Ternary conditional operator in Python

You can index into a tuple:

(falseValue, trueValue)[test]

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

(falseValue, trueValue)[test == True]

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

<condition> and <true_value> or <false_value>

This feels more hacky than the new A if B else C syntax mentioned elsewhere, and is generally considered to be a Bad Thing. Although it does have the benefit of evaluating expressions left to right, which is clearer in my opinion.

From the documentation:

Conditional expressions (sometimes called a “ternary operator”) have the lowest priority of all Python operations.

The expression x if C else y first evaluates the condition, C (not x); if C is true, x is evaluated and its value is returned; otherwise, y is evaluated and its value is returned.

See PEP 308 for more details about conditional expressions.

New since version 2.5.

expression1 if condition else expression2

>>> a = 1
>>> b = 2
>>> 1 if a > b else -1 
-1
>>> 1 if a > b else -1 if a < b else 0
-1

For Python 2.5 and newer there is a specific syntax:

[on_true] if [cond] else [on_false]

In older Pythons a ternary operator is not implemented but it's possible to simulate it.

cond and on_true or on_false

Though, there is a potential problem, which if cond evaluates to True and on_true evaluates to False then on_false is returned instead of on_true. If you want this behavior the method is OK, otherwise use this:

{True: on_true, False: on_false}[cond is True] # is True, not == True

which can be wrapped by:

def q(cond, on_true, on_false)
    return {True: on_true, False: on_false}[cond is True]

and used this way:

q(cond, on_true, on_false)

It is compatible with all Python versions.

@up:

Unfortunately, the

(falseValue, trueValue)[test]

solution doesn't have short-circuit behaviour; thus both falseValue and trueValue are evaluated regardless of the condition. This could be suboptimal or even buggy (i.e. both trueValue and falseValue could be methods and have side-effects).

One solution to this would be

(falseValue, trueValue)[test]()

(execution delayed until the winner is known ;)), but it introduces inconsistency between callable and non-callable objects. In addition, it doesn't solve the case when using properties.

And so the story goes - choosing between 3 mentioned solutions is a trade-off between having the short-circuit feature, using at least python 2.5 (IMHO not a problem anymore) and not being prone to "trueValue-evaluates-to-false" errors.

You might often find

cond and on_true or on_false

but this lead to problem when on_true == 0

>>> x = 0
>>> print x == 0 and 0 or 1 
1
>>> x = 1
>>> print x == 0 and 0 or 1 
1

where you would expect for a normal ternary operator this result

>>> x = 0
>>> print 0 if x == 0 else 1 
0
>>> x = 1
>>> print 0 if x == 0 else 1 
1

Though Pythons older than 2.5 are slowly drifting to history, here is a list of old pre-2.5 ternary operator tricks: "Python Idioms", search for the text 'Conditional expression' . Wikipedia is also quite helpful Ж:-)