not, and & or

We've already dicusses the and, or and not operators in the previous chapter. Let's see how they can be combined.

not

The not operator is a unary operator, meaning it only takes one operand. It negates the truth value of the operand.

Condition	not Condition
not True	False
not False	True

The operand of the not operator can be any expression that evaluates to a Boolean value.

06_not.py

server_status = "Healthy"

if not server_status == "Healthy":
    print("CRITICAL ERROR: Server down!!")

This executes as follows:

1. server_status is compared with the string "Healthy', which returns True,
2. The result of server_status == "Healthy" is negated, so True becomes False.
3. The script does nothing since there is no else branch.

and

The and operator is a binary operator, meaning it takes two operands. It evaluates to True if both operands are True, and False otherwise.

Operand 1	Operand 2	Operand 1 and Operand 2
True	True	True
True	False	False
False	True	False
False	False	False

07_and.py

server_status = "Healthy"
environment = "Production"

# Evaluates to: (not True) and True => False and True => False
# Nothing is printed
if not server_status == "Healthy" and environment == "Production":
    print("CRITICAL ERROR: Server down!!")

server_status = "Down"
    
# Evaluates to: True and True => True
# The alert is printed    
if server_status == "Down" and environment == "Production":
    print("CRITICAL ERROR: Server down!!")

or

The or operator is also a binary operator, meaning it takes two operands. It evaluates to True if at least one of the operands is True, and False only if both operands are False.

Operand 1	Operand 2	Operand 1 or Operand 2
True	True	True
True	False	True
False	True	True
False	False	False

08_or.py

is_root_user = True
is_administrator = True

if is_root_user or not is_administrator:
    print("This script can only be executed by members of the administrator group that aren't the root user.")

# Do some administrative task
# ...

Order of operations

When combining multiple logical operators in a single expression, it's important to understand the order of operations. The three operators are evaluated in the following order:

1. ==
2. not
3. and
4. or

So the expression

if not status == "Healthy" and env == "Production":

is evaluated as

if (not (status == "Healthy")) and (env == "Production"):