What is a Variable?
A variable is a named container that stores a value.
Think of it like a labeled box:
- The label is the variable name
- The contents are the value
age = 20
name = "Alex"
Creating Variables
Use = to assign a value to a variable.
# Storing a number
credits = 15
# Storing text (a "string")
major = "Biology"
# Storing a decimal
gpa = 3.75
The variable name goes on the left, the value on the right.
Naming Rules
Variable names must:
- Start with a letter or underscore
- Contain only letters, numbers, underscores
- Not be a Python keyword (like
if,for,print)
# Good names
student_name = "Jordan"
age2 = 21
_private = "secret"
# Bad names (these will error!)
2nd_place = "silver" # Can't start with number
my-name = "Alex" # No hyphens allowed
for = 5 # 'for' is reserved
Naming Conventions
By convention, Python uses snake_case:
# Good (snake_case)
first_name = "Alex"
total_credits = 45
# Works but not preferred
firstName = "Alex" # camelCase
FIRSTNAME = "Alex" # SCREAMING_CASE (used for constants)
Choose descriptive names:
# Good
student_count = 25
average_score = 87.5
# Avoid
x = 25
temp = 87.5
Data Types
Python has several built-in types:
| Type | Description | Example |
|---|---|---|
int | Whole numbers | 42, -7, 0 |
float | Decimal numbers | 3.14, -0.5 |
str | Text (strings) | "Hello", 'World' |
bool | True/False | True, False |
Integers (int)
Whole numbers, positive or negative:
age = 20
temperature = -5
population = 1000000
You can do math with integers:
sum = 5 + 3 # 8
diff = 10 - 4 # 6
product = 6 * 7 # 42
quotient = 15 // 3 # 5 (integer division)
Floats (Decimal Numbers)
Numbers with decimal points:
price = 19.99
pi = 3.14159
temperature = 98.6
Division always gives a float:
result = 7 / 2 # 3.5
result = 6 / 2 # 3.0 (still a float!)
Strings (Text)
Text enclosed in quotes (single or double):
name = "Alice"
greeting = 'Hello, World!'
message = "It's a beautiful day"
Combine strings with +:
first = "Hello"
second = "World"
combined = first + " " + second # "Hello World"
Booleans (True/False)
Only two possible values:
is_student = True
has_graduated = False
Result from comparisons:
5 > 3 # True
10 == 10 # True
"a" == "b" # False
We'll use these a lot in the next lecture!
Checking Types
Use type() to see a value's type:
print(type(42)) # <class 'int'>
print(type(3.14)) # <class 'float'>
print(type("hello")) # <class 'str'>
print(type(True)) # <class 'bool'>
Type Conversion
Convert between types:
# String to integer
age_str = "25"
age_int = int(age_str) # 25
# Integer to string
count = 42
count_str = str(count) # "42"
# String to float
price_str = "19.99"
price = float(price_str) # 19.99
Note: This is important for input()—it always returns a string.
The print() Function
Display output to the screen:
print("Hello!")
print(42)
print(3.14)
Print multiple values:
name = "Alex"
age = 20
print("Name:", name)
print(name, "is", age, "years old")
The input() Function
Get input from the user:
name = input("What is your name? ")
print("Hello,", name)
Note: input() always returns a string.
age = input("How old are you? ")
# age is "25" (string), not 25 (int)
# Convert if you need a number:
age = int(input("How old are you? "))
Putting It Together
# Get user information
name = input("Enter your name: ")
birth_year = int(input("Enter your birth year: "))
# Calculate age
current_year = 2025
age = current_year - birth_year
# Display result
print("Hello,", name)
print("You are approximately", age, "years old")
Key Takeaways
- Variables store values with meaningful names
- Types include
int,float,str,bool - Use
snake_casefor variable names input()returns strings—convert withint()orfloat()print()displays output
Lab Preview
In Lab 1, you'll:
- Create your first Python program
- Practice with variables and types
- Use
input()andprint() - Do basic calculations