Conditional Statements

Topic — Running different code depending on whether a condition is true.

Conditional statements let a program take a decision. Python evaluates the condition, gets True or False, and runs the matching block.

---

Indentation is the block

Python has no braces. Indentation is what makes a line part of the if block — four spaces by convention.

age = 20
if age >= 18:
    print("Eligible to vote")   # indented → inside the if
print("Done")                   # not indented → always runs

Output

Eligible to vote
Done

The colon at the end of the if line is mandatory. Both of these are errors:

if age >= 18      # SyntaxError: expected ':'
if age >= 18:
print("Eligible") # IndentationError: expected an indented block

---

Plain if

Runs the block only when the condition is true. When it's false, nothing happens.

age = 20
if age >= 18:
    print("Eligible to vote")

Output

Eligible to vote

With age = 16 this program prints nothing at all — there's no alternative branch.

Comparison operators

These are what produce the True/False a condition needs:

Operator	Means	Example	Result
==	equal to	20 == 20	True
!=	not equal to	20 != 18	True
>	greater than	20 > 18	True
<	less than	20 < 18	False
>=	greater than or equal	18 >= 18	True
<=	less than or equal	16 <= 18	True

danger

= is assignment, == is comparison

if age = 18: is a SyntaxError. This is the most common beginner mistake, and Python deliberately refuses to guess what you meant.

---

if...else

Gives the false case somewhere to go. Exactly one of the two blocks runs.

age = 16
if age >= 18:
    print("Eligible")
else:
    print("Not Eligible")

Output

Not Eligible

---

if...elif...else

For more than two outcomes. elif is short for "else if", and you can chain as many as you need.

marks = 75
if marks >= 90:
    print("A")
elif marks >= 75:
    print("B")
else:
    print("C")

Output

B

A full grade scale:

for mk in [95, 80, 60, 30]:
    if mk >= 90:
        grade = "A"
    elif mk >= 75:
        grade = "B"
    elif mk >= 50:
        grade = "C"
    else:
        grade = "Fail"
    print(f"{mk:>3} -> {grade}")

Output

 95 -> A
 80 -> B
 60 -> C
 30 -> Fail

Order matters — the first true branch wins

This is the single biggest trap in an elif chain. Python checks conditions top to bottom and stops at the first match. It does not look for the best match.

marks = 95

if marks >= 50:
    print("Pass")
elif marks >= 75:
    print("Good")
elif marks >= 90:
    print("Excellent")
else:
    print("Fail")

Output

Pass

95 satisfies all three conditions, but marks >= 50 is checked first, so the other two are never reached. The fix is to order the conditions from most restrictive to least:

marks = 95

if marks >= 90:
    print("Excellent")
elif marks >= 75:
    print("Good")
elif marks >= 50:
    print("Pass")
else:
    print("Fail")

Output

Excellent

elif chain vs separate if statements

They are not the same thing. An elif chain runs at most one block; separate if statements each get tested independently, so several can run.

marks = 95

if marks >= 50:
    print("Pass")
if marks >= 75:
    print("Good")
if marks >= 90:
    print("Excellent")

Output

Pass
Good
Excellent

Three lines instead of one. Use elif when the cases are mutually exclusive, separate ifs when they're independent checks.

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Nested if

An if inside another if. The inner condition is only tested when the outer one passed.

age = 22
if age >= 18:
    if age >= 21:
        print("Can drive and vote")

Output

Can drive and vote

Each level adds four more spaces of indentation. A more useful version covers the inner false case too:

age, has_license = 22, True

if age >= 18:
    if has_license:
        print("May drive")
    else:
        print("Old enough, but no licence")
else:
    print("Too young")

Output

May drive

tip

Nesting more than two deep usually means the conditions should be combined with and instead. if age >= 18 and has_license: says the same thing as the nested version, flatter.

---

Logical operators

Operator	True when	Example
and	both sides are true	age >= 18 and has_license
or	either side is true	marks >= 90 or sports
not	flips the value	not has_license

age = 20
has_license = True
if age >= 18 and has_license:
    print("Can drive")

marks = 95
sports = False
if marks >= 90 or sports:
    print("Scholarship")

if age >= 18 and not has_license:
    print("Get a licence")

Output

Can drive
Scholarship

The third if prints nothing: age >= 18 is True, but not has_license is False, and and needs both.

Short-circuit evaluation

and stops at the first false value; or stops at the first true one. The rest is never evaluated:

def loud():
    print("  (evaluated)")
    return True

print("A:", False and loud())   # loud() never runs
print("B:", True or loud())     # loud() never runs
print("C:", True and loud())    # loud() does run

Output

A: False
B: True
  (evaluated)
C: True

This is what makes if items and items[0] == "x": safe — if items is empty, the second half is skipped rather than raising IndexError.

Chained comparisons

Python lets you write range checks the way maths does — no and needed:

age = 35
print(18 <= age < 60)      # → True
print(0 < 5 < 10 < 20)     # → True

18 <= age < 60 means exactly 18 <= age and age < 60, but reads better and evaluates age only once.

---

Truthiness in conditions

A condition doesn't have to be a comparison. Any value works — Python asks whether it's truthy.

name = ""
if name:
    print("has name")
else:
    print("empty")

items = [1, 2]
if items:
    print("list has", len(items), "items")

Output

empty
list has 2 items

0, 0.0, "", [], {}, None and False are falsy; everything else is truthy. So if items: is the idiomatic way to say "if the list isn't empty" — you don't need if len(items) > 0:. See Values and Types.

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The ternary (conditional expression)

For picking between two values, a one-liner is often clearer than four lines:

n = 25
print("EVEN" if n % 2 == 0 else "ODD")

Output

ODD

The shape is value_if_true if condition else value_if_false. It's an expression, so it can go inside an f-string — which is how the f-strings page builds status columns.

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Worked example — leap year

Three rules at once: divisible by 4, except centuries, unless divisible by 400.

for year in [2000, 1900, 2024, 2023]:
    leap = year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)
    print(f"{year}: {leap}")

Output

2000: True
1900: False
2024: True
2023: False

The parentheses are doing real work — and binds tighter than or, so without them 1900 would come out True.

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match — Python 3.10+

For comparing one value against many fixed options, match reads better than a long elif chain. case _ is the catch-all.

command = "start"
match command:
    case "start":
        print("Starting")
    case "stop":
        print("Stopping")
    case _:
        print("Unknown")

Output

Starting

match and case are soft keywords — still legal as variable names, but don't use them as such. See Variables and Naming.

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Common Mistakes

#	Mistake	Wrong	Right
1	= instead of ==	if age = 18: → SyntaxError	if age == 18:
2	Missing colon	if age >= 18 → SyntaxError	if age >= 18:
3	Body not indented	IndentationError: expected an indented block	indent by 4 spaces
4	elif chain in the wrong order	>= 50 before >= 90 prints Pass for 95	most restrictive condition first
5	Separate ifs where elif was meant	all matching branches run	use elif for mutually exclusive cases
6	and/or precedence	a % 4 == 0 and b or c	parenthesise: ... and (b or c)
7	Comparing to True	if flag == True:	if flag:
8	if len(items) > 0:	verbose	if items:

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Summary

Form	Runs when	Blocks that can run
if	condition true	0 or 1
if...else	always one branch	exactly 1
if...elif...else	first true condition	exactly 1
separate ifs	each tested independently	0 to all
x if c else y	expression, not statement	produces a value
match/case	value matches a case	exactly 1

Key takeaways

• Indentation defines the block; the colon is mandatory
• elif stops at the first true condition — order from most restrictive down
• Separate ifs can all fire; an elif chain fires at most once
• and needs both sides, or needs either, not flips
• and/or short-circuit, which makes guard conditions like if items and items[0] safe
• 18 <= age < 60 is legal and clearer than joining two comparisons with and
• Any value can be a condition — if items: beats if len(items) > 0:

See also: Loops for repeating a block · Values and Types for truthiness

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Run It Yourself

conditionals.py

# 1. Positive / negative / zero
for number in [7, -3, 0]:
    if number > 0:
        print(f"{number:>3}: Positive Number")
    elif number < 0:
        print(f"{number:>3}: Negative Number")
    else:
        print(f"{number:>3}: Zero")

# 2. Even or odd, as a ternary
for number in [10, 7]:
    print(f"{number} is {'EVEN' if number % 2 == 0 else 'ODD'}")

# 3. Largest of three
a, b, c = 12, 45, 30
if a >= b and a >= c:
    largest = a
elif b >= a and b >= c:
    largest = b
else:
    largest = c
print(f"largest of {a}, {b}, {c} is {largest}")

# 4. Driving eligibility — combined instead of nested
age, has_license = 22, True
if age >= 18 and has_license:
    print("May drive")
elif age >= 18:
    print("Old enough, but no licence")
else:
    print("Too young")

Output

  7: Positive Number
 -3: Negative Number
  0: Zero
10 is EVEN
7 is ODD
largest of 12, 45, 30 is 45
May drive

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Practice Questions

From the Unit 1 question bank. Tags and marks are explained on the Python index.

Unit 1 § H — Conditional Statements

H1. [PROG] Accept a number and print "Positive Number" if it is greater than zero. Use a plain if. [2]

H2. [PROG] Accept a number and check whether it is even or odd using if...else. [3]

H3. [PROG] Accept three numbers and display the largest, using if...elif...else. [4]

H4. [PROG] Accept marks and display the grade: A (90+), B (75–89), C (50–74), Fail (below 50). [4]

H5. [PROG] Accept a person's age and whether they hold a driving licence. Using a nested if, display whether they may legally drive. [4]

H6. [OUT] More than one condition is true here. Which branch wins, and why? [3]

marks = 95

if marks >= 50:
    print("Pass")
elif marks >= 75:
    print("Good")
elif marks >= 90:
    print("Excellent")
else:
    print("Fail")

Then rewrite it so it prints Excellent.

H7. [OUT] [3]

age = 20
license = True

if age >= 18 and license:
    print("Can drive")

marks = 95
sports = False

if marks >= 90 or sports:
    print("Scholarship")

if age >= 18 and not license:
    print("Get a licence")

H8. [PROG] Accept a year and determine whether it is a leap year. (Divisible by 4, except centuries, unless divisible by 400.) [4]

H9. [THEORY] What is the difference between using several separate if statements and one if...elif...else chain? When does it change the output? [3]

Unit 1 § L — Mini Challenges

L1. [PROG] Write one program that: [8]

• accepts a student's name
• accepts marks in three subjects
• calculates the average
• displays the grade using if...elif...else
• prints the name, average (to 2 decimal places), and grade in a neat, aligned layout