Python Blocks: Indentation and Colons

🎯 Learning Objective

After this lesson, you will be able to:

• Open Python blocks with a colon and use consistent indentation.
• Recognize which statements belong to a branch from their indentation level.
• Indent nested blocks without relying on braces.
• Use pass for an intentionally empty block and initialize values safely when they are needed later.

Short Explanation

Python uses indentation as syntax. A colon opens a block, and the indented statements below it belong to that block:

if is_ready:
    print("Deploy")

Four spaces per indentation level is the usual convention. Python does not use {} to mark blocks, and inconsistent indentation can change the program or stop it from running.

Nested decisions add another indentation level. When the inner block ends, move back to the outer level.

Python also rejects an empty block. Use pass when you need a legal placeholder without behavior.

An if block does not create a separate variable scope, but that does not guarantee an assignment happened. If a value must exist after the block, initialize it before the condition or assign it in every possible branch.

💡 Tip: Read indentation as structure: same level means same block; deeper means nested; moving left closes a block.

Clean Code Examples

1. Read block membership from indentation

Both indented lines belong to the successful branch.

checks_passed = True

if checks_passed:
    print("Validation complete")
    print("Release unlocked")

print("Workflow finished")

Output:

Validation complete
Release unlocked
Workflow finished

The final line is not indented, so it runs after the condition.

2. Indent a nested decision

The second condition is checked only when the first one passes.

account_exists = True
email_verified = True

if account_exists:
    if email_verified:
        print("Login allowed")

Output:

Login allowed

The inner print() is two indentation levels deep.

3. Keep an empty block valid with pass

pass does nothing, but it satisfies Python's requirement that a block contain a statement.

feature_enabled = False

if feature_enabled:
    pass

print("Configuration loaded")

Output:

Configuration loaded

4. Initialize a value before a conditional update

Create a safe default before the branch when the value is printed later.

is_enterprise = False
support_label = "Standard support"

if is_enterprise:
    support_label = "Priority support"

print(support_label)

Output:

Standard support

The variable always exists, even when the branch does not run.

⚠️ Common Mistakes

1. Do not use braces instead of indentation.
2. Do not align a nested statement with its parent branch.
3. Do not leave a block empty when pass is required.
4. Do not assign a value only inside a branch when later code needs it regardless of the branch result.