Key Takeaways — Chapter 3: Python Basics

Variables

A variable is a named container: revenue = 45000
Names: lowercase with underscores (monthly_revenue), descriptive, no keywords
Reassignment replaces the old value: balance += 500 (shorthand for balance = balance + 500)

The Five Core Types

Type	Example	Business Use
`int`	`200`	Counts, quantities
`float`	`29.99`	Prices, rates, percentages
`str`	`"Acme Corp"`	Names, codes, descriptions
`bool`	`True`, `False`	Flags, conditions
`None`	`None`	Missing/unknown values

Type Conversion Quick Reference

int("45000")          # → 45000 (string to int)
float("29.99")        # → 29.99 (string to float)
str(200)              # → "200" (number to string)
int(29.99)            # → 29 (truncates, doesn't round!)

Arithmetic Operators

Operator	Use	Example
`+`, `-`, `*`, `/`	Basic math	`revenue - cost`
`//`	Floor division	`100 // 12` → `8`
`%`	Remainder	`100 % 12` → `4`
`**`	Power	`(1+rate) ** years`

Compound growth formula: initial * (1 + rate) ** years

F-String Format Codes

f"{revenue:,}"        # 1,450,000
f"{revenue:,.2f}"     # 1,450,000.00
f"{margin:.1%}"       # 34.2%
f"{value:>10}"        # Right-aligned in 10 chars

Comparison and Logical Operators

# Comparison → returns bool
revenue > target        # Greater than?
revenue >= target       # Greater than or equal to?
status == "Active"      # Equal to?

# Logical → combines bools
is_vip and is_paid      # Both must be True
over_budget or at_risk  # At least one True
not is_active           # Invert the bool

Common Pitfalls

String numbers: "200" + "200" = "200200" (not 400!) — convert first: int("200") + int("200") = 400
Float imprecision: 0.1 + 0.2 = 0.30000000000000004 — use Decimal for financial calculations
Case sensitivity: "Midwest" == "midwest" is False — normalize with .lower() or .upper()
None is not zero: None == 0 is False

Next: Chapter 4 — Control Flow: How to make your programs make decisions.