Conditional if statements

The if statement is used to implement decisions and branching in a program. One or more instructions are only executed if a condition matches:

name = 'Emily'
if name == 'Emily':
    studies = 'Physics'
studies

'Physics'

There must be an if block, zero or more elif’s and an optional else block:

    if name == 'Emily':
        studies = 'Physics'
    elif name == 'Maria':
        studies = 'Computer Science'
    elif name == 'Sarah':
        studies = 'Archaeology'
    else:
        studies = '-- not registered yet --'

Below is an example that demonstrates the Python syntax for an if-else control statement. For a value assigned to a variable x, the program prints a message and modifies x; the message and the modification of x depend on the initial value of x:

x = -10.0  # Initial x value

if x > 0.0:  
    print('Initial x is greater than zero')
    x -= 20.0
elif x < 0.0:  
    print('Initial x is less than zero')
    x += 21.0
else: 
    print('Initial x is not less than zero and not greater than zero, therefore it must be zero')
    x *= 2.5

# Print new x value
print("New x value:", x)

Initial x is less than zero
New x value: 11.0

Code blocks

After an if statement, all indented commands are treated as a code block, and are executed in the context of the condition.

The next unindented command is executed in any case.

Comparison operators

An if expression may contain comparison operators like:

a =1
b =2

    a == b
    

False

    a != b

True

    a < b

True

    a > b

False

    a <= b

True

    a >= b

False

On lists and strings you can also use:

b = [1,3]
c = 2
d = 1
a in b

True

Multiple expressions can be combined with boolean logic (and, or, not):

    a or b

1

    a and b

[1, 3]

    not a

False

    (a or b) and not (c or d)

False

Boolean value of variables

Each variable can be interpreted as a boolean (True/False) value. All values are treated as True, except for:

    False

False

    0

0

    []

[]

    ''

''

    {}

{}

    set()    

set()

    None