6.6. Logical Operators

Recall that an operator carries out an action on one or more operands (values). Math operators (+, -, *, /, //, **, %) perform calculations. Boolean operators (like the comparisons == and <) return a value of either True or False.

Example

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name = input('Please enter a username: ')

if len(name) > 5:
   print("Welcome, " + name + "!")
else:
   print("Invalid username.")

The expression len(name) > 5 compares the length of the string stored in name to the value 5.

If False, the program prints Invalid username. If True, the program prints the welcome message.

What if we wanted to set another limit to the length of name? We could replace line 3 with if len(name) < 10, but then we would lose the first comparison. Fortunately, we can perform both checks at the same time.

Three boolean operators allow us to make more complicated comparisons in a single if statement. These are called logical operators, and there are only three—and, or, and not.

6.6.1. Logical and

Let’s take the two boolean expressions from above:

  1. len(name) > 5 returns True when name contains more than 5 characters.

  2. len(name) < 10 returns True when name contains less than 10 characters.

A compound boolean expression is a boolean expression built out of smaller ones. Python allows us to combine expressions by using the and operator.

len(name) > 5 and len(name) < 10

Run the following code and examine the output. Try each of the name suggestions to see how they change the output.

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name = 'Bob' # Other names to try: 'Username', 'Rumpelstiltskin'

print(name)
print('Longer than 5 characters: ', len(name)>5)
print('Shorter than 10 characters: ', len(name)<10)
print('len(name) > 5 and len(name) < 10: ', len(name)>5 and len(name)<10)

repl.it

A compound expression returns only ONE boolean value, which depends on the results from BOTH of the smaller comparisons. len(name) > 5 and len(name) < 10 is true only if len(name) is greater than 5 AND, at the same time, len(name) is less than 10.

Take-Home Idea

  1. Logical and combines two conditions.

  2. The combined expression is True only if both conditions return True.

  3. If either condition is False, the overall expression is False.

Tip

The meaning of and resembles its use in English. The sentence “Roses are red and violets are blue,” is true as a whole because roses are actually red, and violets are blue.

On the other hand, the sentence “Roses are red and violets are green,” is false as a whole. While roses are indeed red, violets are NOT green.

Let’s look at another example.

Example

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num = 5
print(num > 0 and num < 10)

print(7 > num and num == 3)

print(num*5 > 100 and 'dog' == 'cat')

Console Output

True
False
False

In line 2, num > 0 and num < 10 evaluates to True because both num > 0 and num < 10 are each True.

In line 4, the expression 7 > num and num == 3 evaluates to False because one of the two comparisons, num == 3, is False.

Line 6 evaluates to False because both comparisons return False. Notice that we can mix and match data types however we like, as long as both sides of the and expression are themselves boolean expressions.

6.6.2. Logical or

Python’s logical or also combines two boolean expressions. In this case, however, the resulting expression is True if either of the conditions are True. If both conditions are False, the overall expression is False.

For the compound expression num - 2 == 0 or num - 3 == 0, only one part has to be true for the overall result to be True.

Let’s look at another code example. Change the value of num to see when each combined expression returns True.

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num = 5

print(num > 0 or num < 10)
print(7 > num or num == 3)
print(num*5 > 100 or 'dog' == 'cat')

repl.it

Using num = 5, lines 2 and 4 both return True because at least one of the two comparisons is True. Line 6 returns False because both of the comparisons are False.

Tip

Logical or also resembles its English use. The sentence “Pigs can fly, or dogs can run,” is true as a whole. Even though pigs cannot fly, dogs CAN run. Only one of the two statements has to be true in order for the whole sentence to be true.

When both of the statements joined by or are false, the statement as a whole is false. “Pigs can fly or the Earth is flat,” is a false statement.

6.6.3. Logical not

The logical not operator takes a single operand and flips its boolean value. If a comparison returns False, then applying not changes the result to True (and vice versa).

Examples

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print(not True)
print(not False)

num = 5

print( not(num < 7) )
print( not('dog' == 'cat') )
print( not(num*5 > 100 or 'dog' == 'cat') )

Console Output

False
True
False
True
True

6.6.4. Longer Combinations

In the examples above, we used the and and or operators to combine two smaller boolean expressions. However, we can use the operators to combine as many expressions as we want!

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num = 5
python = 'Awesome!'

print(num > 0 and num < 10 and 'dog' == 'cat')
print(num > 7 or num == 3 or 'dog' == 'cat' or python == 'Awesome!')

Console Output

False
True

Warning

Here is a VERY common mistake programmers make when they try to combine boolean expressions.

What if we have a variable num and we want to check if its value is 5, 6, or 7?

  1. If we try to describe this out loud, we might say, “num is equal to 5 or 6 or 7”.

  2. If we translate this into Python as num == 5 or 6 or 7, we get an error when we run the code.

To prevent this error, we must combine three separate equality comparisons, num == 5 or num == 6 or num == 7. This may seem like a lot of extra typing, but it is necessary.

6.6.5. Check Your Understanding

Question

What is returned by the following boolean expression?

4 < 3 or 2 < 3

  1. True

  2. False

  3. "True"

  4. "False"

Question

What is the correct Python expression for checking to see if a number stored in the variable num is between 0 and 5.

  1. num > 0 and < 5

  2. num > 0 or < 5

  3. num > 0 and num < 5

  4. num > 0 or num < 5

Question

Predict if each of the following expressions evaluates to True or False.

  1. 12 * 2 == 24

  2. 'dog' == 'cat' or 'dog' == 'Dog'

  3. 12%2 == 0 and len('flower') < 6

  4. 'a' in 'xyz' and len('flower') >= 6 or 5 + 5 == 10