Conditionals

What are conditionals?

Conditionals are statements that let your program make decisions based on values and conditions. Python uses if, elif, and else to control which code runs, allowing your program to respond differently to different situations.

Why this matters

Conditionals are what make programs intelligent and interactive. Without them, your code would always execute the same way every time. With conditionals, you can check user input, validate data, handle different cases, and create programs that adapt to their environment. Almost every useful program uses conditionals, whether it's checking if a user is logged in, validating input, choosing which calculation to perform, or determining what message to display. Mastering conditionals is essential for writing programs that do more than just execute a fixed sequence of steps.

Basic syntax

The basic form of a conditional statement looks like this:

if <condition>:
    <code to run>

The condition is any expression that evaluates to True or False (or a truthy or falsy value). If the condition is truthy, Python executes the indented block. If it's falsy, Python skips the block and moves on to the next statement.

important

Indentation matters! The code that runs when the condition is true must be indented (usually 4 spaces). Python uses indentation to know which code belongs to the if statement. If you don't indent properly, you'll get an error.

If statements

The simplest conditional runs code only when a condition is truthy:

age = 18

if age >= 18:
    print("You are an adult")

This checks if age is greater than or equal to 18. If it is, Python prints the message. Notice the indentation! The print() statement is indented to show it belongs to the if statement.

If the condition is falsy, nothing happens—Python skips the indented block entirely:

age = 15

if age >= 18:
    print("You are an adult")  # This won't print because age is 15

print("This always prints")  # This runs regardless of the if statement

if / else

Add else to provide an alternative when the condition is falsy:

age = 15

if age >= 18:
    print("You are an adult")
else:
    print("You are a minor")

if / elif / else

Use elif (short for "else if") to check multiple conditions in order:

score = 85

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

How it works:

1. Python checks the first if condition (score >= 90). If it's true, it runs that block and stops.
2. If the first condition is false, Python checks the first elif (score >= 80). If it's true, it runs that block and stops.
3. This continues for each elif.
4. If none of the conditions are true, Python runs the else block (if there is one).

In the example above, score is 85, so:

• score >= 90 is false (85 is not >= 90)
• score >= 80 is true (85 is >= 80)
• Python prints "Grade: B" and stops. It skips over the remaining conditions.

Important: Only one block runs, even if multiple conditions could be true. Python always uses the first matching condition.

Truthiness in conditionals

Python evaluates conditions based on truthiness (which you learned about in the truthiness guide). This means you can use any value in a condition, not just True or False.

These values are falsy (treated as False):

• False
• None
• 0 (or 0.0)
• Empty collections: "", [], {}, set(), ()

Everything else is truthy (treated as True).

name = "Alice"

if name:  # Non-empty string is truthy, so this runs
    print(f"Hello, {name}")

items = []

if items:  # Empty list is falsy, so this doesn't run
    print("List has items")
else:
    print("List is empty")  # This runs instead

This is why you can write if name: instead of if name != "":. This makes your code shorter and more Pythonic!

Comparison operators

Use comparison operators to build conditions that compare values:

Operator	Meaning	Example	Result
==	Equal to	5 == 5	True
!=	Not equal to	5 != 3	True
>	Greater than	5 > 3	True
<	Less than	5 < 10	True
>=	Greater than or equal to	5 >= 5	True
<=	Less than or equal to	5 <= 5	True

x = 5

if x == 5:   # Is x equal to 5? Yes!
    print("x is 5")

if x != 3:   # Is x not equal to 3? Yes!
    print("x is not 3")

if x > 3:    # Is x greater than 3? Yes!
    print("x is greater than 3")

These operators return True or False, which makes them perfect for use in if statements.

Logical operators

Combine conditions with and, or, and not:

age = 25
has_license = True

if age >= 18 and has_license:
    print("You can drive")

if age < 18 or not has_license:
    print("You cannot drive")

and

Both conditions must be truthy for the whole expression to be truthy:

x = 5

if x > 0 and x < 10:
    print("x is between 0 and 10")

This only prints if both x > 0 is true and x < 10 is true. If either condition is false, nothing prints.

or

At least one condition must be truthy for the whole expression to be truthy:

day = "Saturday"

if day == "Saturday" or day == "Sunday":
    print("It's the weekend")

This prints if the day is Saturday or Sunday (or both, though a day can't be both). If neither condition is true, nothing prints.

not

Inverts the truthiness. True converts to False and False to True:

is_raining = False

if not is_raining:
    print("Let's go outside")

Since is_raining is False, not is_raining is True, so the message prints. Think of not as asking "is this false?"

Nested conditionals

You can put if statements inside other if blocks. This is called "nesting":

age = 25
has_license = True

if age >= 18:
    if has_license:
        print("You can drive")
    else:
        print("You need a license")
else:
    print("You're too young to drive")

How it works:

1. First, Python checks if age >= 18. If true, it goes inside that block.
2. Inside that block, it checks if has_license is true.
3. If both are true, it prints "You can drive".
4. If age >= 18 is false, it skips to the else and prints "You're too young to drive".

Notice the double indentation—the inner if statement is indented twice because it's inside the outer if block.

tip

You can often simplify nested conditionals using and:

if age >= 18 and has_license:
    print("You can drive")
elif age >= 18:
    print("You need a license")
else:
    print("You're too young to drive")

This is usually easier to read than nesting!

Conditional expressions (ternary operator)

Python supports a compact one-line way to assign a value based on a condition. This is called a "ternary operator" or "conditional expression":

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

Syntax: value_if_true if condition else value_if_false

This is equivalent to the longer version:

if age >= 18:
    status = "adult"
else:
    status = "minor"

note

This is a more advanced feature. If you find it confusing, stick with the regular if/else statements for now. You can always learn this later once you're more comfortable with conditionals.

Common patterns

Checking membership

You can check if a value is in a collection using the in operator:

color = "red"

if color in ["red", "green", "blue"]:
    print("Primary color")

This checks if color is one of the values in the list. You'll learn more about lists and the in operator in the [data structures section](../data strcutures/lists).

Checking type (with isinstance)

You can check what type a value is using isinstance():

value = 42

if isinstance(value, int):
    print("It's an integer")
elif isinstance(value, str):
    print("It's a string")

isinstance(value, int) returns True if value is an integer, False otherwise. This is useful when you need to handle different types of data differently.

Multiple conditions

Sometimes you want to check more than one thing at a time and handle different combinations of conditions.

temperature = 75
is_sunny = True

if temperature > 70 and is_sunny:
    print("Perfect weather for a picnic")
elif temperature > 70:
    print("Warm but cloudy")
elif is_sunny:
    print("Sunny but cool")
else:
    print("Not ideal weather")

Best practices

Here are some tips for writing clear, readable conditionals:

1. Keep conditions simple: If a condition is complex, store it in a variable with a clear name:

   # Good: Clear and readable
   is_valid = age >= 18 and has_license
   
   if is_valid:
       print("You can drive")

2. Use elif for mutually exclusive conditions: If only one condition should be true, use elif instead of multiple if statements:

   # Good: Only one will run
   if score >= 90:
       print("A")
   elif score >= 80:
       print("B")
   
   # Avoid: Multiple could run
   if score >= 90:
       print("A")
   if score >= 80:  # This might also run!
       print("B")

3. Avoid deeply nested conditionals: If you find yourself nesting many if statements, consider using and to combine conditions:

   # Good: Flat structure
   if age >= 18 and has_license and not is_suspended:
       print("You can drive")
   
   # Avoid: Too nested (hard to read)
   if age >= 18:
       if has_license:
           if not is_suspended:
               print("You can drive")

4. Use truthiness when it makes sense: if name: is cleaner than if name != "", but use explicit comparisons when the distinction matters (like checking for None specifically):

    name = "Wolf"
   
   # Good: uses truthiness
   if name:
       print("Hello,", name)
   
   # Also good: explicit comparison to None
   if user is not None:
       print("User is set")