Conditional Logic

Overview

In the previous lesson, you learned how to manipulate strings and transform text data. So far, your programs have run the same code every time. In this lesson, you'll learn how to make your programs make decisions — executing different code depending on whether a condition is true or false.

Learning Objective

By the end of this lesson, you will have learned how to:

• Use if-else statements and logical operators (and, or, not).
• Control program flow based on boolean conditions.

Key terms

Conditional statement: A programming construct that executes different code blocks based on whether a condition evaluates to True or False.

if statement: Runs a block of code only when its condition is True.

elif: Short for "else if" — adds additional conditions to check when the preceding if is False.

else: A catch-all block that runs when none of the preceding conditions are True.

Boolean expression: An expression that evaluates to either True or False.

Logical operator: A keyword (and, or, not) used to combine or negate boolean expressions.

Comparison operator: A symbol used to compare two values, such as ==, !=, >, <, >=, <=.

Starter Code

This lesson encourages experimentation. Use the included Colaboratory notebook to run the code as you learn, or read the lesson and then freely experiment with the notebook's code.

Introduction

Every meaningful program needs to respond differently to different situations. A weather app shows an umbrella icon when it's raining but a sun when it's clear. A data cleaning script skips rows that are missing values but processes rows that are complete. This decision-making ability comes from conditional logic.

In Python, conditional logic is built around if, elif, and else statements. Combined with logical operators, these tools give you precise control over which code runs and when.

Comparison Operators

Before writing conditions, you need to know how to compare values. Python's comparison operators return True or False:

score = 85
print(score >= 90)   # Output: False
print(score > 60)    # Output: True
print(score == 100)  # Output: False

The if Statement

The simplest conditional runs a block of code only when its condition is True:

temperature = 38

if temperature > 37.5:
    print("You have a fever.")

The indented code below if only runs when the condition evaluates to True. If the condition is False, Python skips that block entirely.

if / else

Add an else block to specify what should happen when the condition is False:

score = 72

if score >= 60:
    print("You passed!")
else:
    print("You did not pass.")

# Output: You passed!

if / elif / else

Use elif (short for "else if") to check multiple conditions in sequence. Python evaluates each condition from top to bottom and executes the first one that is True:

score = 85

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
elif score >= 60:
    print("Grade: D")
else:
    print("Grade: F")

# Output: Grade: B

Once one condition matches, Python skips all the remaining conditions.

Logical Operators

Logical operators let you combine multiple conditions into one expression.

and — Both Conditions Must Be True

age = 22
has_id = True

if age >= 18 and has_id:
    print("Entry allowed.")
else:
    print("Entry denied.")

# Output: Entry allowed.

and returns True only when both conditions are True. If either is False, the result is False.

or — At Least One Condition Must Be True

is_member = False
has_coupon = True

if is_member or has_coupon:
    print("Discount applied!")
else:
    print("No discount available.")

# Output: Discount applied!

or returns True when at least one condition is True.

not — Reverses the Boolean Value

is_weekend = False

if not is_weekend:
    print("It's a weekday — time to study!")

# Output: It's a weekday — time to study!

not flips True to False and False to True.

Combining Logical Operators

You can combine all three operators in a single expression. Use parentheses to make precedence explicit:

temperature = 22
is_sunny = True
is_windy = False

if temperature > 20 and is_sunny and not is_windy:
    print("Perfect day for a picnic!")
else:
    print("Maybe stay inside.")

# Output: Perfect day for a picnic!

Checking Membership with in

The in operator checks whether a value exists in a collection (like a string or list):

valid_categories = ["sports", "science", "technology", "art"]
category = "science"

if category in valid_categories:
    print(f"'{category}' is a valid category.")
else:
    print(f"'{category}' is not recognized.")

# Output: 'science' is a valid category.

Putting It Together: A Data Science Example

Here's a practical example that classifies a dataset entry based on multiple conditions:

def classify_bmi(weight_kg, height_m):
    """Classify BMI according to standard categories."""
    bmi = weight_kg / (height_m ** 2)

    if bmi < 18.5:
        category = "Underweight"
    elif bmi < 25:
        category = "Normal weight"
    elif bmi < 30:
        category = "Overweight"
    else:
        category = "Obese"

    return bmi, category

bmi, label = classify_bmi(70, 1.75)
print(f"BMI: {bmi:.1f} — {label}")  # Output: BMI: 22.9 — Normal weight

Conclusion

In this lesson, you learned how to use if, elif, and else statements to make your programs respond to different situations. You also learned how to use comparison operators to evaluate conditions and logical operators (and, or, not) to combine them. Conditional logic is a cornerstone of programming — it will appear in every data processing pipeline, classification task, and validation step you write. In the next lesson, you'll learn about lists, Python's most versatile data structure for storing collections of values.

Practice

Coding Assessment

Practice the concepts from this lesson using this Colaboratory notebook. After completing the exercises, save your notebook to GitHub and submit the link for grading.

Knowledge Check

x = 15
if x > 20:
    print("Big")
elif x > 10:
    print("Medium")
else:
    print("Small")

items = ["apple", "banana", "cherry"]
if "grape" not in items:
    print("Grape is not in the list.")
else:
    print("Grape is in the list.")