🐍 Welcome + Python Mental Model

Why it matters

A clean mental model prevents 80% of beginner bugs and makes problem-solving faster.

The idea

Think of Python as a smart calculator plus a notebook. Expressions create values; names point to those values; statements do work.

The interpreter reads your file top-to-bottom. Indentation isn't decoration — it's syntax. Two spaces in, two spaces out: that's a block.

Try it

A name is a label stuck on a value. Re-binding x doesn't change 5, it just points the label elsewhere.

x = 5
print("x is", x)
x = "now I'm a string"
print("x is", x, "of type", type(x).__name__)

Blocks are defined by indentation. Mix tabs and spaces and Python will rightfully refuse to run.

for i in range(3):
  if i % 2 == 0:
      print(i, "even")
  else:
      print(i, "odd")

Quick check

• Q: What does type(3/2) return? A: float
• Q: Why does indentation matter? A: It defines code blocks (if / for / def).

Mini drills

• Print your name and age on separate lines.
• Evaluate 7 // 2 and 7 % 2.
• Try "hi" * 3.

Do's and don'ts

• Do keep indentation consistent (4 spaces).
• Do use the playground to test small ideas.
• Don't mix tabs and spaces.

Common mistakes

• Mistake: Forgetting a colon after if / for / def. Fix: Add :.
• Mistake: Inconsistent indentation. Fix: Use 4 spaces everywhere.
• Mistake: Expecting print() to return a value. Fix: print() returns None.

Key takeaways

• Python reads code top-to-bottom.
• Indentation creates blocks (be consistent: spaces, not tabs).
• Expressions produce values; statements do work.
• Names bind to objects — the object is what has the type.