/ˌeɪˌɛsˈtiː/
noun … “Structural map of code for analysis and execution.”
AST, short for Abstract Syntax Tree, is a tree representation of the syntactic structure of source code in a programming language. Each node in the tree denotes a construct occurring in the source, such as expressions, statements, operators, or function calls, abstracted from concrete syntax details like punctuation or formatting. ASTs are essential in Compiler design, Interpreter execution, static analysis, and code transformation tools.
During parsing, the source code is tokenized and transformed into an AST. The tree captures the hierarchical relationships between constructs: for example, a function call node may have child nodes representing the function identifier and its arguments. This abstraction allows subsequent stages—semantic analysis, optimization, or Bytecode generation—to operate on a structured and unambiguous representation of the program.
Key characteristics of AST include:
- Hierarchical structure: nodes reflect the nested, logical composition of the program.
- Language-agnostic manipulation: many tools transform ASTs without concern for concrete syntax.
- Facilitates static analysis: allows type checking, linting, and code quality inspection.
- Supports code transformation: used in refactoring, transpilation, and optimization workflows.
Workflow example: In Python, the built-in ast module can parse source code into an AST object. A developer analyzing a script might traverse the AST to detect unused variables or modify function calls for optimization. Similarly, a Compiler generates an AST before producing Bytecode, ensuring accurate representation of control flow and expressions.
import ast
source = "x = a + b * c"
tree = ast.parse(source)
print(ast.dump(tree))This snippet produces a structured tree reflecting assignment and arithmetic operations, which can be analyzed or transformed programmatically.
Conceptually, an AST is like a blueprint of a building. It does not show colors, textures, or furniture (concrete syntax), but it precisely maps the structural relationships between rooms and supports engineers (the Compiler or Interpreter) in building or modifying the final structure (executable program).
See Compiler, Interpreter, Bytecode, Python.