扩展Python内置的Str

Question

我正在尝试创建一个继承自 str 的新类，但由于 str 是不可变的，我遇到了一些困难。

class DerivedClass(str):

    def __new__(cls, string):
        ob = super(DerivedClass, cls).__new__(cls, string)
        return ob

    def upper(self):
        #overridden, new functionality. Return ob of type DerivedClass. Great.
        caps = super(DerivedClass, self).upper()
        return DerivedClass(caps + '123')

derived = DerivedClass('a')

print derived.upper() #'A123'
print type(derived.upper()) #<class '__main__.DerivedClass'>
print derived.lower() #'a' 
print type(derived.lower()) #<type 'str'>  

对于那些不需要新功能的继承方法，比如 derived.lower()，有没有简单又符合 Python 风格的方法可以返回一个 DerivedClass 类型的对象（而不是 str）？还是说我只能像处理 derived.upper() 那样，手动重写每个 str.method() 呢？

编辑：

#Any massive flaws in the following?

class DerivedClass(str):
    def __new__(cls, string):
        ob = super(DerivedClass, cls).__new__(cls, string)
        return ob

    def upper(self):
        caps = super(DerivedClass, self).upper()
        return DerivedClass(caps + '123')

    def __getattribute__(self, name):
        att = super(DerivedClass, self).__getattribute__(name)

        if not callable(att):
            return att

        def call_me_later(*args, **kwargs):
            result = att(*args, **kwargs)
            if isinstance(result, basestring):
                return DerivedClass(result)
            return result
        return call_me_later

Answer 1

你们的想法都差不多，但逐个检查的方法在重写很多函数时就不太好用了。

from functools import partial

class DerivedClass(str):
    def __new__(cls, string):
        ob = super(DerivedClass, cls).__new__(cls, string)
        return ob

    def upper(self):
        caps = super(DerivedClass, self).upper()
        return DerivedClass(caps + '123')

    def __getattribute__(self, name):
        if name in ['__dict__', '__members__', '__methods__', '__class__']:
            return object.__getattribute__(self, name)
        func = str.__getattribute__(self, name)
        if name in self.__dict__.keys() or not callable(func):
            return func

        def call_me_later(*args, **kwargs):
            result = func(*args, **kwargs)
            # Some str functions return lists, ints, etc
            if isinstance(result, basestring):
                return DerivedClass(result)
            return result

        return partial(call_me_later)

_{(这个建议是由jarret hardie在评论中提出的。)}

Answer 2

这是一个很好的类装饰器的用法——大致上（代码未经测试）：

@do_overrides
class Myst(str):
  def upper(self):
    ...&c...

还有

def do_overrides(cls):
  done = set(dir(cls))
  base = cls.__bases__[0]
  def wrap(f):
    def wrapper(*a, **k):
      r = f(*a, **k)
      if isinstance(r, base):
        r = cls(r)
      return r
  for m in dir(base):
    if m in done or not callable(m):
      continue
    setattr(cls, m, wrap(getattr(base, m)))

Answer 3

你可以按照Zr40的建议，通过重写__getattribute__来实现这个功能。不过，你需要让getattribute返回一个可以调用的函数。下面的示例应该能满足你的需求；它使用了functools.partial这个工具来简化操作，不过如果你愿意，也可以不使用这个工具来实现：

from functools import partial

class DerivedClass(str):

    def __new__(cls, string):
        ob = super(DerivedClass, cls).__new__(cls, string)
        return ob

    def upper(self):
        #overridden, new functionality. Return ob of type DerivedClass. Great.
        caps = super(DerivedClass, self).upper()
        return DerivedClass(caps + '123')

    def __getattribute__(self, name):
        func = str.__getattribute__(self, name)
        if name == 'upper':
            return func

        if not callable(func):
            return func

        def call_me_later(*args, **kwargs):
            result = func(*args, **kwargs)
            # Some str functions return lists, ints, etc
            if isinstance(result, basestring:
                return DerivedClass(result)
            return result

        return partial(call_me_later)