Conditional control flow: The if-statement

Conditional statements allow us to branch execution based on the value of an expression. The form of the statement is the if keyword, followed by an expression, terminated by a colon to introduce a new block. Let's try this at the REPL:

>>> if True:

Remembering to indent four spaces within the block, we add some code to be executed if the condition is True, followed by a blank line to terminate the block:

...     print("It's true!")
...
It's true!

At this point the block will execute, because self-evidently the condition is True. Conversely, if the condition is False, the code in the block does not execute:

>>> if False:
...     print("It's true!")
...
>>>

The expression used with the if-statement will be converted to a bool just as if the bool() constructor had been used, so:

>>> if bool("eggs"):
...     print("Yes please!")
...
Yes please!

If the value is exactly equivalent to something, we then use the if command as follows:

>>> if "eggs":
...     print("Yes please!")
...
Yes please!

Thanks to this useful shorthand, explicit conversion to bool using the bool constructor is rarely used in Python.