Appendix B — Python basics

To assign a value to a variable, use the assignment operator =. The only variable types we’re interested in are numbers and strings (strings are basically pieces of text).

Assign the number 5 to the variable x:

x = 5

Assign the string “Hello” to the variable y:

y = "Hello"

Strings need to be surrounded by double or single quotes.

Comments start with a hash:

# This is a comment
y = 'Hello'  # You can also use single quotes for strings

Python’s comparison operator to test for equality is ==. The following conditional statement means: If x equals 5, assign the value 10 to the variable z.

if x == 5:
    z = 10

Python’s comparison operator to test for inequality is !=. The following conditional statement means: If x is not equal to 5, assign the value 10 to the variable z.

if x != 5:
    z = 10

Note that the conditional statement needs to be followed by a colon. The next line (indicating what to do if x is indeed 5) needs to be indented by a consistent number of spaces (four spaces is recommended). Python does insist on these spaces!

Note that you can have more than just a single condition:

if x == 5:
    z = 10
elif x == 6:  # elif stands for else if
    z = 20
else:
    z = 100

This means: If x equals 5, assign the value 10 to the variable z, if x equals 6, assign the value 20 to the variable z and in all other cases, assign the value 100 to the variable z.

In Python, truth values can be expressed using Boolean constants: True (note the upper-case T!) indicates that something is true, False (note the upper-case F!) that something is false.