AST: Tree of Code

Python's ast module gives a tree:

Each node has:

• A type (Assign, BinOp, Name, FunctionDef, ...).
• Children (lists of nodes).
• Location (lineno, col_offset, end_lineno, end_col_offset).

Visitor pattern:

generic_visit recurses; without it, you stop descent at this node.

NodeVisitor vs NodeTransformer:

• Visitor: read-only.
• Transformer: returns new node to REPLACE the visited one (for autofixers).

For other languages: tree-sitter is a portable parser library — same visitor model works across 50+ languages.

Important: AST loses some info — comments, blank lines, exact whitespace. Style linters need a CST (concrete syntax tree) or token stream alongside.