if and else

Programs that choose

Everything you have written so far runs every line, always. Real programs branch: unlock the phone if the passcode matches, otherwise show an error.

Python's if statement runs a block of code only when a condition is True:

age = 20
if age >= 18:
    print("You are an adult.")
else:
    print("You are a minor.")

The pieces:

• if is followed by a condition, any expression that produces a boolean (lesson 4-1)
• The colon : announces that an indented block follows
• The indented lines (4 spaces) are what runs when the condition is True
• else: provides the block that runs when it is False

Indentation is not decoration in Python. The spaces at the start of a line are how Python knows which lines belong inside the if. Lines indented the same amount form one block.

A second worked example: the gate everyone uses

Every login screen on earth is, at its core, one if/else: compare what the user typed against the stored secret, and branch. Here it is with input() from lesson 1-3 supplying the condition's left side:

password = input()
if password == "open sesame":
    print("Access granted")
else:
    print("Access denied")

Note the comparison is == (asking, from lesson 4-1), never = (storing). Real systems add one crucial twist — they store a scrambled version of the password rather than the real one — but the branching logic is exactly this.