Python for Data Science · Lesson 4 of 29

Control Flow: if, elif, else, Nested Conditions & match

Make programs decide. Learn indentation, if/elif/else, comparison and logical conditions, truthiness, conditional expressions (ternary), nested decisions, the match statement, and how Python's if maps to Excel's IF/IFS and SQL's CASE WHEN, with data-flavoured examples.

● Python class 3✦ Python 3.13Beginner⏱ 90 min

⬇ Download PDF

By the end of this lesson you can

Write correct if / elif / else blocks and explain why indentation matters
Combine conditions with and, or, not, in, chained comparisons and truthiness
Use conditional expressions and the match statement
Order conditions correctly and avoid common branching bugs
Translate a spreadsheet or SQL rule into Python

Datasets used

employees_small.csv

Decisions and indentation

A program normally runs line by line. Control flow lets it choose between paths. In Python, a colon : starts a block and indentation (4 spaces by convention) says which lines belong to it.

Python

marks = 82
if marks >= 40:
    print("Pass")
    print("Well done!")
print("This line always runs")

Output

Pass
Well done!
This line always runs

Python

if True:
print("not indented")        # IndentationError: expected an indented block after 'if' statement

Common mistakesIndentation rules

Use 4 spaces (never mix tabs and spaces). Everything in the same block has the same indentation. A missing colon or wrong indentation causes SyntaxError / IndentationError.

if / elif / else

Python

marks = 82
if marks >= 90:
    grade = "A+"
elif marks >= 75:
    grade = "A"
elif marks >= 60:
    grade = "B"
elif marks >= 40:
    grade = "C"
else:
    grade = "Fail"
print(marks, "->", grade)

Output

82 -> A

Python checks the conditions top to bottom and runs the first one that is true, then skips the rest. This is Excel's nested IF / IFS and SQL's CASE WHEN (Excel Lesson 5, SQL Lesson 7).

Common mistakesOrder matters

If you test marks >= 40 first, an 82 gets "C" because the first true branch wins. Put the most specific / strictest condition first.

Python

for m in (95, 82, 61, 45, 12):
    g = "A+" if m >= 90 else "A" if m >= 75 else "B" if m >= 60 else "C" if m >= 40 else "Fail"    # chained conditional expression
    print(m, g)

Output

95 A+
82 A
61 B
45 C
12 Fail

Conditional expression (ternary)

value_if_true if condition else value_if_false is Python's one-line IF():

Python

salary = 97000
band = "High" if salary >= 100000 else "Standard"
bonus = salary * 0.1 if band == "High" else 0
print(band, bonus)

Output

Standard 0

Conditions

Comparison and logical operators

Python

age, city, income = 29, "Pune", 85000
print(age >= 18 and city == "Pune")                    # both must be true
print(city in ("Pune", "Mumbai") or income > 100000)   # at least one
print(not (age < 18))
print(18 <= age < 60)                                   # chained comparison
if 18 <= age < 60 and income >= 50000 and city in ("Pune", "Mumbai", "Delhi"):
    print("Eligible for the metro premium plan")

Output

True
True
True
True
Eligible for the metro premium plan

Truthiness

Python treats "empty" and zero-like values as False (Lesson 2), so you can test them directly:

Python

name = ""
items = [3, 5]
note = None
if not name: print("name is empty")
if items: print("items has", len(items), "elements")
if note is None: print("note is missing")                 # use `is None`, not `== None`
if 0 < len(items) <= 5: print("small list")

Output

name is empty
items has 2 elements
note is missing
small list

Membership and string tests

Python

status = "Shipped"
if status in ("Shipped", "Delivered"): print("order has left the warehouse")
email = "[email protected]"
if "@" in email and email.endswith(".in"): print("Indian email address")

Output

order has left the warehouse
Indian email address

Nested conditions

Python

is_member, cart_value = True, 3200
if cart_value > 0:
    if is_member:
        discount = 0.1 if cart_value >= 3000 else 0.05
    else:
        discount = 0.02 if cart_value >= 3000 else 0
    print(f"Discount: {discount:.0%}")
else:
    print("Cart is empty")

Output

Discount: 10%

Prefer flat conditions with and to deeply nested ones when they read clearly:

Python

def shipping_fee(value, is_member):
    if value <= 0: return 0
    if is_member and value >= 500: return 0
    if value >= 1500: return 0
    return 49
print([shipping_fee(v, m) for v, m in [(0, True), (600, True), (600, False), (1600, False)]])

Output

[0, 0, 49, 0]

Returning early (guard clauses) keeps code flat and readable.

match: structural pattern matching (3.10+)

match compares one value with several patterns: a clean alternative to a long chain of == tests.

Python

def describe_status(code):
    match code:
        case 200 | 201:
            return "success"
        case 301 | 302:
            return "redirect"
        case 404:
            return "not found"
        case int(c) if 500 <= c < 600:
            return "server error"
        case _:
            return "unknown"
print([describe_status(c) for c in (200, 404, 503, 999)])

Output

['success', 'not found', 'server error', 'unknown']

Python

def handle(command):
    match command.split():
        case ["load", filename]:  return f"loading {filename}"
        case ["save", filename]:  return f"saving {filename}"
        case ["quit"]:            return "bye"
        case _:                   return "unknown command"
print(handle("load sales.csv"), "|", handle("quit"), "|", handle("fly away"))

Output

loading sales.csv | bye | unknown command

Data examples

Rule-based bonus (the same as an Excel IF + AND)

Python

import pandas as pd
emp = pd.read_csv("employees_small.csv")
def bonus(salary, rating, dept):
    if rating >= 5: rate = 0.15
    elif rating >= 4: rate = 0.10
    elif rating >= 3: rate = 0.05
    else: rate = 0
    if dept == "Sales": rate += 0.02             # extra for Sales
    return round(salary * rate)
emp["bonus"] = [bonus(s, r, d) for s, r, d in zip(emp.salary, emp.rating, emp.department)]
emp[["name", "department", "salary", "rating", "bonus"]].head(8)

Output

	name	department	salary	rating	bonus
0	Kavya Iyer	HR	97000	4	9700
1	Imran Reddy	Finance	157000	3	7850
2	Harpreet Kulkarni	Finance	131000	4	13100
3	Diya Iyer	HR	90000	4	9000
4	Simran Iyer	HR	139000	4	13900
5	Saanvi Rao	HR	83000	4	8300
6	Isha Mehta	Sales	57000	3	3990
7	Aarav Kulkarni	HR	101000	2	0

8 rows × 5 columns

(In pandas you would use np.where, np.select or .apply: Lesson 19. Writing the rule once as a function makes it testable.)

A small decision tree: loan pre-check

Python

def loan_precheck(age, income, existing_emi, credit_score):
    reasons = []
    if age < 21 or age > 60: reasons.append("age outside 21–60")
    if income < 25000: reasons.append("income below ₹25,000")
    if existing_emi > 0.5 * income: reasons.append("EMIs exceed 50% of income")
    if credit_score < 650: reasons.append("credit score below 650")
    return ("Eligible", []) if not reasons else ("Review", reasons)
print(loan_precheck(34, 70000, 20000, 730))
print(loan_precheck(19, 22000, 15000, 610))

Output

('Eligible', [])
('Review', ['age outside 21–60', 'income below ₹25,000', 'EMIs exceed 50% of income', 'credit score below 650'])

Collecting all failing reasons is more useful than stopping at the first, and shows how a series of independent if statements (not elif) is sometimes right.

`if`/`elif` vs several `if`s

Pattern	Use when
`if … elif … else`	Choices are mutually exclusive: only one branch should run
Several separate `if`	Each condition is independent: more than one may apply (collecting reasons, applying several rules)

Common mistakes

Mistake	Fix
`if x = 5:` (assignment)	`if x == 5:`
`if x == 3 or 4:` (always true: `4` is truthy)	`if x in (3, 4):`
`if x == None:`	`if x is None:`
Comparing floats with `==`	`math.isclose`
Forgetting `elif` ordering	strictest first
`if len(items) > 0:` (verbose)	`if items:`
Deep nesting	guard clauses / `and` / helper functions

Do it with AIDo it with AI

Take an Excel rule from your own work, such as a nested IF for commission, and ask an assistant to convert it to Python with test cases. Then write five edge-case tests yourself (boundaries like exactly 50,000, blanks, negative values) and see if the code survives.

Summary

Key takeaways

A colon starts a block; indentation (4 spaces) defines it. if / elif / else runs the first true branch.
Combine conditions with and, or, not, in, chained comparisons; empty/zero/None are falsy; test None with is None.
Use the ternary a if cond else b for simple choices and the match statement for many exact cases and patterns.
Order conditions from strictest to loosest; prefer flat code with guard clauses; use separate ifs when several rules can apply.
Wrap rules in functions so they can be tested and reused.

Quick quiz6 questions · tap an answer to check it

1. What does x if cond else y do?

Answer: B. It is Python's conditional (ternary) expression.

2. Which condition is correct for "x is 3 or 4"?

Answer: B. x == 3 or 4 is always truthy because 4 is truthy.

3. With marks = 82, which grade does if marks>=40: "C" elif marks>=75: "A" give?

Answer: A. The first true branch wins, so the order is wrong.

4. Which is the best way to test for None?

Answer: B. is None tests identity and avoids surprises with falsy values like 0.

5. What is a guard clause?

Answer: B. Guard clauses keep code flat and readable.

6. What does match do?

Answer: B. Structural pattern matching arrived in Python 3.10.

ExerciseRules in code

Write delivery_charge(order_value, city, express): free if order ≥ ₹1,500; otherwise ₹49 in metros (Delhi, Mumbai, Bengaluru, Chennai, Kolkata, Hyderabad) and ₹79 elsewhere; express adds ₹100 unless the value is ≥ ₹5,000. Test five cases including boundaries.

Show solution

Python

METROS = {"Delhi", "Mumbai", "Bengaluru", "Chennai", "Kolkata", "Hyderabad"}
def delivery_charge(value, city, express=False):
    fee = 0 if value >= 1500 else (49 if city in METROS else 79)
    if express and value < 5000: fee += 100
    return fee
tests = [(1500, "Pune", False), (1499, "Delhi", False), (1499, "Pune", True), (5000, "Pune", True), (100, "Mumbai", True)]
print([delivery_charge(*t) for t in tests])      # [0, 49, 179, 0, 149]

Interview cornerQuestions you may be asked

Q: What is the difference between elif and multiple if statements? elif branches are mutually exclusive (only the first true runs); separate ifs are all evaluated, so several can run.

Q: How do you write a switch-case in Python? Use match/case (3.10+), or a dictionary mapping values to functions or results.

Q: What are falsy values in Python? False, None, 0, 0.0, "", [], {}, (), set(), and objects whose __bool__ returns False or __len__ returns 0.

Q: How do you replace nested if/else cleanly? Guard clauses, combined conditions, lookup tables (dicts), or polymorphism.

Control Flow: if, elif, else, Nested Conditions & match

Decisions and indentation

if / elif / else

Conditional expression (ternary)

Conditions

Comparison and logical operators

Truthiness

Membership and string tests

Nested conditions

match: structural pattern matching (3.10+)

Data examples

Rule-based bonus (the same as an Excel IF + AND)

A small decision tree: loan pre-check

if/elif vs several ifs

Common mistakes

Summary

Practise more online

`if`/`elif` vs several `if`s