Understanding Python's "Truthiness"

(Adapted from a post I wrote on Quora...)

In many languages, the expression which follows the if keyword must be a Boolean expression–that is, it must evaluate to True or False. Of course we can write such if statements in Python:

>>> a = 1 
>>> if a == 1: 
...     print('yes') 
... 
yes 

The if statement at line 2 clearly evaluates to True, i.e., the value of the variable a is equal to 1.

Python, like some other programming languages, allows you to put any expression after the if, e.g.,

>>> if a: 
...     print('truthy') 
... 
truthy 

(The above would not work in my second favorite language, Go, but it works in Python.)

The question is–Why does it work?

Python follows a model of truthiness–that is, any non-zero value is considered True, whereas any zero value is considered False. Let’s see it in action:

>>> if 0: 
...     print('this should not be printed') 
... 
>>> if 0.0: 
...     print('ditto for this') 
... 
>>> if -3: 
...     print('-3 is True') 
... 
-3 is True 

If the above is confusing, think of it as if we had written

!= False

after those numbers, i.e.,

>>> if -3 != False: 
...     print('-3 is True') 
... 
-3 is True 

Of course, we’d never write the above, but it is equivalent.

It turns out the story is a bit more interesting. Any non-empty container is considered True (and conversely, any empty container is considered False).

What’s a container? Well, it’s an object that contains 0 or more things. In this case, the circular definition serves us well:

• A list is a container:

>>> my_list = ['this', 'that', 'other'] 
>>> if my_list: 
...     print('list is non-empty') 
... 
list is non-empty 

• A string is a container:

>>> s = 'this is a string' 
>>> if s: 
...     print('string is non-empty') 
... 
string is non-empty 

• A dictionary is a container:

>>> d = { 'one': 1 } 
>>> if d: 
...     print('dict is non-empty') 
... 
dict is non-empty 

…and tuples and sets are also containers.

We can demonstrate the False condition similarly:

>>> if []: 
...     print("[] is considered False, so this won't be printed") 
... 
>>> 
>>> if '': 
...     print("'' is considered False, so this won't be printed") 
... 
>>> 

…and so on.

Next time you write an if statement in Python, remember the concept of truthiness!

One final note…sometimes the brevity introduced by truthiness is not such a good idea. Consider the first example above–checking to see integer variable a is not 0.

>>> if a: 
...     print('…') 
... 

While it’s correct, one might argue that the above syntax obscures the fact that a could be negative. I’d probably write (and indeed have written in the past):

if a != 0 

in a case where I wanted the reader to understand that a could be negative. It’s a matter of taste, to be sure, but I always recommend that beginners err on the side of clarity. Of course, that applies to experts too! Remember the wise words of Harold Abelson: “Programs must be written for people to read, and only incidentally for machines to execute.”