How can I intercept calls to python’s “magic” methods in new style classes?

by

For performance reasons, Python always looks in the class (and parent classes’) __dict__ for magic methods and does not use the normal attribute lookup mechanism. A workaround is to use a metaclass to automatically add proxies for magic methods at the time of class creation; I’ve used this technique to avoid having to write boilerplate call-through methods for wrapper classes, for example.

class Wrapper(object):
    """Wrapper class that provides proxy access to an instance of some
       internal instance."""

    __wraps__  = None
    __ignore__ = "class mro new init setattr getattr getattribute"

    def __init__(self, obj):
        if self.__wraps__ is None:
            raise TypeError("base class Wrapper may not be instantiated")
        elif isinstance(obj, self.__wraps__):
            self._obj = obj
        else:
            raise ValueError("wrapped object must be of %s" % self.__wraps__)

    # provide proxy access to regular attributes of wrapped object
    def __getattr__(self, name):
        return getattr(self._obj, name)

    # create proxies for wrapped object's double-underscore attributes
    class __metaclass__(type):
        def __init__(cls, name, bases, dct):

            def make_proxy(name):
                def proxy(self, *args):
                    return getattr(self._obj, name)
                return proxy

            type.__init__(cls, name, bases, dct)
            if cls.__wraps__:
                ignore = set("__%s__" % n for n in cls.__ignore__.split())
                for name in dir(cls.__wraps__):
                    if name.startswith("__"):
                        if name not in ignore and name not in dct:
                            setattr(cls, name, property(make_proxy(name)))

Usage:

class DictWrapper(Wrapper):
    __wraps__ = dict

wrapped_dict = DictWrapper(dict(a=1, b=2, c=3))

# make sure it worked....
assert "b" in wrapped_dict                        # __contains__
assert wrapped_dict == dict(a=1, b=2, c=3)        # __eq__
assert "'a': 1" in str(wrapped_dict)              # __str__
assert wrapped_dict.__doc__.startswith("dict()")  # __doc__

More Related Contents:

  1. How to sum all range output in python 3
  2. Python for in range()
  3. __init__() missing 1 required positional argument ‘address’
  4. How to specify multiple return types using type-hints
  5. Are lists thread-safe?
  6. What are type hints in Python 3.5?
  7. How can I change a global variable from within a function?
  8. KeyError: ‘TCL_Library’ when I use cx_Freeze
  9. pysftp vs. Paramiko
  10. Scraping Google Finance (BeautifulSoup)
  11. Image rotation while moving
  12. Extending list returns None [duplicate]
  13. How do I detect if the mouse is hovering over a button? PyGame button class is not displaying the text or changing colour on hover
  14. List returned by map function disappears after one use
  15. ‘is’ operator behaves differently when comparing strings with spaces
  16. Get contents of a Tkinter Entry widget
  17. How can I flush the output of the print function?
  18. Type hint for a function that returns only a specific set of values
  19. Checking the strength of a password (how to check conditions)
  20. SSL: CERTIFICATE_VERIFY_FAILED with Python3
  21. ImportError: No module named ‘google’
  22. How to downgrade python from 3.7 to 3.6
  23. from __future__ import annotations
  24. Updating OptionMenu from List
  25. “Permission Denied” trying to run Python on Windows 10
  26. Using asyncio.Queue for producer-consumer flow
  27. Are there any reasons not to use an OrderedDict?
  28. Changing one dict changes all dicts in a list of dicts [duplicate]
  29. What is internal representation of string in Python 3.x
  30. Run process with realtime output to a Tkinter GUI

Leave a Comment