Python元编程实战：动态代码生成与AST操作高级技巧

一、Python元编程核心概念

传统编程

编写静态代码
直接操作数据
编译时确定行为
有限的运行时灵活性

元编程

代码生成代码
操作程序结构
运行时改变行为
极高的灵活性

动态类生成示例

def create_class(class_name, attributes):
    """动态创建类工厂"""
    def __init__(self, **kwargs):
        for k, v in kwargs.items():
            setattr(self, k, v)
    
    cls_dict = {
        '__init__': __init__,
        '__slots__': tuple(attributes),
        '__repr__': lambda self: f"{class_name}({', '.join(
            f'{k}={getattr(self, k)!r}' for k in attributes
        )})"
    }
    
    return type(class_name, (), cls_dict)

# 使用示例
Person = create_class('Person', ['name', 'age'])
p = Person(name='Alice', age=30)
print(p)  # 输出: Person(name='Alice', age=30)

二、AST操作实战

1. 代码安全分析器

import ast

class SecurityVisitor(ast.NodeVisitor):
    UNSAFE_FUNCS = {'eval', 'exec', 'open', '__import__'}
    
    def visit_Call(self, node):
        if isinstance(node.func, ast.Name) and node.func.id in self.UNSAFE_FUNCS:
            print(f"⚠️ 安全警告: 检测到危险函数调用 - {node.func.id}")
        self.generic_visit(node)

def analyze_code_security(code):
    """分析代码中的安全隐患"""
    tree = ast.parse(code)
    SecurityVisitor().visit(tree)

# 示例检测
code = """
import os
eval('os.system("rm -rf /")')
"""
analyze_code_security(code)  # 输出: ⚠️ 安全警告: 检测到危险函数调用 - eval

2. 动态SQL生成器

class QueryBuilder:
    def __init__(self, table):
        self.table = table
        self._select = []
        self._where = []
    
    def select(self, *columns):
        self._select.extend(columns)
        return self
    
    def where(self, condition):
        self._where.append(condition)
        return self
    
    def build(self):
        if not self._select:
            self._select = ['*']
        
        query = f"SELECT {', '.join(self._select)} FROM {self.table}"
        if self._where:
            query += " WHERE " + " AND ".join(self._where)
        return query

# 链式调用示例
query = (QueryBuilder("users")
    .select("name", "email")
    .where("age > 18")
    .where("status = 'active'")
    .build())
print(query)  # 输出: SELECT name, email FROM users WHERE age > 18 AND status = 'active'

三、ORM框架实战案例

迷你ORM实现

class ModelMeta(type):
    """元类：将类属性映射为数据库字段"""
    def __new__(cls, name, bases, attrs):
        fields = {}
        for k, v in attrs.items():
            if isinstance(v, Field):
                fields[k] = v
                v.name = k
        
        attrs['_fields'] = fields
        attrs['_tablename'] = attrs.get('__tablename__', name.lower())
        return super().__new__(cls, name, bases, attrs)

class Field:
    """字段描述符"""
    def __init__(self, type_, default=None):
        self.type = type_
        self.default = default
        self.name = None
    
    def __get__(self, instance, owner):
        return instance.__dict__.get(self.name, self.default)
    
    def __set__(self, instance, value):
        if not isinstance(value, self.type):
            raise TypeError(f"Expected {self.type}, got {type(value)}")
        instance.__dict__[self.name] = value

class Model(metaclass=ModelMeta):
    """模型基类"""
    def __init__(self, **kwargs):
        for k, v in kwargs.items():
            setattr(self, k, v)
    
    @classmethod
    def create_table_sql(cls):
        columns = []
        for name, field in cls._fields.items():
            columns.append(f"{name} {field.type.__name__.upper()}")
        return f"CREATE TABLE {cls._tablename} ({', '.join(columns)})"

# 使用示例
class User(Model):
    __tablename__ = 'app_users'
    id = Field(int)
    name = Field(str)
    email = Field(str, default='')

print(User.create_table_sql())
# 输出: CREATE TABLE app_users (id INTEGER, name STRING, email STRING)