Python One Line And/Or

How do the Boolean and and or operators work in the context of Python one-liners?

You may know the standard use of the logical operators applied to Boolean values:

>>> True and False
False
>>> False or True
True

But there’s more to these operators that only experts in the art of writing concise Python one-liners know.

For instance, the following use of the or operator applied to non-Boolean values is little known:

>>> 'hello' or 42
'hello'
>>> [] or 42
42

Similarly, the following use of the and operator often causes confusion in readers of advanced Python one-liners:

>>> 'hello' and 42
42
>>> [] and 42
[]

How do the and and or operator work when applied to non-Boolean operands?

To understand what is going on, you need to look at the definitions of the Boolean operators:

OperatorDescription
a or bReturns b if the expression a evaluates to False using implicit Boolean conversion. If the expression a evaluates to True, the expression a is returned.
a and bReturns b if the expression a evaluates to True using implicit Boolean conversion. If the expression a evaluates to False, the expression a is returned.

Study these explanations thoroughly! The return value is of the same data type of the operands—they only return a Boolean value if the operands are already Boolean!

This optimization is called short-circuiting and it’s common practice in many programming languages. For example, it’s not necessary to evaluate the result of the second operand of an and operation if the first operand evaluates to False. The whole operation must evaluate to False in this case because the logical and only returns True if both operands are True.

Python goes one step further using the property of implicit Boolean conversion. Every object can be implicitly converted to a Boolean value. That’s why you see code like this:

l = []
if l:
    print('hi')
else:
    print('bye')
# bye

You pass a list into the if condition. Python then converts the list to a Boolean value to determine which branch to visit next. The empty list evaluates to False. All other lists evaluate to True, so the result is bye.

Together, short circuiting and implicit Boolean conversion allow the logical operators and and or to be applied to any two Python objects as operands. The return value always is one of the two operands using the short circuiting rules described in the table.

Try it yourself in our interactive code shell:

Exercise: Guess the output! Then check if you were right! Create your own crazy operands and evaluate them by executing the code in your browser.

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