Logical Operators

Python supports three logical operators: and, or and not. They must be written in lower case only — AND or Or will not work, because Python is case-sensitive and its keywords are fixed. A logical operator evaluates to either True or False, based on the logical value of the operand(s) on either side of it.

Every value has a truth value

Before combining conditions, understand a key idea: every value in Python is logically either True or False. By default, all values count as True except:

• None
• False itself
• 0 (zero)
• empty collections — the empty string "", the empty tuple (), the empty list [], the empty dictionary/set {}
• and a few other special values

So if num1 = 10 and num2 = -20, then both num1 and num2 are logically True (any non-zero number is), while num3 = 0 is logically False. The built-in function bool() shows the truth value of anything:

>>> bool(10)
True
>>> bool(-20)
True
>>> bool(0)
False

Logical AND: and

The condition becomes True only if both operands are True.

>>> True and True
True
>>> True and False
False
>>> False and False
False

With ordinary values, the truth-value rule applies:

>>> num1 = 10
>>> num2 = -20
>>> bool(num1 and num2)
True          # both non-zero, so both True
>>> num3 = 0
>>> bool(num1 and num3)
False         # num3 is 0, which is logically False

Logical OR: or

The condition becomes True if any one (at least one) of the two operands is True.

>>> True or True
True
>>> True or False
True
>>> False or False
False

Again with variables:

>>> num1 = 10
>>> num3 = 0
>>> bool(num1 or num3)
True          # num1 is True, so the whole condition is True

Logical NOT: not

not takes a single operand and reverses its logical state — True becomes False and False becomes True.

>>> num1 = 10
>>> bool(num1)
True
>>> not num1
False

Quick reference

| Operator | Operation | Result |

|---|---|---| …