Python · Beginner

Conditions: if, elif and else in Python

In short: An if statement runs its indented block only when its condition is true; elif adds further conditions tested in order, and else runs when none matched. Python runs the first block whose condition holds and skips the rest of the chain, so branch order matters.

Choosing a path

Real programs need to react: charge one price below a weight limit and another above it, reject an empty name, stop when stock is gone. The if statement makes a block conditional: Python evaluates the expression after if, runs the indented block when it is true, and otherwise skips it and continues after it.

else attaches an alternative block that runs only when the if condition was false. elif ("else if") tests a second condition when the first failed, then a third, and so on. The whole chain is one statement: Python tests the conditions from the top, runs the block of the first true one, and ignores every later branch even if its condition would also have held. That is why ordering matters, and why a chain differs from a series of independent if statements, each of which is tested regardless of the others.

The condition does not have to be a comparison. Python accepts any value and asks whether it is truthy. Numbers are truthy unless they are zero; strings, lists and other collections are truthy unless they are empty; None is falsy. So if items: reads as "if there are any items", and if not name: as "if the name is missing or empty". Comparisons, and, or and not from the operators lesson build richer conditions, and in checks membership: if choice in ("y", "yes"):.

Blocks can nest: an if inside another if's block is indented one level further. Two levels are fine; at four it is usually clearer to combine conditions with and or move part of the logic into a function.

Syntax

if condition:
    block            # runs when condition is truthy
elif other_condition:
    block            # tested only if the first was false
else:
    block            # runs when nothing above matched

label = "even" if n % 2 == 0 else "odd"   # conditional expression

Any number of elif branches may follow an if; else is optional and must come last. The one-line conditional expression picks one of two values and belongs inside larger expressions; it does not run blocks.

A price with three tiers

Parcel postage depends on weight. The program is run with the input 2.4.

weight = float(input())
if weight <= 1:
    cost = 4
elif weight <= 5:
    cost = 7
else:
    cost = 12
print("Weight:", weight, "kg")
print("Cost:", cost)

Weight: 2.4 kg
Cost: 7

2.4 fails the first test and passes the second, so cost becomes 7 and the else branch is skipped. Because the chain stops at the first true condition, the second branch need not say 1 < weight <= 5; passing the first test already rules out the light parcels. Swap the two tests and every parcel up to 5 kg would cost 7, because weight <= 5 would catch the light ones first.

Truthiness in conditions

None of these conditions contains a comparison operator.

unread = []
name = ""
retries = 3
note = None
if unread:
    print("You have messages")
else:
    print("No new messages")
if not name:
    print("Name is required")
if retries:
    print("Retries left:", retries)
if note is None:
    print("No note attached")

No new messages
Name is required
Retries left: 3
No note attached

The empty list and the empty string count as false, the non-zero integer as true. is None tests for None specifically, which matters when 0 or an empty string would be a legitimate value that must not be treated as "nothing".

Combining and nesting conditions

A library loan check. The program is run with the input lines member and 3.

status = input()
books_out = int(input())
if status == "member" and books_out < 5:
    print("Loan approved")
    if books_out == 4:
        print("This is your last slot")
elif status == "member":
    print("Limit reached: return a book first")
else:
    print("Membership required")
kind = "heavy reader" if books_out >= 3 else "casual reader"
print(kind)

Loan approved
heavy reader

The first branch checks two things at once with and. The nested if runs only inside that branch, and here it is false (3 is not 4), so nothing extra prints. The elif does not repeat the books_out check: reaching it already means the member has five or more books out. The last line is a conditional expression that evaluates to one of two strings.

Common mistakes

• Ordering the branches from widest to narrowest

  Why it goes wrong: With if x <= 100: before elif x <= 10:, the second branch can never run: every value that satisfies it also satisfied the first. Python does not warn about unreachable branches.

  Fix: Order the tests so that each is narrower than the ones below it, or make each condition self-contained with both bounds.

   Python · fixCopy

  if score >= 90:
      grade = "A"
  elif score >= 75:
      grade = "B"
  else:
      grade = "C"

• Comparing to True or False explicitly

  Why it goes wrong: if done == True: works but is redundant, and if flag == "True": compares to a string and never matches a bool. The condition already is a truth value.

  Fix: Write if done: and if not done:.

• Using = inside a condition

  Why it goes wrong: if count = 0: is a SyntaxError because = assigns. Python refuses it precisely to catch this slip.

  Fix: Compare with ==. To assign and test in one go, the walrus operator := (Python 3.8 and later) exists: if (n := len(items)) > 10:.

• Independent ifs where a chain was meant

  Why it goes wrong: Two separate if statements both run their tests, so one value can trigger both blocks. With if x > 0: followed by if x > 10:, the number 15 prints from both.

  Fix: Use elif when the branches are alternatives and exactly one should run.

What counts as false

Where you use this

Validating input is the first place conditions earn their keep. A sign-up form should refuse an empty username, reject an age outside a sensible range, and only then create the account. Written as a chain, each check reports one clear problem and stops, so the user sees the first thing to fix. The same shape appears in pricing rules, in access control (if role == "admin" or owner == user:), and inside loops that decide whether to skip an item or stop, which the loops lesson covers next.

name = input().strip()
age = int(input())
if not name:
    print("Name is required")
elif age < 13:
    print("Too young to register")
else:
    print("Account created for", name)

Key points

• if runs a block when its condition is truthy; else covers the other case.
• An if/elif/else chain runs exactly one block: the first whose condition is true.
• Order branches from most specific to least specific.
• Empty collections, empty strings, 0 and None are false; nearly everything else is true.
• Test for None with is None, not == None.
• x if cond else y is an expression that picks a value; it does not replace blocks.

Try it yourself

The program labels a temperature as Cold or Hot. Add an elif so that temperatures from 10 up to and including 24 print Mild, with Cold below 10 and Hot from 25 upward.

temp = int(input())
if temp < 10:
    print("Cold")
else:
    print("Hot")

temp = int(input())
if temp < 10:
    print("Cold")
elif temp <= 24:
    print("Mild")
else:
    print("Hot")

Practise this

• Choose a delivery windowEasy

  Classify a parcel into a delivery window with clear Python if, elif and else conditions.

  ConditionsNot started

Open the Python playground

Related lessons

• Operators in PythonPython's arithmetic, comparison, logical, assignment and membership operators, with the precedence rules and the difference between /, // and %.8 minCompleted
• Loops in Python: for and whileHow a for loop walks through a sequence and a while loop repeats until a condition fails, with range(), enumerate(), break and continue explained by example.9 minCompleted
• Data Types in PythonThe core Python data types int, float, str, bool and None: how to check a type with type() and convert between types with int(), float() and str().8 minCompleted

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