ast

ast 模块用于将 Python 源码解析为抽象语法树（Abstract Syntax Tree），是理解 CPython 编译流程的核心工具。源码经过 tokenize → parse（生成 AST）→ compile（生成字节码）→ 执行，AST 正是其中承上启下的关键环节。

ast

解析与查看 AST

import ast

source = """
def add(a, b):
    return a + b
"""

tree = ast.parse(source)
print(ast.dump(tree, indent=2))
"""
Module(
  body=[
    FunctionDef(
      name='add',
      args=arguments(
        posonlyargs=[],
        args=[
          arg(arg='a'),
          arg(arg='b')],
        kwonlyargs=[],
        kw_defaults=[],
        defaults=[]),
      body=[
        Return(
          value=BinOp(
            left=Name(id='a', ctx=Load()),
            op=Add(),
            right=Name(id='b', ctx=Load())))],
      decorator_list=[])],
  type_ignores=[])
"""

遍历 AST 节点

ast.NodeVisitor 用于只读遍历，ast.NodeTransformer 用于修改 AST。

import ast

source = """
x = 1
y = x + 2
print(y)
z = x * y + 3
"""

class NameCollector(ast.NodeVisitor):
    """收集所有变量名"""
    def __init__(self):
        self.names = set()

    def visit_Name(self, node):
        self.names.add(node.id)
        self.generic_visit(node)

tree = ast.parse(source)
collector = NameCollector()
collector.visit(tree)
print(collector.names)
# {'x', 'y', 'print', 'z'}

修改 AST

import ast

class ConstantDoubler(ast.NodeTransformer):
    """将所有整数字面量翻倍"""
    def visit_Constant(self, node):
        if isinstance(node.value, int):
            node.value *= 2
        return node

source = "result = 1 + 2 + 3"
tree = ast.parse(source)
tree = ConstantDoubler().visit(tree)
ast.fix_missing_locations(tree)

code = compile(tree, "<string>", "exec")
namespace = {}
exec(code, namespace)
print(namespace["result"])  # 12 (= 2 + 4 + 6)

安全地求值表达式

ast.literal_eval 只允许基本字面量，比 eval() 安全得多。

import ast

print(ast.literal_eval("[1, 2, 3]"))        # [1, 2, 3]
print(ast.literal_eval("{'a': 1, 'b': 2}")) # {'a': 1, 'b': 2}
print(ast.literal_eval("(True, None, 3.14)")) # (True, None, 3.14)

try:
    ast.literal_eval("__import__('os').system('echo hacked')")
except (ValueError, SyntaxError) as e:
    print(f"被拒绝: {e}")

代码分析：函数复杂度统计

import ast

source = """
def simple():
    return 1

def complex_func(x):
    if x > 0:
        for i in range(x):
            if i % 2 == 0:
                yield i
    elif x < 0:
        raise ValueError("negative")
    else:
        return 0
"""

class ComplexityAnalyzer(ast.NodeVisitor):
    """统计函数中的分支与循环数量"""
    def __init__(self):
        self.results = {}
        self._current_func = None
        self._complexity = 0

    def visit_FunctionDef(self, node):
        old_func, old_complexity = self._current_func, self._complexity
        self._current_func = node.name
        self._complexity = 1
        self.generic_visit(node)
        self.results[node.name] = self._complexity
        self._current_func, self._complexity = old_func, old_complexity

    def visit_If(self, node):
        self._complexity += 1
        self.generic_visit(node)

    def visit_For(self, node):
        self._complexity += 1
        self.generic_visit(node)

    visit_While = visit_For

analyzer = ComplexityAnalyzer()
analyzer.visit(ast.parse(source))
for name, score in analyzer.results.items():
    print(f"{name}: 圈复杂度 = {score}")
# simple: 圈复杂度 = 1
# complex_func: 圈复杂度 = 4

编译与执行

import ast

source = "print('Hello from AST!')"
tree = ast.parse(source, mode="exec")
code = compile(tree, filename="<ast>", mode="exec")
exec(code)
# Hello from AST!

expr_tree = ast.parse("2 ** 10", mode="eval")
result = eval(compile(expr_tree, "<ast>", "eval"))
print(result)  # 1024

AST 的实际应用

• 代码格式化工具（如 Black）基于 AST 确保格式化不改变语义
• 静态分析工具（如 Pylint、mypy）使用 AST 检查代码问题
• AI 代码生成：分析/验证 LLM 生成代码的语法正确性
• CPython 贡献：理解 AST 是参与编译器优化的前提