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

[xSicKxBot]

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:

Code:

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

Simple enough. Yet, that’s not the whole story: you can use the

Code:

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
  Code:

  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

Code:

and

operator work?

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

Operation |
Definitions

Code:

x or y

|
if x is false, then y, else x

Code:

x and y

|
if x is false, then x, else y

Code:

not x

|
if x is false, then

Code:

True

, else

Code:

False

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

Code:

x and y

operation, if the value of

Code:

x

is evaluated to

Code:

True

, Python simply returns the value of

Code:

y

. It doesn’t even look at what the value of

Code:

y

actually is. If you’re using Boolean operators

Code:

x

and

Code:

y

, this is expected behavior because if

Code:

x

is

Code:

True

, then the

Code:

y

determines whether

Code:

x

and

Code:

y

is

Code:

True

.

This leads to the interesting behavior: if

Code:

x

and

Code:

y

are objects, the result of the operation

Code:

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

Code:

False

. Otherwise, it’s

Code:

True

. So an empty list, an empty string, or a 0 integer value are all

Code:

False

. Most other values will be

Code:

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

Code:

x

and

Code:

y

. What’s the result of the operation

Code:

x and y

?

The answer is simple: the result is

Code:

y

if

Code:

x

is non-empty (and, thus, evaluates to

Code:

True

).

What If You Apply the AND Operator To Two Lists?

Here’s an example for two list objects:

Code:

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

The first argument of the

Code:

and

operation is non-empty and evaluates to

Code:

True

. Therefore, the result of the operation is the second list argument

Code:

[0, 0, 0, 0]

.

But what if the first argument is empty?

Code:

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

The result is the first argument (and not a Boolean value

Code:

False

). If you’re in doubt why, consult the above definition again:

Code:

x and y

: if x is false, then x, else y

Summary

You’ve learned that the

Code:

and

operator returns the first operand if it evaluates to

Code:

False

, otherwise the second operand.

You’ve also learned that you can use the

Code:

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

Code:

False

.

If you find this interesting, feel free to check out my upcoming Python book that shows you hundreds of small Python tricks like this one:



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