ternary_operators.rst

Ternary Operators

Ternary operators are more commonly known as conditional expressions in Python. These operators evaluate something based on a condition being true or not. They became a part of Python in version 2.4

Here is a blueprint and an example of using these conditional expressions.

Blueprint:

value_if_true if condition else value_if_false

Example:

is_nice = True
state = "nice" if is_nice else "not nice"

It allows to quickly test a condition instead of a multiline if statement. Often times it can be immensely helpful and can make your code compact but still maintainable.

Another more obscure and not widely used example involves tuples. Here is some sample code:

Blueprint:

(if_test_is_false, if_test_is_true)[test]

Example:

nice = True
personality = ("mean", "nice")[nice]
print("The cat is ", personality)
# Output: The cat is nice

This works simply because True == 1 and False == 0, and so can be done with lists in addition to tuples.

The above example is not widely used and is generally disliked by Pythonistas for not being Pythonic. It is also easy to confuse where to put the true value and where to put the false value in the tuple.

Another reason to avoid using a tupled ternery is that it results in both elements of the tuple being evaluated, whereas the if-else ternary operator does not.

Example:

condition = True
print(2 if condition else 1/0)
#Output is 2

print((1/0, 2)[condition])
#ZeroDivisionError is raised

This happens because with the tupled ternary technique, the tuple is first built, then an index is found. For the if-else ternary operator, it follows the normal if-else logic tree. Thus, if one case could raise an exception based on the condition, or if either case is a computation-heavy method, using tuples is best avoided.

ShortHand Ternary

In python there is also the shorthand ternary tag which is a shorter version of the normal ternary operator you have seen above.

Syntax was introduced in Python 2.5 and can be used in python 2.5 or greater.

Example

>>> True or "Some"
True
>>>
>>> False or "Some"
'Some'

The first statement (True or "Some") will return True and the second statement (False or "Some") will return Some.

This is helpful in case where you quickly want to check for the output of a function and give a useful message if the output is empty:

>>> output = None
>>> msg = output or "No data returned"
>>> print(msg)
No data returned

Or as a simple way to define function parameters with dynamic default values:

>>> def my_function(real_name, optional_display_name=None):
>>>     optional_display_name = optional_display_name or real_name
>>>     print(optional_display_name)
>>> my_function("John")
John
>>> my_function("Mike", "anonymous123")
anonymous123