Summary - AST
- Python course
- Expert
- Lesson 18/47
- 5 min
Combine parse, NodeVisitor and compile to write a code analysis tool.
Recap: AST tools
In this group you learned the AST toolchain. You turned code into a tree with ast.parse, did quick scans with ast.walk, and walked orderly with NodeVisitor.
You changed nodes with NodeTransformer and recompiled and ran with compile. Now you will combine these into a code analysis tool: a FunctionLister visitor that prints the name of every function definition in the tree. This is the core idea of linters and documentation tools.
Practice Write a code analysis tool
Import the module with import ast. Write a FunctionLister(ast.NodeVisitor) class. In visit_FunctionDef(self, node) do print(node.name) and call self.generic_visit(node). Build the tree with ast.parse("def a():\n pass\ndef b():\n pass") and run it with FunctionLister().visit(tree).
Starter code
# write a function lister
# walk the tree and print namesSample solution
import ast
class FunctionLister(ast.NodeVisitor):
def visit_FunctionDef(self, node):
print(node.name)
self.generic_visit(node)
tree = ast.parse("def a():\n pass\ndef b():\n pass")
FunctionLister().visit(tree)Expected output
a b
What you learned
In this group you went deep into Python's abstract syntax tree. You learned to turn code into a tree with ast.parse and to do quick scans with ast.walk.
You saw orderly walking with NodeVisitor, node modification with NodeTransformer and recompilation with compile. In the summary you combined these into a code analysis tool. This knowledge is the foundation for writing linters, formatters and code generators.
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which tool for which job?
- ast.parse to build a tree
- NodeTransformer to delete a node
- ast.walk to modify a node
- compile to run a tree Correct answer
Explanation: answer: compile to run a tree. ast.walk only walks, it does not modify.
2 Match all AST concepts
| Term | Meaning |
|---|---|
ast.parse |
Produces a tree |
NodeVisitor |
Walks orderly |
NodeTransformer |
Changes nodes |
compile |
Turns into code |
3 Final blank
class FunctionLister(ast.NodeVisitor):
def visit_FunctionDef(self, node):
print(node.____)
self.generic_visit(node)class FunctionLister(ast.NodeVisitor):
def visit_FunctionDef(self, node):
print(node.name)
self.generic_visit(node)Explanation: node.name carries the function's name.