Python If...Else

Learn how to use conditional statements in Python to make decisions in your code.

Python Conditions and If statements

Python supports the usual logical conditions from mathematics:

  • Equals: a == b
  • Not Equals: a != b
  • Less than: a < b
  • Less than or equal to: a <= b
  • Greater than: a > b
  • Greater than or equal to: a >= b

These conditions can be used in several ways, most commonly in "if statements" and loops.

An "if statement" is written by using the if keyword.

Example

a = 33
b = 200
if b > a:
    print("b is greater than a")

In this example we use two variables, a and b, which are used as part of the if statement to test whether b is greater than a. As a is 33, and b is 200, we know that 200 is greater than 33, and so we print to screen that "b is greater than a".

Indentation

Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.

Example

a = 33
b = 200
if b > a:
    print("b is greater than a") # you will get an error

Elif

The elif keyword is Python's way of saying "if the previous conditions were not true, then try this condition".

Example

a = 33
b = 33
if b > a:
    print("b is greater than a")
elif a == b:
    print("a and b are equal")

In this example a is equal to b, so the first condition is not true, but the elif condition is true, so we print to screen that "a and b are equal".

Else

The else keyword catches anything which isn't caught by the preceding conditions.

Example

a = 200
b = 33
if b > a:
    print("b is greater than a")
elif a == b:
    print("a and b are equal")
else:
    print("a is greater than b")

In this example a is greater than b, so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that "a is greater than b".

You can also have an else without the elif:

Example

a = 200
b = 33
if b > a:
    print("b is greater than a")
else:
    print("b is not greater than a")

Short Hand If

If you have only one statement to execute, you can put it on the same line as the if statement.

Example

if a > b: print("a is greater than b")

Short Hand If ... Else

If you have only one statement to execute, one for if, and one for else, you can put it all on the same line:

Example

a = 2
b = 330
print("A") if a > b else print("B")

This technique is known as Ternary Operators, or Conditional Expressions.

You can also have multiple else statements on the same line:

Example

a = 330
b = 330
print("A") if a > b else print("=") if a == b else print("B")

And

The and keyword is a logical operator, and is used to combine conditional statements:

Example

a = 200
b = 33
c = 500
if a > b and c > a:
    print("Both conditions are True")

Or

The or keyword is a logical operator, and is used to combine conditional statements:

Example

a = 200
b = 33
c = 500
if a > b or a > c:
    print("At least one of the conditions is True")

Not

The not keyword is a logical operator, and is used to reverse the result of the conditional statement:

Example

a = 33
b = 200
if not a > b:
    print("a is NOT greater than b")

Nested If

You can have if statements inside if statements, this is called nested if statements.

Example

x = 41

if x > 10:
    print("Above ten,")
    if x > 20:
        print("and also above 20!")
    else:
        print("but not above 20.")

The pass Statement

if statements cannot be empty, but if you for some reason have an if statement with no content, put in the pass statement to avoid getting an error.

Example

a = 33
b = 200

if b > a:
    pass

elif and Nesting

score = 82

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

print(grade)   # B

Python checks branches top to bottom and runs the first that is true, then skips the rest.

The Ternary (Conditional) Expression

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

# useful for defaults and inline choices
label = "even" if n % 2 == 0 else "odd"

Python has no switch; use elif chains, a dictionary of actions, or match (next chapter).

Try It Yourself

Exercise 1: Print "FizzBuzz" if n is divisible by both 3 and 5, else "Fizz", "Buzz", or the number.

Show solution
n = 15
if n % 15 == 0:
    print("FizzBuzz")
elif n % 3 == 0:
    print("Fizz")
elif n % 5 == 0:
    print("Buzz")
else:
    print(n)

Exercise 2: Use a ternary to store "pass" if mark >= 50 else "fail".

Show solution
mark = 63
result = "pass" if mark >= 50 else "fail"
print(result)   # pass

Key Takeaways

  • Use if/elif/else; the first true branch runs.
  • Indentation defines each branch's body.
  • The ternary A if cond else B chooses inline.
  • No switch in Python — use elif, dicts, or match.
← Previous: Dictionary MethodsNext: Match →

📘 Real-World Deep Dive

Branching is the core of any program. Python's <code>if/elif/else</code> pattern reads top-to-bottom and short-circuits on the first match — the disciplined version uses one expression per branch.

Real-Life Scenario

A billing rule engine: route each invoice through a series of increasingly-specific predicates until a rule fires.

Real-Life Example

from dataclasses import dataclass

@dataclass
class Invoice:
    customer_type: str   # "vip" | "edu" | "regular"
    region: str          # "EU" | "US" | "Other"
    total: float

def discount(inv: Invoice) -> tuple[float, str]:
    pct, reason = 0.0, "no-discount"
    if inv.customer_type == "vip":
        pct, reason = 0.25, "vip-tier"
    elif inv.customer_type == "edu":
        pct, reason = 0.20, "edu-tier"
    elif inv.total >= 10_000:
        pct, reason = 0.10, "volume"
    elif inv.region == "EU":
        pct, reason = 0.05, "eu-region"
    return round(inv.total * (1 - pct), 2), reason

invoices = [
    Invoice("vip",     "US",    4_000),
    Invoice("edu",     "EU",    2_000),
    Invoice("regular", "EU",    1_000),
    Invoice("regular", "Other", 25_000),
    Invoice("regular", "US",    250),
]
for inv in invoices:
    final, why = discount(inv)
    print(f"{inv.customer_type:<8} {inv.region:<6} {inv.total:>8.2f} -> {final:>8.2f}  ({why})")

Expected Output

vip      US      4000.00 ->   3000.00  (vip-tier)
edu      EU      2000.00 ->   1600.00  (edu-tier)
regular  EU      1000.00 ->    950.00  (eu-region)
regular  Other  25000.00 ->  22500.00  (volume)
regular  US       250.00 ->    250.00  (no-discount)

Common mistakes

  • Long if/elif chains hurt readability; a rule table (list[dict]) or strategy pattern is cleaner.
  • Comparing floats with == is fragile; use a tolerance (abs(a - b) < 1e-9).
  • Truthiness on None vs. 0 vs. "" all collapse to False — be explicit when the difference matters.

🚀 Performance & Best Practices

  • Order if/elif with the most likely branch first; everything below it then short-circuits.
  • Use dict dispatch: DISCOUNT.get(inv.customer_type, 0).
  • For mutually exclusive cases the match statement is more readable in Python 3.10+.

🧪 Try It Yourself

  1. Refactor discount() to look up rules from a list of (predicate, pct, reason) tuples.
  2. Combine vip + EU rules into a stacking rule and re-run.
  3. Add an audit-log line for every rule that *almost* matched.

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