How do the Boolean
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
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:
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
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
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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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
To help students reach higher levels of Python success, he founded the programming education website Finxter.com that has taught exponential skills to millions of coders worldwide. He’s the author of the best-selling programming books Python One-Liners (NoStarch 2020), The Art of Clean Code (NoStarch 2022), and The Book of Dash (NoStarch 2022). Chris also coauthored the Coffee Break Python series of self-published books. He’s a computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.
His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.