How Python Runs Your Code

How Python turns your text into actions, plus the core ideas every later lesson leans on.

Values and types

• A program is instructions run in order, top to bottom.
• Every value has a type that decides what operations do.
• Core types: integer (42), float (3.14), string ("hi"), boolean (True/False).
• print(...) is a function: a named action you call on a value.
• + adds numbers but joins strings: 2 + 3 -> 5, "2" + "3" -> "23".
• Mixing types that don’t fit raises an error, not a guess: "2" + 3 -> TypeError.

Variables

• A variable is a name pointing at one value; create with = (“assign”, not “equals”).
• Read score = score + 5 right-to-left: compute the right side, then rebind the name.
• Names: lowercase words joined by underscores, describe the value, cannot start with a digit.

How a file runs

.py text  ->  parse (grammar)  ->  compile to bytecode  ->  execute (top to bottom)
                 |
                 +-- SyntaxError: stop, nothing runs

• Parse checks grammar first; a malformed line gives SyntaxError and nothing runs.
• Valid code compiles to bytecode, then executes one instruction at a time.
• Runtime errors (like TypeError) surface only when the bad line is reached.

Comments

• Text after # is ignored by Python: notes for humans.

Common pitfalls

• = assigns, == compares (gives True/False).
• 7 / 2 -> 3.5 (float); 7 // 2 -> 3 (whole-number division); % is the remainder.
• Indentation is syntax, not style; inconsistent indentation is a SyntaxError.