9. Conditional if-elif-else statement in Python

Summary

- The conditional if construction allows you to make program branches, thus you can control the progress of the program.

- The keywords if, elif and else are used in the conditional construction.

- The keyword if is always specified first in the conditional construction.

- Using elif, you can add other conditions that will be fulfilled after checking the if condition.

- The else block will be executed only if all the conditions in if and elif are false.

- In Python, the code block is indented.

- True values in Python include any non-zero number, any non-empty string, and any non-empty object.

- The in operator allows you to determine the presence of an element in a sequence, for example, in a row or list.

- Boolean operators and, or and not return one of the operands, not a Boolean value.

- Other instructions, including conditional ones, can be used inside conditional instructions.

In the previous lesson we learned how to format strings using the format() method and f-strings. Now let's take a little break from working with types in order to better understand the need for a theory from next lesson.

Conditional if construction

The conditional if construction allows you to branch the program when a given condition is met. We touched it a little bit in the introductory lesson, and now let's look at it in more detail.

In this construction, the keyword if is mandatory, in turn, the keywords elif and else are used if necessary. Important points:

• the keyword if is always specified first in the conditional construction;
• after the keyword if there should be a condition and, if it is true (returns True), then the code in the block is executed;
• using elif, you can add other conditions that will be fulfilled after checking the if condition;
• elif cannot exist without if. The keyword elif is specified only after if;
• the else block will be executed only if all the conditions in if and elif are false (return False).

It is easier to consider an example:

a = 2

if (a == 2):
    print("Two")
elif (a == 3):
    print("Three")
else:
    print("Another number") 

As you have already understood, the program will output "Two", since the condition a == 2 is true (i.e. returns True):

print(a == 2) # => True        

Please note that the condition written in parentheses is followed by a "colon" sign, and the subsequent block of code is indented (tabulated). In Python, the code block is indented, otherwise the interpreter will consider it an error. For example:

# Правильно
if (a == 2):
    print("Digit")
    print("Two")

# Неправильно
if (a == 2):
print("Digit")
print("Two") 

In the following example, the word "Digit" will be output only if a is equal to 2, and the word "Two" will be output regardless of the condition:

if (a == 2):
    print("Digit")
print("Two") 

Now let's figure out what is true in Python (True) and what is false (False).

The true values in Python include:

• any number is not equal to zero;
• any non-empty string;
• any non-empty object.

False values in Python include:

• zero (0);
• None;
• an empty line;
• an empty object.

For example, to find out an empty list or not, you can do this:

l = [1, 2, 3, 4, 5]
if (l):
    print("Not empty")
else:
    print("Empty") 

Comparison operators have already been considered earlier. In order not to come back to you, here they are:

• a == b – checks if both operands are equal and returns true if so;
• a != b – checks if both operands are equal and returns true if they are different;
• a > b – returns true if the left operand (a) is greater than the right operand (b);
• a < b – returns true if the left operand (a) is less than the right operand (b);
• a >=b – returns true if the left operand (a) is greater than or equal to the right operand (b);
• a <=b – returns true if the left operand (a) is less than or equal to the right operand (b).

Note that equality is checked using the double equal symbol. One "equals" character indicates an assignment operation. Another example with comparison operators:

if a > b:
    print("a is greater than b")
elif a < b:
    print("b is greater than a")
else:
    print("a and b are equal")        

By the way, the parentheses that were used in the conditions of the first example are optional, i.e. you can write the condition without parentheses.

The in operator

Python has a convenient in operator that allows you to determine the presence of an element in a sequence, for example, in a row or list:

'a' in 'abcdef' # => True
'Hello' in 'Hello, World!' # => True
1 in [1, 2, 3, 4, 5] # => True        

and, or, not operators

Boolean operators and, or and not they do not return a boolean value, they will always return one of the operands, for example:

'a' and 'b' # => b
'a' and 'b' and 'c' # => c    

When using the and operator, the values are checked from left to right. We have already found out what is true and what is false. The characters a, b, c refer to "any non-empty string". So, if all the elements are true, then the last element will be returned.

0 and 'b' # => 0
'a' and [] and 'c' # => []        

There are false objects in the example above. In this case, the and operator will return the first false object. In the case of the or operator, the first true object will be returned and, only if all are false, the last false object will be returned.

I'll say it again: the and and or operators they do not return True and False, but one of the operands (although, in a conditional construction, this will mean the same thing).

The not operator inverts the logical value of the operand. If the operand is false, True will be returned, otherwise False. The not operator in Python returns True or `False', not an operand:

not 0 and 'b' # => b
'a' and not []# => True 

Nested conditional instructions

In Python, as in other programming languages, other instructions can be used inside conditional instructions, including the same conditional ones. That is, you can put one instruction into another indefinitely, within reason, of course. The main thing is not to forget about the mandatory margins:

if (a > 0): # 0 spaces
    if (a < 3): # 4 spaces or one tab
        if (a == 2): # 8 spaces or two tabs
            ... # 12 spaces or three tabs 

In this lesson, we learned how to use the conditional statement if-elif-else in Python, remembered the list of true and false objects, and got acquainted with the operators in, and, or and not, which can significantly simplify writing code.

In the next lesson we will learn how to determine which type of data an object belongs to and convert from one to another.

Test task