Variables and Assignment

1. What is a Variable?

In everyday life, you use labels all the time. A contact in your phone labeled "Mom" holds a phone number. A folder on your desktop labeled "Homework" holds files. Variables work the same way in programming -- they give names to data so you can find and use it later.

In Python, you create a variable using the assignment operator (=):

age = 25
name = "Alice"
temperature = 98.6

When Python sees age = 25, it creates a space in memory, stores the value 25 there, and labels it age. From that point on, whenever you use age in your code, Python knows you mean 25.

age = 25
print(age)

2. Creating Variables with Assignment

The assignment operator = works from right to left. Python first evaluates the value on the right side, then stores it in the variable on the left side.

# Python evaluates the right side first, then assigns to the left
x = 10          # Store 10 in x
y = 3 + 7       # Evaluate 3 + 7 = 10, store in y
z = x + y       # Evaluate x + y = 20, store in z

print(x)
print(y)
print(z)

You can also create multiple variables at once:

# Multiple assignments on separate lines
first_name = "Alice"
last_name = "Smith"
age = 25

# Multiple assignments on one line (less common, but valid)
a, b, c = 1, 2, 3
print(a, b, c)

3. Variable Naming Rules

Python has strict rules about what you can name your variables. If you break these rules, Python will give you an error.

# VALID variable names
age = 25
first_name = "Alice"
_private = 42
item2 = "banana"
MAX_SIZE = 100

# INVALID variable names (these cause errors!)
# 2nd_place = "Silver"    # Cannot start with a number
# first name = "Alice"    # Cannot contain spaces
# my-var = 10             # Cannot contain hyphens
# class = "Math"          # 'class' is a reserved keyword

Python has about 35 reserved keywords that you cannot use as variable names. Some common ones include: if, else, for, while, True, False, None, and, or, not, class, def, return, import.

Name = "Alice"
name = "Bob"
NAME = "Charlie"

print(Name)
print(name)
print(NAME)

4. Naming Conventions (Best Practices)

While Python's rules tell you what you can do, conventions tell you what you should do. Following conventions makes your code easier to read.

# Good: snake_case (Python convention)
student_name = "Alice"
total_price = 29.99
is_enrolled = True

# Avoid: other styles (valid but not Pythonic)
studentName = "Alice"      # camelCase - used in JavaScript
StudentName = "Alice"      # PascalCase - used for class names
STUDENT_NAME = "Alice"     # ALL_CAPS - used for constants

5. Reassigning Variables

One of the most powerful features of variables is that you can change their values at any time. When you assign a new value to an existing variable, the old value is replaced.

score = 0
print("Starting score:", score)

score = 10
print("After earning points:", score)

score = score + 5
print("After bonus:", score)

The line score = score + 5 might look strange mathematically, but remember that = means assignment. Python evaluates the right side first (score + 5 = 10 + 5 = 15), then stores that result back in score.

Python also lets you change the type of data a variable holds:

my_var = 42           # my_var holds an integer
print(my_var)

my_var = "hello"      # now my_var holds a string
print(my_var)

6. Choosing Meaningful Variable Names

Good variable names make your code read almost like English. Compare these two versions of the same program:

x = 40
y = 15.50
z = x * y
print(z)

hours_worked = 40
hourly_rate = 15.50
weekly_pay = hours_worked * hourly_rate
print(weekly_pay)

Both programs produce the same result, but the second one tells a story. You can immediately understand what the program calculates without any comments.

Guidelines for choosing good names:

• Use descriptive names: student_count instead of sc
• Avoid single letters except for simple loop counters
• Use nouns for data: temperature, name, total
• Use verb phrases for booleans: is_valid, has_permission
• Keep names reasonably short but clear