Python Truth Value Testing is Awesome

2016 February 25

I’m constantly amazed by the simplicity and readability Python provides. (See The Zen of Python.) As the industry evolves, more and more codes written each day, how to read code fast is a rising question every developer would face. Imaging the times when you faced a new code base and need to ramp up the knowledge in order to work with it. Lot’s of reading, isn’t it? This is why I think Python is a great modern programming language. Let’s talk about one of the features Python provides — the truth value testing.

(Go ahead and read the docs in Python’s official documentation. You cannot avoid reading documentations while developing software.)

Basically the doc tells you that any object could be used for if-condition, while-condition, or boolean operation. Let’s look at the following code:

if 1:
    print('non-zero number is truthy!')
if [1]:
    print('non-empty list is truthy!')
if {'attr': 'value'}:
    print('non-empty dict is truthy!')
if 'a_string':
    print('non-empty string is truthy!')
if 0:
    # print('0 is not truthy!')
if '':
    # print('empty string is not truthy!')
if []:
    # print('empty list is not truthy!')
if {}:
    # print('empty dict is not truthy!')

This feature gives us a very consistent way of writing code. Unlike writing code in Java, I don’t need to remember whether the return type is a null pointer or a boolean false when calling a method. In Python, a simple truth testing would give me the answer I want, because they both gives me a False.

The truth testing feature also makes a very sweet combo with the Boolean Operators. Consider the code below:

ENV_VAR = os.environ.get(ENV_VAR) or <default_value>

This code reads: get the ENV_VAR value from the OS’s environment, if can’t find it, just use a default value. It looks pretty neat, isn’t it? Imaging this code written in Java:

String ENV_VAR;
if (getEnvVar("ENV_VAR") != null):
    SECRET_KEY = getEnvVar("ENV_VAR");
    SECRET_KEY = <default_value>;

Needless to say, lot’s of noise for my eyes when reading it.

Why this would work is because the Python or-operator does something slightly difference than what we learn on the truth table. When evaluating the line x or y, what Python does is to do if x is false, then y, else x. Again, the ‘if x is false’ statement wouldn’t work without the truth value testing feature in Python.