Python Variables, Data Types and Operators

Programs work with data: names, prices, dates, product lists. To store that data and reuse it, we use variables. In this lesson you'll learn how to create them, which basic data types Python has, and how to use operators to calculate and compare. By the end you'll write a small end-of-day sales report for a taco truck in Texas.

What a variable is

A variable is a name that points to a value stored in memory. You create one with the = sign, which in programming means "assign," not "equals":

name = "Emma"
age = 29
price = 49.99
active = True

print(name)
print(age + 1)

Emma
30

Unlike languages such as Java or C#, Python doesn't make you declare the type; it figures it out from the value. A variable can also change its value while the program runs:

stock = 10
stock = stock - 3
print(stock)  # 7

Read stock = stock - 3 from right to left: "take the current value of stock, subtract 3, and store the result back in stock." Python has no true constants, but by convention values that shouldn't change are written in capital letters: SALES_TAX = 0.0825. Anyone reading your code knows not to touch them.

Rules for naming variables

• Names can contain letters, numbers and underscores, but can't start with a number (2nd_price is invalid; price_2 works).
• They're case-sensitive: age and Age are different variables.
• They can't be reserved words such as if, for, class or True.
• The Python convention is snake_case: lowercase words joined by underscores (total_price, due_date), not camelCase.
• Choose names that explain the content: total_sales beats ts or x. You'll read your code far more often than you write it.

Python's basic data types

You can check the type of any value with type():

print(type(29))       # <class 'int'>
print(type(49.99))    # <class 'float'>
print(type("29"))     # <class 'str'>

Watch the last one: "29" in quotes is text, not a number, and you can't add 1 to it. That's also why ZIP codes like "02134" should be stored as strings: as an int, the leading zero of a Boston ZIP code would disappear.

Converting between types

int("29")              # 29       (text → integer)
float("49.99")         # 49.99    (text → decimal)
str(29)                # '29'     (number → text)
int(9.99)              # 9        (chops off the decimals, doesn't round)
round(9.99)            # 10
round(243.56249, 2)    # 243.56

Converting is essential whenever data comes from a user, a web form or a file, because it almost always arrives as text. You'll see this in the next lesson with input().

When conversion fails: TypeError and ValueError

These two errors show up as soon as you work with real data. The first one happens when you mix text and numbers:

quantity = "12"          # arrived as text from a web form
total = quantity + 1

Traceback (most recent call last):
  File "order.py", line 2, in <module>
    total = quantity + 1
            ~~~~~~~~~^~~
TypeError: can only concatenate str (not "int") to str

Read it as: "I can only concatenate (join) text with text, not with an int." The fix is to convert first: int(quantity) + 1. The second error appears when the text doesn't look like a number. In the U.S. the usual culprits are dollar signs and thousands separators copied from a spreadsheet or a bank statement:

float("$49.99")     # ValueError: could not convert string to float: '$49.99'
float("1,250.50")   # ValueError: could not convert string to float: '1,250.50'
int("12.5")         # ValueError: invalid literal for int() with base 10: '12.5'

ValueError means "the type is right (it's text), but its content can't be used." Python only understands plain digits with a dot for decimals. In the next lesson you'll clean strings with .replace("$", "") and .replace(",", ""). To turn "12.5" into an integer, convert it to float first: int(float("12.5")) gives 12.

Arithmetic operators

The % operator is more useful than it looks: n % 2 == 0 tells you whether a number is even, and minutes % 60 gives the leftover minutes after full hours. Python follows the usual order of operations (exponents, then multiplication and division, then addition and subtraction), so use parentheses whenever you want to be explicit: price * (1 - discount).

Shorthand assignment operators

total = 100
total += 20   # same as total = total + 20  → 120
total -= 5    # 115
total *= 2    # 230

Comparison and logical operators

Comparison operators compare values and return True or False. They're the foundation of the decisions you'll write in lesson 4:

age = 20
print(age >= 18)               # True
print(age == 20)               # True  (== compares; = assigns)
print(age != 30)               # True  (not equal)
print(age > 18 and age < 65)   # True
print(age < 18 or age > 65)    # False
print(not True)                # False

and is True only when both sides are True; or is True when at least one side is; not flips the value. Python even lets you chain comparisons the way you'd write them in math class: 18 <= age < 65.

Full example: a final price calculator

SALES_TAX = 0.0825
base_price = 250.0
discount = 0.10

discounted_price = base_price * (1 - discount)
tax = discounted_price * SALES_TAX
final_price = discounted_price + tax

print("Discounted price:", discounted_price)
print("Sales tax:", round(tax, 2))
print("Total due:", round(final_price, 2))

Discounted price: 225.0
Sales tax: 18.56
Total due: 243.56

Notice the order: in most states, sales tax applies to the price after a store discount, so the discount comes first. Also notice that print() accepts several values separated by commas and displays them with a space in between.

Real case: closing out a taco truck in San Antonio

Rosie runs a taco truck in San Antonio, Texas, and every night she does the math in a notebook. This program uses everything in the lesson: constants in capital letters, ints and floats, //, % and a comparison that returns a bool.

# End-of-day cash count for Rosie's Taco Truck (San Antonio, Texas)
TACO_PRICE = 3            # dollars
SODA_PRICE = 2
AGUA_FRESCA_PRICE = 4
TORTILLAS_PER_PACK = 30

tacos = 185
sodas = 45
aguas_frescas = 30
supply_cost = 320.0       # meat, tortillas, produce and propane

taco_sales = tacos * TACO_PRICE
drink_sales = sodas * SODA_PRICE + aguas_frescas * AGUA_FRESCA_PRICE
total_sales = taco_sales + drink_sales
profit = total_sales - supply_cost
margin = profit / total_sales * 100

tortillas = tacos * 2     # every taco is served on two tortillas
packs = tortillas // TORTILLAS_PER_PACK
loose = tortillas % TORTILLAS_PER_PACK

print("Total sales:", total_sales)
print("Profit:", profit)
print("Margin (%):", round(margin, 1))
print("Full packs:", packs, "| loose tortillas:", loose)
print("Did we beat the $750 goal?", total_sales >= 750)

Total sales: 765
Profit: 445.0
Margin (%): 58.2
Full packs: 12 | loose tortillas: 10
Did we beat the $750 goal? True

Three things to notice in the output:

• 765 is an int, but 445.0 is a float: subtracting a float (supply_cost) turns the result into a float. Mixing ints and floats always gives a float.
• // and % work as a team: 370 tortillas are 12 full packs plus 10 loose ones, so Rosie actually needs to buy 13 packs.
• The comparison returns a bool (True). In lesson 4 you'll use values like this to make decisions, such as printing a congratulations message.

Common mistakes with variables and types

take_home_pay = 3800
fixed_expenses = 2650

left_over = take_home_pay - fixed_expenses       # 1150
percentage = left_over / take_home_pay * 100     # 30.26...
print("Left over:", left_over)
print("Percentage:", round(percentage, 1), "%")

print(5000 // left_over)
print(5000 / left_over)
print(left_over >= take_home_pay * 0.20)

Left over: 1150
Percentage: 30.3 %
4
4.3478260869565215
True

Summary

• Variables are created with =, and their names use snake_case; constants go in CAPITALS by convention.
• Basic types: int, float, str, bool and None; check them with type().
• Convert with int(), float() and str(); round with round(), not int().
• / divides with decimals, // floor-divides, % gives the remainder and == compares.
• TypeError means types that can't be mixed; ValueError means text that doesn't look like a number, such as "$49.99".

Next lesson: Python Strings, F-Strings and input(). You'll index and slice text, use string methods, format dollars with f-strings and ask the user for data.