Simple if Statements

Simple if Statements#

Note

Source: Adapted from the C# edition (decisions/ifstatements.rst and decisions/ifelse.rst). Suitcase and clothes examples are Python translations of the original C# programs. The conditional expression (ternary) form is a Python-specific original addition.

An if statement lets a program choose whether to execute a block of code based on a condition. Python’s syntax uses a colon and indentation — no braces, no parentheses around the condition.

One-Branch if#

weight = float(input("Enter suitcase weight (lbs): "))
if weight > 50:
    print("Overweight: $25 extra charge.")
print("Thank you for flying with us.")

Try this with input 30 (no extra message) and 55 (extra charge printed). The indented print only runs when the condition is true. The final print always runs.

General form:

if condition:
    statement(s)

The condition is any expression that evaluates to a Boolean. The indented block (the body) executes only when the condition is True.

Warning

Python uses indentation to define blocks. Every statement in the if body must be indented by the same amount (4 spaces is the standard). A TabError or IndentationError means the indentation is inconsistent.

Two-Branch if-else#

When you want to do one thing if the condition is true and something different if it is false, add an else clause:

temperature = float(input("Enter temperature (°F): "))
if temperature > 60:
    print("Wear light clothes.")
else:
    print("Bring a jacket.")

Exactly one of the two blocks executes. The general form:

if condition:
    statement(s)    # executed when condition is True
else:
    statement(s)    # executed when condition is False

Multi-Statement Bodies#

The body can contain any number of statements, all indented to the same level:

balance = float(input("Account balance: $"))
if balance < 0:
    transfer = -balance
    backup_account -= transfer
    balance += transfer
    print("Balance was negative; funds transferred from backup.")
print(f"Current balance: ${balance:.2f}")

if as an Expression (Conditional Expression)#

Python also has a one-line form called a conditional expression (sometimes called a ternary operator):

label = "positive" if x > 0 else "non-positive"

This sets label to "positive" when x > 0, otherwise "non-positive". Use it for simple choices; a full if/else is clearer for anything more complex.

Simple If Exercises#

Predict the output for each fragment, then test in Python:

  1. With v = "Hi" and again with v = "Hello there":

    v = input("Enter a word: ")
if len(v) > 3:
    v = v + v
print("Now we have", v)

  2. With x = 7 and again with x = -3:

    x = int(input("Enter an integer: "))
print("The magnitude of", x, "is", end=" ")
if x < 0:
    x = -x
print(x)