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:
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.
>>> 'true' if True else 'false' 'true' >>> 'true' if False else 'false' '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.
You can index into a tuple:
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:
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?
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