What is a Selection Statement?

1A program that decides

Think about a shop bill. If the total is over ₹500, delivery is free. If it is not, delivery is charged. The same shop, the same rule, two different bills — and the shopkeeper does something different in each case.

You could not write that with what you know so far. A sequential program does the same thing every time it runs. To write the rule you need a way to say: run these lines only when this is true.

2It all hangs on the condition

The condition is the question being asked. It is nothing new: it is an expression whose value is True or False, exactly like the ones you built with the relational and logical operators.

# a condition is just an expression whose value is True or False
age = 19

print(age >= 16)
print(age == 16)
print(age < 16 or age > 60)

True
False
False

Those three lines print their answers because we asked them to. Inside a selection statement nothing is printed — Python works the condition out for itself, and uses the True or False to decide what to run next.

3The shape of a selection statement

Every selection statement is built from the same parts. Tap them below — including the empty space at the start of the third line, which is a part in its own right.

Two of those parts are new punctuation and both are compulsory. The colon ends the header. The indentation marks the block that belongs to it.

4Indentation is not decoration

In many languages, curly brackets say where a block starts and ends, and the spacing is only for looks. Python has no brackets here. The spacing is the language: a line indented under the header is inside the block, and a line back at the margin is outside it.

Which means one stray press of the space bar changes what your program does. Both programs below have a bill of 200, and only one of them prints anything at all:

bill = 200.0

if bill > 500:
    print('You get free delivery!')

print('Amount to pay:', bill)

Amount to pay: 200.0

# the last line has been indented by mistake
bill = 200.0

if bill > 500:
    print('You get free delivery!')
    print('Amount to pay:', bill)

The second program prints nothing. Its last line is now indented, so it is part of the block — and the block was skipped, because 200 is not more than 500. Same statements, same order, one extra level of indentation, and the output disappears. No error is reported, because nothing is wrong with the program; it is simply not the program you meant to write.

A block is not limited to one statement. Every line indented under the header belongs to it, however many there are:

# a block can hold as many statements as you like
marks = 91

if marks >= 90:
    print('Excellent!')
    print('You are in the top grade.')

print('Marks =', marks)

Excellent!
You are in the top grade.
Marks = 91

5Selection comes in three shapes

Now the part that decides which one you write. All three start with if and a condition, so they look alike on the page. What separates them is how many different things your program can end up doing.

6Recap