Python If Statements: Making Your Code Decide

Python If Statements: Making Your Code Decide

An if statement lets your program choose. If a condition is true, run this block. Otherwise, run that one, or nothing. Without it, every program does the exact same thing every time, which is not much of a program.

Why this matters

Most business rules are if statements in disguise. Members get a discount. Orders above a limit need approval. Late payments get a reminder. If you can write clear conditions, you can turn rules from a meeting into working code. If you write them carelessly, the code makes the wrong decision quietly, every time.

The basic form

temperature = 36

if temperature > 35:
    print("Too hot. Stay inside.")
print("This line always runs.")

Two things to notice. The line with if ends with a colon. And the code that belongs to the if is indented, four spaces by convention. The indentation is not decoration. In Python, it is how the interpreter knows which lines are inside the if and which are not.

else and elif

score = 78

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "Needs improvement"

print(f"Score {score}: {grade}")

Python checks the conditions from top to bottom and runs only the first one that is true. Then it skips the rest. That is why the order matters. If you put score >= 70 first, a score of 95 would get a "C", because 95 is also greater than 70.

Combining conditions

age = 17
with_parent = True

if age >= 18 or with_parent:
    print("Allowed in")
else:
    print("Not allowed")

Use and when everything must be true, or when one is enough. If a condition becomes hard to read, give the pieces names first:

subtotal = 1500
is_member = False
has_coupon = True

qualifies = subtotal >= 1000 and (is_member or has_coupon)
if qualifies:
    print("Discount applied")

Truthy and falsy values

Python treats some values as false in a condition even though they are not False: 0, an empty string "", an empty list [], and None. Everything else counts as true.

name = ""
if name:
    print(f"Hello, {name}")
else:
    print("You did not enter a name.")

This is handy for checking empty input. But be careful with numbers: if 0 is a valid value, if quantity: treats it as "nothing entered". In that case, write if quantity is not None: instead.

The one-line form

age = 20
status = "adult" if age >= 18 else "minor"
print(status)

Good for simple two-way choices. Anything longer, use the normal form. Saving one line is not worth a line nobody can read.

Common mistakes

• Missing colon. if x > 5 without : is a SyntaxError.
• Wrong order of elif. Broad conditions before specific ones swallow the specific cases.
• Using = instead of ==. Python stops you, but read the error, it tells you.
• Comparing text with different case. "Yes" == "yes" is False. Clean input with .lower() first.

What happens if you skip this step

A condition can work for your example and still fail at a boundary. The program may run without crashing while applying the wrong discount or rejecting a valid input. Test values below, at and above each cutoff, and check that every branch produces the intended result before relying on it.

Quick check: in the grading example, what grade does a score of exactly 80 get, and why?