Does Python have a ternary conditional operator?


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


Answers



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

a if condition else b

First condition is evaluated, then either a or b is returned based on the Boolean value of condition
If condition evaluates to True a is returned, else b is returned.

For example:

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

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

  • The order of the arguments is different from many other languages (such as C, Ruby, Java, etc.), which may lead to bugs when people unfamiliar with Python's "surprising" behaviour use it (they may reverse the order).
  • 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.

If you're having trouble remembering the order, then remember that if you read it out loud, 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:




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(<expression>)]



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?




Putting a simple if-then statement on one line

That's more specifically a ternary operator expression than an if-then, here's the python syntax

value_when_true if condition else value_when_false

Better Example: (thanks Mr. Burns)

'Yes' if fruit == 'Apple' else 'No'

Now with assignment and contrast with if syntax

fruit = 'Apple'
isApple = True if fruit == 'Apple' else False

vs

fruit = 'Apple'
isApple = False
if fruit == 'Apple' : isApple = True



count = 0 if count == N else N+1

- the ternary operator. Although I'd say your solution is more readable than this.




Moreover, you can still use the "ordinary" if syntax and conflate it into one line with the colon.

if i > 3: print("We are done.")

or

field_plural = None
if field_plural is not None: print("insert into testtable(plural) '{0}'".format(field_plural)) 



Python-equivalent of short-form “if” in C++

a = '123' if b else '456'



While a = 'foo' if True else 'bar' is the more modern way of doing the ternary if statement (python 2.5+), a 1-to-1 equivalent of your version might be:

a = (b == True and "123" or "456" )

... which in python should be shortened to:

a = b is True and "123" or "456"

... or if you simply want to test the truthfulness of b's value in general...

a = b and "123" or "456"

? : can literally be swapped out for and or




My cryptic version...

a = ['123', '456'][b == True]



Python condense if/else to one line?

An example of Python's way of doing "ternary" expressions:

i = 5 if a > 7 else 0

translates into

if a > 7:
   i = 5
else:
   i = 0

This actually comes in handy when using list comprehensions, or sometimes in return statements, otherwise I'm not sure it helps that much in creating readable code.

The readability issue was discussed at length in this recent SO question better way than using if-else statement in python.

It also contains various other clever (and somewhat obfuscated) ways to accomplish the same task. It's worth a read just based on those posts.




Python's if can be used as a ternary operator:

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



Only for using as a value:

x = 3 if a==2 else 0

or

return 3 if a==2 else 0