Python AND Operator On Two Objects or Lists [What’s The Result?]

You may already know Python’s and operator when applied to two Booleans:

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

Simple enough. Yet, that’s not the whole story: you can use the and operator even on complex data types such as lists or custom objects. So you may ask (and rightly so):

What If You Apply the AND Operator To Two Objects?

To understand the output, you have to understand two things:

• How does the and operator work?
• What’s the truth value of any object – such as a list?

Let’s answer those two questions quickly.

How does the and operator work?

Let’s start with the official documentation for Boolean operators:

You must understand the deeper meaning of those definitions: all of them are short-circuit which means that as soon as the condition is fullfilled, they will abort further processing.

In the x and y operation, if the value of x is evaluated to True, Python simply returns the value of y. It doesn’t even look at what the value of y actually is. If you’re using Boolean operators x and y, this is expected behavior because if x is True, then the y determines whether x and y is True.

This leads to the interesting behavior: if x and y are objects, the result of the operation x and y will be an object, too! (And not a Boolean value.)

In combination with the next piece of Python knowledge, this leads to an interesting behavior:

What’s the truth value of any object – such as a list?

The Python convention is simple: if the object is “empty”, the truth value is False. Otherwise, it’s True. So an empty list, an empty string, or a 0 integer value are all False. Most other values will be True.

Now, you’re equipped with the basics to understand the answer to the following question:

What If You Apply the AND Operator To Two Objects?

Say, you’ve got two non-Boolean objects x and y. What’s the result of the operation x and y?

The answer is simple: the result is y if x is non-empty (and, thus, evaluates to True).

What If You Apply the AND Operator To Two Lists?

Here’s an example for two list objects:

>>> [1, 2, 3] and [0, 0, 0, 0]
[0, 0, 0, 0]

The first argument of the and operation is non-empty and evaluates to True. Therefore, the result of the operation is the second list argument [0, 0, 0, 0].

But what if the first argument is empty?

>>> [] and [0, 0, 0, 0]
[]

The result is the first argument (and not a Boolean value False). If you’re in doubt why, consult the above definition again:

x and y: if x is false, then x, else y

Summary

You’ve learned that the and operator returns the first operand if it evaluates to False, otherwise the second operand.

You’ve also learned that you can use the and operator even for non-Boolean types in which case the result will be an object, not a Boolean value.

Finally, you’ve also learned that an empty object usually evaluates to False.

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