The if Statement

1How it is written

if condition:
    statement
    statement
next statement

The header names the condition and ends in a colon. The indented lines under it are the block. The line at the margin below is not part of the block, and that line is the one worth watching in everything that follows.

2A program that decides

A shop gives free delivery on bills over ₹500. Every customer is told their total; only some are told about the delivery.

# a big order earns free delivery
bill = float(input('Bill amount: '))

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

print('Amount to pay:', bill)

Bill amount: 750
You get free delivery!
Amount to pay: 750.0

Now the same program, run again by a customer who spent less:

# the same program, a different customer
bill = float(input('Bill amount: '))

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

print('Amount to pay:', bill)

Bill amount: 200
Amount to pay: 200.0

One program, two runs, two different outputs. Nothing was rewritten between them. That is the whole point of selection — and notice what did not happen on the second run: no error, no warning, no blank line where the message would have been. The block was skipped, and the program went straight on to the line below it.

3Watch the condition decide

Drag the bill through the boundary and watch two things: the block, which appears and disappears, and the last line, which never does.

Bill amount: 750
You get free delivery!
Amount to pay: 750.0

4Many possibilities, one outcome

The bill could be any amount — that is thousands of possibilities. But the program only ever does one extra thing about them: it prints the free-delivery message. There is no second message for small bills, and none is wanted. That is what makes a plain if the right shape here.

The condition can be as complicated as the rule needs. Here two questions are joined with and, so several scenarios are being tested at once — and there is still only one outcome:

# one outcome: you are told about the club, or nothing is said
age = int(input('Your age: '))
marks = float(input('Your marks: '))

if age >= 16 and marks >= 60:
    print('You can join the coding club')

print('Thank you for applying')

Your age: 17
Your marks: 72
You can join the coding club
Thank you for applying

# the same program, an applicant who is too young
age = int(input('Your age: '))
marks = float(input('Your marks: '))

if age >= 16 and marks >= 60:
    print('You can join the coding club')

print('Thank you for applying')

Your age: 15
Your marks: 90
Thank you for applying

Excellent marks, and still nothing — because and needs both halves to be True. Everybody gets the thank-you either way.

5The two mistakes everybody makes once

Both are punctuation, both stop the program before it runs a single line, and both name themselves clearly once you have seen them.

The missing colon.

age = 19

if age >= 16
    print('You are eligible to drive!')

  File "no_colon.py", line 3
    if age >= 16
                ^
SyntaxError: expected ':'

The missing indentation.

age = 19

if age >= 16:
print('You are eligible to drive!')

  File "no_indent.py", line 4
    print('You are eligible to drive!')
    ^
IndentationError: expected an indented block after 'if' statement on line 3

Both are syntax errors, so nothing runs at all — not even the correct line above. Python does not need to know the age to spot them; it cannot read the file. And the second message is unusually helpful: it names the statement, the line number and what it wanted.

6Try it

Run it with a bill over 500, then with one under. Then try moving the last line four spaces to the right and run it again — you should get nothing at all from a small bill.

# a big order earns free delivery
bill = float(input('Bill amount: '))

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

print('Amount to pay:', bill)

7Recap