Python Basics

Basic operators, syntax, conditionals

if True: Huh? If what is true?

When well-written, Python code can usually be made to read like English.

if (2 < 3) and ("me" == "you"):
    # if-body
else:
    # else-body

We read this as "if two is less than three and the string 'me' is the same as 'you', then do whatever is in the if-body. Otherwise, execute the code in the else-body".

On the other hand, this piece of code can look confusing:

if False:
    # if-body
else:
    # else-body

"If false? Huh? If what is false? Did we miss out something?", you ask. This following snippet would have made more sense:

if is_odd(x) is False:
    print("x is an even number")

How it works

The Python interpreter cannot tell which branch to run from the line if (2 < 3) and ("me" == "you"): directly. It will first evaluate the expression between the if keyword and the colon : and reduce it to either True (or equivalent) or False (or equivalent like 0). Only then will it know which block of code to execute.

In this sense, we can say that the interpreter only knows how to execute either

if True:
    # if-body
else:
    # else-body

or

if False:
    # if-body
else:
    # else-body

The interpreter combines this ability with its ability to simplify expressions like (2 < 3) and ("me" == "you") into True or False in order to process our English-like if-statements.

Examples

Let's call the python interpreter Py. Say 'Hi!' to Py!

User: Py, execute this for me

if (2 < 3) and ("me" == "you"):
    # if-body
else:
    # else-body

Py: Hmm, I don't know if I should execute the if or else branch yet. Give me a moment, let me simplify things a bit. 2 < 3 is True and "me" == "you" is False. Easy-peasy-lemon-squeezy.

if True and False:
    # if-body
else:
    # else-body

Py: *scratches head* Hold on a second. I need to simplify some more:

if False:
    # if-body
else:
    # else-body

Py: There. We should execute the else block!

Note: This is a simplification of what really goes on, but we hope it helps your understanding (: