What is a Variable?¶

Try This First: Before reading, open Python and type x = 5 then print(x). Now type x = "hello" then print(x). Notice how x changed? That is what a variable does.

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A variable is a name that holds a value. You create it by writing a name, then =, then the value.

name = "Alice"
age = 30
is_student = True

Embedding variables in strings (f-strings)¶

You can put variables directly inside a string by placing an f before the opening quote, then wrapping any variable name in curly braces {} where you want its value to appear. This is the most common and readable way to format strings in modern Python.

name = "Alice"
age = 30
greeting = f"Hello, {name}! You are {age} years old."
print(greeting)  # Output: Hello, Alice! You are 30 years old.

The f tells Python: "treat anything in curly braces as a variable, not plain text." Without the f, {name} is just printed as the literal characters {name}.

print(f"Hello, {name}")   # Output: Hello, Alice   ✅
print("Hello, {name}")    # Output: Hello, {name}  ❌ (missing f)

Visualize It¶

See how Python stores variables in memory step by step: Open in Python Tutor

Think of it like a labeled jar. The label is the name. The contents are the value. You can: - Look at the contents: print(name) shows Alice - Replace the contents: name = "Alice" — now it holds "Alice" - Use the contents in calculations: next_year = age + 1

Rules for naming variables¶

Must start with a letter or underscore (not a number)
Can contain letters, numbers, and underscores
Case-sensitive: Name and name are different variables
Convention: use lowercase_with_underscores (called "snake_case")

Good names vs bad names¶

# Good — describes what the value represents
student_count = 42
max_temperature = 98.6
is_active = True

# Bad — vague or misleading
x = 42
temp = 98.6  # temp... temperature? temporary?
flag = True  # flag for what?

Common mistakes¶

Using = instead of ==:

x = 5       # This STORES 5 in x
x == 5      # This CHECKS if x equals 5 (returns True or False)

Forgetting quotes for text:

name = Alice    # Error! Python thinks Alice is a variable name
name = "Alice"  # Correct — quotes make it text (a string)

Using a variable before creating it:

print(score)     # Error! score does not exist yet
score = 100      # This line creates it
print(score)     # Now it works

Practice¶

Quick check: Take the quiz

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