Logical operations

Computers are funny creatures. They think in terms of 1*s and *0*s, *True and False. While Python has several numeric types, there is only one logical type: boolean. A boolean can only take two values: True or False. And this is all you need, if you are logical…

Bool type

Booleans are a built-in data type in Python. Take care to note that True and False are both capitalized.

>>> True
True
>>> true
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'true' is not defined
>>>

If you type it incorrectly, you will receive a name error.

Booleans are commonly encountered when comparing two objects. For example, to compare two numbers use the double equals (==) operator:

>>> a = 3
>>> b = 5
>>> a == b
False
>>>

We get False since a and b are different integers. Notice that you use one equal sign to assign numbers to a variable, and a double equal sign to compare them. In addition to testing if two numbers are the same you can test if they are different usint the not equal operator.

>>> a != b
True
>>>

This comparison returns True because it is true that a does not equal b. By the way, the exclamation mark is commonly used as a logical not symbol in programming languages. So this symbol literally reads as not equal.

And finally, in addition to comparing two numbers for equality or inequality, you can test to see if one is larger than the other.

>>> a > b
False
>>> a < b
True

>>>

Is a greater than b ? No. This is a false statement.

Is a less than b ? Yes. This is a true statement.

Falsy values

If you inspect the type of True and False:

>>> type(True)
<class 'bool'>
>>> type(False)
<class 'bool'>
>>>

You see the type is bool. This suggests another way to create booleans: by passing values to the boolean constructor. For example, let’s convert some numbers to booleans:

>>> bool(28)
True
>>> bool(-2.71828)
True
>>> bool(0)
False
>>>

In Python, 0 is converted to False, while every other number is converted to True.

We can also convert strings to booleans. For example,

>>> bool("Turing")
True
>>> bool(" ")
True
>>> bool("")
False
>>>

In Python, the empty string is converted to False, while every other string is converted to True.

This is a general principle in Python. When converting a value to a boolean, trivial values are converted to False, while non-trivial values are converted to True.

Just as you can convert objects to booleans you can convert booleans to other types of objects:

>>> str(True)
'True'
>>> str(False)
'False'

If you convert True to a string, it returns “True”, but notice this is surrounded by quotes, so it is a string. The boolean value does not have quotes.

You can also convert booleans to numbers. If you convert True to an integer, you get 1 and if you convert False to an integer you get 0.

>>> int(True)
1
>>> int(False)
0
>>> 5 + True
6
>>>

Look what happens if you add a number and a boolean: Python recognizez that you are trying to add True to an integer, so it first converts it to an integer then adds. What do you think 10 * False will be ?

>>> 10 * False
0
>>>

Like before, Python recognizez you are trying to perform an arithmetic operation so it converts False to the number 0 then multiplies. Is this something you will use ? Probabily not. But it does highlight that Python treats 1 as True and 0 as False, and vice-versa. In computer science, this is a fundamental fact.

If statement

When coding in Python you will frequently encounter a fork in the road, depending on the values of certain data you may want to go in one direction or the other, there may be even more than two directions for you to choose, the if-then-else statements help you navigate these situations:

If you want to learn more:

then continue reading this lesson

else try the next lesson.

if-then

In this example we will collect a string then test its length to see if it has at least 8 characters. This is something you may need to do when validating new passwords.

To begin, create a file called if_then.py and type in the following.

password = input("Please enter a test string: ")

if len(password) < 8:
        print("Your string is too short.")
        print("Please enter a string with at least 8 characters.")

The first line uses the input() function which will prompt the user to enter a string and then store the value in the variable password.

Next we use the if-then command to see if the length of the provided string is less than 8 characters long. If true the following indented lines are executed, if not these lines are skipped.

Note

Did you noticed that the if line ends with a colon (:) and the following lines are indented. This is how you identify a code-block in Python. This is a big difference from other languages such as Java or C++.

In those languages indentation does not matter and you group code with braces, in Python you start a new code-block with a colon and group the commands with indentation. You can use any size indentation as long as the commands line up.

Now save this file and run the program from the command line and enter the word magic.

$ python3 if_then.py
Please enter a test string: magic
Your string is too short.
Please enter a string with at least 8 characters.
$ _

Because this word has fewer than 8 characters the if statement is true (the length is less than 8) so the following code-block is executed and the two lines are printed.

Run the program again and enter a longer string like fantastic.

$ python3 if_then.py
Please enter a test string: fantastic
$ _

Because fantastic has more than 8 characters the if statement is false so the following code-block is skipped.

if-then-else

Let’s see another example. Create a file called if_then2.py. This time we will prompt the user to enter a number and we will test to see if it is even or odd.

First, the input() function returns a string, so we want to convert it to an integer. We will do this using the int() constructor. If the user does not type an integer this will cause an error. We will learn how to handle errors in a future lesson.

value = input("Please enter an integer: ")
number = int(value)

if number % 2 == 0:
        print("Your number is even.")
else:
        print("Your number is odd.")

Next, if the number you entered is a multiple of two then print Your number is even. else print Your number is odd..

The % operator returns the remainder when you divide the first number by the second one. In this case, we are computing the remainder when you divide the number by 2. If the remainder is 0 then the number is even, otherwise it is odd.

We’re ready to test this code, save the program and execute it in console.

$ python3 if_then2.py
Please enter an integer: 17
Your number is odd.
$ _

Everything worked, the if-then statement correctly identified 17 as odd. Run the program again and enter 50.

$ python3 if_then2.py
Please enter an integer: 50
Your number is even.
$ _

Correct again.

if-elif-else

For our final example we will create an if-then statement that handles more than two cases. Create a file called if_then3.py. We will prompt the user to enter the lengths of the sides of a triangle and we will determine if it is scalene, isosceles or equilateral.

A scalene triangle is one where all three sides are different lengths, an isosceles triangle has two sides of the same length and an equilateral triangle is one where all the sides are equal.

First, we prompt the user to enter the lengths of the three sides. Like before, we need to convert the strings to integers, we will do this in one line this time.

# scalene triangle: all three sides are different
# isosceles triangle: two sides of the same length
# equilateral triangle: all the sides are equal

a = int(input("The length of side a: "))
b = int(input("The length of side b: "))
c = int(input("The length of side c: "))

if a == b and b == c:
        print("This is an equilateral triangle.")
elif a != b and b != c and a != c:
        print("This is a scalene triangle.")
else:
        print("This is an isosceles triangle.")

Next, we compare the sides to determine the type of the triangle:

• if a equals b and b equals c then all three sides are identical, therefore it’s an equilateral triangle
• if a does not equal b and b does not equal c and a does not equal c then all three sides are different, it’s a scalene triangle
• if it is neither scalene nor equilateral then it must be an isosceles triangle.

Note

This example illustrates how to handle more than two cases. Once again, if and else lines end in colons. The code-blocks that follow end in indentation. What’s different is the use of elif which is short for else if. This allows you to try another test. There is no limit on how many else ifs you can use. And finally the else statement is a catch-all, if all of the ifs above fail then this block is executed.

Let’s test our code. Save the file and run it in console. Enter the sides: 3, 4 and 5:

$ python3 if_then3.py
The length of side a: 3
The length of side b: 4
The length of side c: 5
This is a scalene triangle.
$ _

Our program is correct, a triangle with these sides is scalene. Run the program again and enter: 5, 5 and 7:

$ python3 if_then3.py
The length of side a: 5
The length of side b: 5
The length of side c: 7
This is an isosceles triangle.
$ _

Excellent, a triangle with these sides is definitely isosceles. One more, run the program and enter: 4, 4, 4:

$ python3 if_then3.py
The length of side a: 4
The length of side b: 4
The length of side c: 4
This is an equilateral triangle.
$ _

Perfect, these are the sides of an equilateral triangle. By the way, we did not test the three numbers to make sure that they make a valid triangle. For example you could enter negative integers and the program will still run.

Here’s a problem for you to think about: how do you test three numbers to see if they form a triangle?

The if, elif and else statements allow you to handle any number of cases in your code, they let you control the flow of your code.

If you are serious about programming in Python

then you should master these statements

or else …

Conditional expressions

In Python, they are more commonly known as ternary operators. These operators evaluate something based on a condition being true or not. Here you can see a blueprint of using these conditional expressions:

value_for_true if condition else value_for_false

This allows you to shorten a multi-line if statement to a single line, making your code compact but still readable. For example, let’s determine whether the water is solid (frozen) or liquid based on a temperature reading from an external sensor and compare the :

temperature = -12  # external sensor reading

# Using an if statement
if temperature < 0:
    water_state = "solid"
else:
    water_state = "liquid"

# Using the ternary operator
water_state = "solid" if temperature < 0 else "liquid"

Short-hand ternary

In Python, there is also an even shorter version of the normal ternary operator you have seen above. Its blueprint looks like this value or alternative value, and this can allow you to easily provide a default value.

message = input() or "No data provided."
print(message)

This is helpful in case you quickly want to check for the output of a function or the input provided by the user and give a useful message if the value is missing (actually if bool(value) is False).