Does Python have a ternary conditional operator?
Yes, it was added in version 2.5. The expression syntax is:
a if condition else b
condition is evaluated, then exactly one of either
b is evaluated and returned based on the Boolean value of
condition evaluates to
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 "<stdin>", 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:
- The order of the arguments is different from those of the classic
- Some find it "unwieldy", since it goes contrary to the normal flow of thought (thinking of the condition first and then the effects).
- Stylistic reasons. (Although the 'inline
if' can be really useful, and make your script more concise, it really does complicate your code)
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]
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.