在Python 3.0中动态向类添加方法

我正在尝试用 Python 编写一个数据库抽象层，它允许您使用链式函数调用构建 SQL 语句，例如：

results = db.search("book")
          .author("J. K. Rowling")
          .price("<40.00")
          .title("Harry")
          .execute()

但是当我尝试将所需的方法动态添加到数据库类时遇到问题。

这是我的代码的重要部分：

import inspect

def myName():
    return inspect.stack()[1][3]

class Search():

    def __init__(self, family):
        self.family = family
        self.options = ['price', 'name', 'author', 'genre']
        #self.options is generated based on family, but this is an example
        for opt in self.options:
            self.__dict__[opt] = self.__Set__
        self.conditions = {}

    def __Set__(self, value):
        self.conditions[myName()] = value
        return self

    def execute(self):
        return self.conditions

但是，当我运行以下示例时：

print(db.search("book").price(">4.00").execute())

outputs:

{'__Set__': 'harry'}

我是否以错误的方式处理这个问题？有没有更好的方法来获取正在调用的函数的名称或以某种方式制作函数的“硬拷贝”？

您可以简单地添加搜索功能（方法）类创建后:

class Search:  # The class does not include the search methods, at first
    def __init__(self):
        self.conditions = {}

def make_set_condition(option):  # Factory function that generates a "condition setter" for "option"
    def set_cond(self, value):
        self.conditions[option] = value
        return self
    return set_cond

for option in ('price', 'name'):  # The class is extended with additional condition setters
    setattr(Search, option, make_set_condition(option))

Search().name("Nice name").price('$3').conditions  # Example
{'price': '$3', 'name': 'Nice name'}

PS: 这个类有一个__init__()方法不具有family参数（条件设置器在运行时动态添加，但添加到类中，而不是单独添加到每个实例中）。如果Search 具有不同条件设置器的对象需要创建，那么上述方法的以下变体就可以工作（__init__()方法有一个family范围）：

import types

class Search:  # The class does not include the search methods, at first

    def __init__(self, family):
        self.conditions = {}
        for option in family:  # The class is extended with additional condition setters
            # The new 'option' attributes must be methods, not regular functions:
            setattr(self, option, types.MethodType(make_set_condition(option), self))

def make_set_condition(option):  # Factory function that generates a "condition setter" for "option"
    def set_cond(self, value):
        self.conditions[option] = value
        return self
    return set_cond

>>> o0 = Search(('price', 'name'))  # Example
>>> o0.name("Nice name").price('$3').conditions
{'price': '$3', 'name': 'Nice name'}
>>> dir(o0)  # Each Search object has its own condition setters (here: name and price)
['__doc__', '__init__', '__module__', 'conditions', 'name', 'price']

>>> o1 = Search(('director', 'style'))
>>> o1.director("Louis L").conditions  # New method name
{'director': 'Louis L'}
>>> dir(o1)  # Each Search object has its own condition setters (here: director and style)
['__doc__', '__init__', '__module__', 'conditions', 'director', 'style']

参考：http://docs.python.org/howto/descriptor.html#functions-and-methods http://docs.python.org/howto/descriptor.html#functions-and-methods

如果您确实需要知道存储它们的属性名称的搜索方法，您可以简单地将其设置为make_set_condition() with

       set_cond.__name__ = option  # Sets the function name

（就在之前return set_cond）。在执行此操作之前，方法Search.name具有以下名称：

>>> Search.price
<function set_cond at 0x107f832f8>

设置其后__name__属性，你会得到一个不同的名称：

>>> Search.price
<function price at 0x107f83490>

以这种方式设置方法名称可以使涉及该方法的可能错误消息更容易理解。