Conditional Statements in Python

Programs that make decisions

Why programs need decisions

Without conditionals, a program does exactly the same thing every time, no matter what the user types or what happens in the game. Conditionals let one program react differently to different situations. Almost every real program is full of them.

Turning an everyday rule into code

"If your phone battery is below 20%, plug it in" becomes:

if battery < 20:
    print("Plug in your phone.")

The condition is the part after if (battery < 20). The action is the indented line. Practice by turning rules you know (school, games, sports) into this shape.

Comparison operators: True or False?

Comparing more than numbers

• Strings can be compared too. == checks if they are exactly the same, letter for letter and capital for capital: "cat" == "Cat" is False.
• < and > on strings compare in dictionary order: "apple" < "banana" is True. Careful: all capital letters come before all lowercase ones, so "Zebra" < "apple" is also True.
• An int and a float with the same value are equal: 5 == 5.0 is True.

You can store the answer

A comparison is an expression, so its True/False result can be saved: is_adult = age >= 18. Later, if is_adult: reads almost like English.

Reading the symbols: say >= as "is at least" and <= as "is at most." The = always comes second: => is a SyntaxError.

Combining conditions: and, or, not

Truth tables: every possible case

Memory trick: and is picky (everything must be True). or is easygoing (one True is enough).

Order of operations: not goes first, then and, then or. Comparisons like > happen before all three. When you mix and and or, add parentheses so the meaning is clear: (is_weekend or is_holiday) and not raining.

Anatomy of an if statement

temperature = 92
if temperature > 85:
    print("It is hot outside.")
    print("Stay hydrated!")
print("Enjoy your day.")

How Python reads it

1. Python reaches the if line and evaluates the condition: 92 > 85 becomes True.
2. Because it is True, Python goes into the body and runs each indented line in order.
3. When it reaches a line that is back at the left edge, the body is over. Python continues from there.

If the condition had been False, Python would have jumped straight from the if line to print("Enjoy your day.").

The colon is easy to forget. Think of it as saying "here comes the block." Every if, elif, and else line ends with one.

Try it: if in action

Change the temperature and click ▶ Run. Watch which lines turn green (Python ran them) and which fade out (Python skipped them).

Edge values: test the boundary

The number right at the dividing line (here, 85) is called an edge value or boundary. Bugs love to hide there, because > and >= only disagree at that one number.

• 85 > 85 is False (85 is not greater than itself).
• 85 >= 85 is True.

So ask yourself: "Should exactly 85 count as hot?" Then pick the operator that matches. When you test any condition, try a value below, a value above, and the value exactly on the line.

Indentation decides what is inside

In Python, indentation is not just for looks. The spaces at the start of a line tell Python which lines belong to the if statement. The standard is 4 spaces. (In most Python editors, pressing Tab inserts 4 spaces for you.)

score = 50
if score >= 70:
    print("You passed!")
    print("Great job!")

score = 50
if score >= 70:
    print("You passed!")
print("Great job!")

The code is almost the same, but the result is different. In Example B, print("Great job!") is not indented, so it is outside the if and always runs.

Indentation errors you may see

How to tell where a block ends

A block ends at the first line that is indented less than the block. Blank lines in between do not end it. That is why the colon and the indentation work together: the colon says "a block starts," and going back to the left says "the block is done."

Two paths: if-else

if condition:
    # runs when the condition is True
else:
    # runs when the condition is False

Why else has no condition

else means "in every other case." Its condition is already decided: it is simply "the if was False." Writing else score < 70: is a SyntaxError because the condition would be redundant.

Use the opposite carefully

The opposite of score >= 70 is score < 70, not score <= 70. With else, you never have to work out the opposite yourself, which removes a common source of bugs.

Many paths: if-elif-else

if condition1:
    # runs if condition1 is True
elif condition2:
    # runs if condition1 is False and condition2 is True
else:
    # runs if all conditions above are False

Why elif score >= 80 is enough for a B

You might think the B condition should be score >= 80 and score < 90. It does not need the second part! Python only reaches the elif if the if above it was False, which already means the score is below 90. Each elif quietly includes "and everything above me was False."

Trace for a score of 72

Order matters!

This program has a logic error. It runs without crashing, but it gives the wrong answer. Run it with a score of 95.

A rule for ordering conditions

When conditions overlap (a score of 95 is also at least 60, 70, and 80), the one that is hardest to meet must go first. Think of it as sorting candidates from most exclusive to least.

• Checking with >=? Start from the biggest number and go down.
• Checking with <? Start from the smallest number and go up: if score < 60: "F", elif score < 70: "D", and so on.

Both orders work, as long as you stay consistent within one chain.

elif vs. separate if statements

Each separate if is its own decision, so Python checks every one. An elif chain is one decision, so Python stops at the first True. Run both with the same score and compare the output.

What you see when you run both with 95

With separate ifs, a score of 95 passes the A check, then also the B and C checks, so A, B, and C all print. With one chain, Python prints only the A and skips the rest.

A test to decide which to use

Ask: "Could more than one of these be true, and should all of those actions happen?"

• Yes: use separate ifs. Example: give a bonus if the player found a key, and also if they beat the clock.
• No, pick only one: use if-elif-else. Example: a letter grade, a ticket price by age group.

Conditionals with input()

age = input("Age: ")     # age is "15", a string
if age > 13:
    print("Teen")

age = int(input("Age: "))   # age is 15, a number
if age > 13:
    print("Teen")

What int(input("Age: ")) does, from the inside out

1. input("Age: ") shows the prompt and waits. The user types 15 and presses Enter. The result is the string "15".
2. int("15") turns that text into the number 15.
3. age = 15 stores the number, so age > 13 can now compare numbers.

When the user types something that is not a number, like ten or 15.5, int() fails with ValueError: invalid literal for int() with base 10. Use float() if decimals are allowed.

Accepting yes, Yes, or YES: .lower() makes a lowercase copy of the text, so answer.lower() == "yes" matches all three.

Nested if statements

has_ticket = True
age = 12

if has_ticket:
    if age >= 13:
        print("Enjoy the PG-13 movie!")
    else:
        print("Please pick a different movie.")
else:
    print("Please buy a ticket first.")

Trace all three cases

When nesting is the better choice

Combining with and is shorter, but nesting is useful when each "no" needs a different message. Here, someone without a ticket and someone who is too young need to hear different things, so the nested version gives clearer feedback.

Common mistakes and their error messages

Messages shown are from Python 3.13. Older versions of Python word some of these messages a little differently, but they point to the same mistake. Python also shows the line number and a ^ arrow near the problem.

How to fix errors quickly

1. Read the last line of the error. SyntaxError and IndentationError mean the code is written wrong; nothing ran. TypeError means it ran until it tried to compare things that cannot be compared.
2. Go to the line number Python gives. Check for the colon, the double ==, and the indentation.
3. If that line looks fine, check the line above. A missing parenthesis or quote on one line often gets reported on the next.

Check yourself

Predict what each program does, then click Check. This is practice only. Nothing is graded or recorded.

Conditional statements cheat sheet

if score >= 70:
    print("Pass")

if score >= 70:
    print("Pass")
else:
    print("Retry")

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
else:
    print("Keep going")

Before you run your code, check:

• Every if, elif, and else line ends with a colon :
• Every body is indented 4 spaces, and else/elif line up with their if
• Conditions use == to compare, not =
• else has no condition, and it comes last
• In an elif chain, the most specific condition comes first
• Values from input() are converted with int() or float() before comparing with numbers

Key words to know

condition · Boolean (True/False) · comparison operator · and / or / not · if / elif / else · body · nested if · edge value · logic error