Variables & Data Types

name = "Sarah"
age = 28
print(f"My name is {name} and I am {age} years old.")
# Output: My name is Sarah and I am 28 years old.

This is how professional Python developers write string output. It is cleaner, faster, and less error-prone than + concatenation.

Numbers — integers and floats

# Integers — whole numbers
population = 8000000000
negative = -42
zero = 0

# Floats — decimal numbers
pi = 3.14159
price = 19.99
temperature = -40.0

# Arithmetic
print(10 + 3)      # 13 (addition)
print(10 - 3)      # 7 (subtraction)
print(10 * 3)      # 30 (multiplication)
print(10 / 3)      # 3.3333... (division — always returns float)
print(10 // 3)     # 3 (floor division — rounds down)
print(10 % 3)      # 1 (modulo — remainder)
print(10 ** 3)     # 1000 (exponentiation — 10 to the power of 3)

Booleans — true or false

Booleans are the simplest type — only two possible values: True or False. They are named after George Boole, the mathematician who invented Boolean algebra.

is_raining = True
has_umbrella = False
should_go_outside = not is_raining  # False

# Comparison operators return booleans
print(5 > 3)       # True
print(5 == 3)      # False
print(5 != 3)      # True (not equal)
print(5 >= 5)      # True (greater than or equal)

<classifychallenge xp="25" title="Name That Type" items={[""Hello, World!"","42","3.14","True",""99"","0","-17.5","False"]} options={["str","int","float","bool"]} hint="Anything in quotes is a string — even "99" is text, not a number. Whole numbers without quotes are int. Numbers with a decimal point are float. True and False (capitalized, no quotes) are bool.">

There Are No Dumb Questions

"Why does Python have both int and float? Why not just one number type?"

Integers are exact. 10 is always exactly 10. Floats are approximations — 0.1 + 0.2 in Python equals 0.30000000000000004, not 0.3. This is not a Python bug; it is how all computers store decimal numbers (IEEE 754 floating-point). For counting things (users, items, loops), use integers. For measuring things (temperature, price, distance), use floats. For money, use integers representing cents (1999 instead of 19.99) to avoid rounding errors.

"What happens if I put a number in quotes?"

It becomes a string, not a number. 42 is an integer you can do math with. "42" is text — two characters, a 4 and a 2. Python treats them completely differently. 42 + 8 gives 50. "42" + "8" gives "428" (concatenation, not addition). This is the single most common beginner bug.

Type conversion — changing between types

Sometimes you need to convert data from one type to another. Python gives you built-in functions for this:

# String to integer
age_text = "25"
age_number = int(age_text)     # 25 (now you can do math)

# Integer to string
count = 42
count_text = str(count)        # "42" (now you can concatenate)

# String to float
price_text = "19.99"
price = float(price_text)      # 19.99

# Float to integer (drops decimal, does NOT round)
temperature = 98.6
temp_int = int(temperature)    # 98 (not 99!)

# Boolean conversions
print(bool(0))       # False (zero is falsy)
print(bool(1))       # True (any non-zero number is truthy)
print(bool(""))      # False (empty string is falsy)
print(bool("hello")) # True (non-empty string is truthy)

There Are No Dumb Questions

"What happens if I try to convert 'hello' to an integer?"

Python raises a ValueError and crashes. int("hello") makes no sense — there is no number in "hello." You can only convert strings that actually contain valid numbers: int("42") works, int("42.5") crashes (use float() first), int("forty-two") crashes. Always validate user input before converting.

"Why do I need to know about types? Can't Python just figure it out?"

Python does figure out the type automatically when you create a variable. But it does NOT automatically convert between types. "5" + 3 crashes because Python refuses to guess whether you want "53" (string concatenation) or 8 (addition). Other languages (like JavaScript) guess — and that guessing causes silent, hard-to-find bugs. Python's strictness is a feature, not a bug.

Back to the hospital spreadsheet

Those 16,000 lost COVID test results happened because the team did not understand data types. The .xls format had a hard limit on integers in a column — and nobody checked. In Python, you would never hit that wall. A Python list can hold millions of rows. A Python integer can be arbitrarily large. And type() lets you check what you are working with at any point.

Data types are not abstract theory. They are the difference between a working system and a system that silently loses 16,000 lives.

Next up: You now know how to store data. But a program that just stores data is a spreadsheet. In the next module, you will learn control flow — how to make your program think. If the score is above 90, print "A." Loop through every item in a list. Keep asking until the user gets the password right. That is where programming gets powerful.

Key takeaways

• Variables are labeled boxes — a name attached to a value using = (assignment, not equality)
• Four core types: str (text), int (whole numbers), float (decimals), bool (True/False)
• f-strings are the modern way to embed variables in text: f"Hello, {name}!"
• input() always returns a string — convert with int() or float() before doing math
• int() truncates, it does not round — int(3.9) is 3, not 4
• "42" is not the same as 42 — one is text, the other is a number; Python will not guess which you want
• Use snake_case for variable names: first_name, not firstName

Try it yourself

Experiment with variables and data types in this live Python playground:

Now prove you can write it yourself. Complete both test cases: