Truth and flow control in Python

Boolean Expressions

Python has a Boolean type. The two values for this type are True and False. Note the capitalization: the first letter must be capitalized. Most text editors, including IDLE, will highlight True and False when properly capitalized.

The bool() Function

If you need to convert something into a boolean value, use the bool() function. For example:

>>> bool(1)
True
>>> bool(0)
False
>>> bool(500)
True
>>> bool(-1)
True

In Python, the integer 0 evaluates to False. All other integers will evaluate to True. As a general rule, try to avoid using Floating Point numbers in boolean expressions: use the int() operator to turn them into integers first.

Strings behave similarly: an empty string evalueates to False, all other strings evaluate to True.

>>> bool("Hello World!")
True
>>> bool("")
False
>>> bool("True")
True
>>> bool("False")
True
>>> bool("0")
True

Note that what is in the string does not matter: It can be "0", "False" or "ReallyReallyFalse": Everything that is not simply an empty string will evaluate to true.

Comparison Operators

Python has a number of built-in comparison operators. They are very versatile and can often be used across data types.

Equality

The Python operator for Equality is ==. Let's try it out:

>>> 1==1
True
>>> "1"=="1"
True
>>> 2+4 == 24
False
>>> "2"+"4" == "24"
True

Flow Control

Now that you know all these great ways to work with booleans, let's use them! Python uses a simple keyword called "if" to determine if it should execute a section of code. You can tell which lines are in this section by the level of indentation. The general format of if goes like this:

if [Boolean Expression]:
    some code
rest of code

This allows for very simple logic. What if you want to have two options, similar to "case" in other languages? Instead of Case, python uses "elif".

if [Boolean Expression]:
    code
elif [Boolean Expression]:
    More code
else:
    Even more code
rest of program

One thing that's important to realize is that if the boolean expression for the first "if" is True, nothing in the elif section will execute, nor will anything in the else section. If there are multiple things that could evaluate to True, use multiple if statements.

That code sample also introduced the "else" statment. Else gets executed if the "if" statement's boolean expression, and the boolean expressions for all elifs (if there are any), evaluate to False.

So, how about some actual code? Here's a simple program; try entering this into IDLE and running it.

name = raw_input("What is your name?")
if name == "":
    print "Hey! You didn't enter a name!"

elif name == "spam" or name == "eggs":
    print "A moose once bit my sister!"

else:
    print "Nice to meet you, %s." % name