Making Decisions with If

Welcome to Making Decisions in Python! This lesson marks an important shift in how our programs behave.

Up to now, every program we have written has run every single line, from top to bottom, no matter what. That works well for calculations and printed messages, but real programs need to react to their data. A weather app should only warn us about heat when it is actually hot. A shopping cart should only mention free shipping when the total is high enough.

In this lesson, we will learn the if statement: the tool that lets Python run certain lines only sometimes. Along the way, we will see why the colon and the indentation matter so much, and how a simple flowchart helps us plan a decision before we write a single line of code.

An if statement always follows the same shape. Here it is as a template:

if condition:
    # indented statement(s) that run only when the condition is True

Three pieces work together here:

• The keyword if, which tells Python that a decision is coming.
• A condition: an expression whose truth value Python tests. In this course, our conditions will usually be comparisons or logical expressions, which produce True or False, or Boolean variables that store one of those values. More generally, Python can test the truth value of many kinds of expressions.
• A colon (:) at the end of the line, which opens the block of code that belongs to the if.

The core rule is short enough to memorize: if Python considers the condition true, the indented block runs; otherwise, Python skips the entire block and moves on.

Let's put that template to work with a temperature reading:

temperature = 32

if temperature > 30:
    print("It is a hot day.")

Python walks through this in three steps. First, it stores 32 in temperature. Next, it evaluates the condition temperature > 30, which is 32 > 30, so the result is True. Because the condition is True, Python runs the indented line and we see:

It is a hot day.

Now imagine we change the first line to temperature = 18. The condition becomes 18 > 30, which is False, so Python skips the indented print() completely. The program still runs without any errors; it simply produces no output at all.

In Python, indentation is not just for looks: it is how Python knows which lines belong to the if. Everything indented under the if line forms one block, and the block ends as soon as the indentation goes back out.

A few practical rules to keep in mind:

• Use four spaces for each level of indentation; this is the standard style in Python.
• Indent every line of the block by the same amount.
• A block may contain as many statements as we need, not just one.

Let's add a second line to our block:

if temperature > 30:
    print("It is a hot day.")
    print("Remember to drink water.")

Both print() calls sit at the same indentation level, so they belong to the same block. That means they run together when temperature > 30 is True, and neither of them runs when it is False. There is no way for just one of them to run.

So what about the lines that come after the block? Let's complete our program with an unindented statement at the end:

temperature = 32

# The indented block runs only when the condition is True
if temperature > 30:
    print("It is a hot day.")
    print("Remember to drink water.")

print("Program finished.")

The final print() starts at the left margin, so it is outside the if and always runs. With temperature = 32, the condition is True, so all three messages appear:

It is a hot day.
Remember to drink water.
Program finished.

If we set temperature = 18 instead, Python skips the whole block but still reaches that last line, and the output shrinks to:

Program finished.

Two small slips account for most broken if statements. The first is forgetting the colon:

if temperature > 30      # SyntaxError: expected ':'
    print("It is a hot day.")

The fix is simply to end the condition line with :. The second is forgetting to indent the body:

if temperature > 30:
print("It is a hot day.")    # IndentationError: expected an indented block

Here, Python opens a block with the :, then finds nothing inside it. Indenting the print() by four spaces resolves the error.

One more thing worth watching: every line in a single block must share the same indentation. Indenting one line by four spaces and the next by six raises an IndentationError, so we should stay consistent.

Each shape maps onto one part of our code: the diamond is the if temperature > 30: line; the "Yes" box holds the two indented print() calls; the "No" arrow is the skipped block; and the final box is the unindented print("Program finished.") that both paths reach.

Let's gather the four takeaways from this lesson: write if, then a condition, then a :; indent the statements that belong to the block; that block runs only when Python considers the condition true; and unindented lines after the block run either way. Notice that in our flowchart, the "No" path does nothing at all: it just skips ahead. Filling that empty path with an alternative action is exactly what else does, and we will explore it next.

Time to make some decisions of your own: in the practices ahead, you will write if statements from scratch, group several lines into one block, check what still runs after the block ends, and turn a flowchart into working Python code.