🏷️ Variables, Types, and I/O

Why it matters

Every bug you fix in Python starts with understanding types, names, and input/output.

The idea

Variables are labels stuck on objects. You can re-label anytime, but the object stays the same — and the object always carries its type.

Try it

type() is your fastest debugger. When numbers behave like strings (or vice versa), type() tells you why.

age = 25
height = 5.9
name = "Ada"
is_engineer = True
print(age, type(age).__name__)
print(height, type(height).__name__)
print(name, type(name).__name__)
print(is_engineer, type(is_engineer).__name__)

input() ALWAYS returns a string. Cast before doing math — otherwise "2" + "3" == "23".

# input() blocked in Pyodide — pretend the user typed "42"
raw = "42"
print("raw:", raw, type(raw).__name__)
n = int(raw)
print("n*2 =", n * 2)

# What happens if you forget to cast?
try:
  print(raw + 1)
except TypeError as e:
  print("TypeError:", e)

Quick check

• Q: What type is input()? A: str
• Q: What happens with int("10.5")? A: It errors — convert to float first.

Mini drills

• Ask for two numbers and print their sum.
• Convert centimeters to meters using input.

Common mistakes

• Mistake: age = input(...) then age + 1. Fix: age = int(input(...)).
• Mistake: Shadowing built-ins like list = []. Fix: Use better names.

Key takeaways

• Variables point to objects; the object owns the type.
• input() always returns a str — cast with int() / float() before math.
• Use type() to diagnose mystery bugs.
• Python is dynamically typed: the same name can re-bind to a different type.