Python Logical Operators

Python logical operators are used to perform logical operations on conditional statements. Like a comparison operation, the result of a logical operation is a boolean value, either True or False.

Table of contents:

• ● What are logical operators
• ● Logical operator and
• ● Logical operator or
• ● Logical operator not

There are three logical operators in python: and, or and not.
The and and or operate on two operands, so they are binary operators.

The not operates on a single operand, so it is a unary operator.

The logical operator and or or is placed between two conditional expressions:

  expression1 and/or expression2

The expression1 & expression2 are the two operands of the logical operator.

The logical and

The and operator results in a boolean value True if both the operands are True. If one of the operands is False, the result is False.

Expression1	Operator	Expression2	Output
True	and	True	True
True	and	False	False
False	and	True	False
False	and	False	False

Let us take two comparison expressions 7 > 2 and 0 < -1. The first expression is True, the second one is False.

True and False has returned a False.

Expression	Output	Reason
5 == 5 and 3 > 2	True	both expressions are True
5 == 5 and 3 == 2	False	expression1 is True, expression2 is False
5 != 5 and 3 > 2	False	expression1 is False, expression2 is True
5 != 5 and 3 == 2	False	both expressions are False

More examples of logical and:

• 3 ** 2 == 9 is evaluated to True,
• this True is assigned to variable x.
• 
  
• 4 >= 4 is evaluated to a True,
• this True is assigned to variable y.
• 
  
• Expression x and y becomes True and True,
• True and True is evaluated to a True.

• "z" == "z" is evaluated to True,
• this True is assigned to variable x.
• 
  
• -1 < -2 is evaluated to False,
• this False is assigned to variable y.
• 
  
• Expression x and y becomes True and False,
• True and False is evaluated to a False.

• 5 - 3 * 2 == 4 is evaluated to False,
• this False is assigned to variable x.
• 
  
• 5 - 3 * 2 == -1 is evaluated to True,
• this True is assigned to variable y.
• 
  
• Expression x and y becomes False and True,
• False and True is evaluated to a False.

• "z" < "Z" is evaluated to False,
• this False is assigned to variable x.
• 
  
• "big" > "small" is evaluated to False,
• this False is assigned to variable y.
• 
  
• Expression x and y becomes False and False,
• False and False is evaluated to a False.

The logical or

The or operator results in a boolean value True if any of the operands is True. If both of the operands are False, the result is False.

Expression1	Operator	Expression2	Output
True	or	True	True
True	or	False	True
False	or	True	True
False	or	False	False

Let us take the comparison expression 7 > 2 or 0 < -1. The first sub-expression is True, the second one sub-expression is False.

True or False has returned a True.

Expression	Output	Reason
5 == 5 or 3 > 2	True	both expressions are True
5 == 5 or 3 == 2	True	expression1 is True, expression2 is False
5 != 5 or 3 > 2	True	expression1 is False, expression2 is True
5 != 5 or 3 == 2	False	both expressions are False

More examples of logical or:

• 3 == 3.0 evaluated to True, assigned to x.
• 4 > -4 evaluated to True, assigned to y.
• 
  
• Expression x or y becomes True or True,
• True or True is evaluated to a True.

• "b" == "b" returns True, assigned to x.
• -4 > -3 returns False, assigned to y.
• 
  
• Expression x or y becomes True or False,
• True or False is evaluated to a True.

• "b" == "B" returns False, assigned to x.
• 1.0 < 1.1 returns True, assigned to y.
• 
  
• Expression x or y becomes False or True,
• False or True is evaluated to a True.

• "cat" < "Cat" returns False, stored in x.
• 21 % 7 == 1 returns False, stored in y.
• 
  
• Expression x or y becomes False or False,
• False or False is evaluated to a False.

The logical not

The logical not operator takes only one operor, which can be a boolean expression (or it can be a python object also, which will be discussed later when python "trythy" and "falsy" is studied). The not is a unary operator, sits before a single operand/expression, and reverses the boolean value of the operand/expression.

The expression not True will result in a False.
The expression not False will result in a True.

Operator	Expression	Output
not	True	False
not	False	True

Again,

Since 2 == 2 is True, not 2 == 2 is False.
Since 2 > 3 is False, not 2 > 3 is True.

Is a given number two-digit odd?

Let us apply the logical and to a problem. Suppose we have to determine whether a given number is a two-digit odd number. That means if the given number is of two digits, and it is odd, the output should be True, else the output should be False.

• In line 3: is_odd gets the value False
• is_odd = number % 2 == 1
• is_odd = 98 % 2 == 1
• is_odd = 0 == 1
• is_odd = False
• In line 4: is_two_digit gets the value True
• is_two_digit = number > 9 and number < 100
• is_two_digit = True and True
• is_two_digit = True
• In line 5: is_two_digit_odd gets the value False
• is_two_digit_odd = is_odd and is_two_digit
• is_two_digit_odd = False and True
• is_two_digit_odd = False

In the third line, the arithmetic sub-expression number % 2 determines the remainder of number / 2.
If it is 1, True is stored in the variable is_odd.
If it is 0, a False is stored.

Observe the fourth line. if the variable number is greater than 9, and it is less than 100, variable is_two_digit will hold the value True, else it will hold a False.

In the fifth line, if both the variables is_two_digit and is_odd hold the value True, True is stored in the variable is_two_digit_odd, else a False is stored.

We finally check the value of is_two_digit_odd; a True will indicate that the input number is a two-digit odd, a False will indicate that it is not a two-digit odd number.

Each print in lines 6, 7 and 8 displays three separate values of type 'int', 'str' and 'int' in a single line.

By changing the value 98 in the second line, you can observe the change in the output window.

--- x ---