean value of condition. If condition evaluates to True, then a is evaluated and returned but b is ignored, or else when b is evaluated and returned but a is ignored.

This allows short-circuiting because when condition is true only a is evaluated and b is not evaluated at all, but when condition is false only b is evaluated and a is not evaluated at all.

For example:

>>> 'true' if True else 'false'
>>> 'true' if False else 'false'

Note that conditionals are an expression, not a statement. This means you can’t use assignment statements or pass or other statements within a conditional expression:

>>> pass if False else x = 3
  File "", line 1
    pass if False else x = 3
SyntaxError: invalid syntax

You can, however, use conditional expressions to assign a variable like so:

x = a if True else b

Think of the conditional expression as switching between two values. It is very useful when you’re in a ‘one value or another’ situation, it but doesn’t do much else.

If you need to use statements, you have to use a normal if statement instead of a conditional expression.

Keep in mind that it’s frowned upon by some Pythonistas for several reasons:

If you’re having trouble remembering the order, then remember that when read aloud, you (almost) say what you mean. For example, x = 4 if b > 8 else 9 is read aloud as x will be 4 if b is greater than 8 otherwise 9.

Official documentation:

Solution 2:

You can index into a tuple:

(falseValue, trueValue)[test]

test needs to return True or False.
It might be safer to always implement it as:

(falseValue, trueValue)[test == True]

or you can use the built-in bool() to assure a Boolean value:

(falseValue, trueValue)[bool()]

Solution 3:

For versions prior to 2.5, there’s the trick:

[expression] and [on_true] or [on_false]

It can give wrong results when on_true
has a false boolean value.1
Although it does have the benefit of evaluating expressions left to right, which is clearer in my opinion.

1. Is there an equivalent of C’s ”?:” ternary operator?

Solution 4:

if else

a = 1
b = 2

1 if a > b else -1 
# Output is -1

1 if a > b else -1 if a < b else 0
# Output is -1

Hope this helps!