Extension method for python built-in types

Question

is it possible to add extension method to python built-in types?
I know that I can add extension method to defined type by simply adding new method by . as following:

class myClass:
    pass

myClass.myExtensionMethod = lambda self,x:x * 2
z = myClass()
print z.myExtensionMethod(10)

But is any way to adding extension method to python built’in types like list, dict, …

list.myExtension = lambda self,x:x * 2
list.myExtension(10)

Asked By: Saeed Afshari

||

Source

Answer 1

No, because I’m pretty sure all the built-in types are written in optimized C and thus can’t be modified with Python. When I try it, I just get:

TypeError: can't set attributes of built-in/extension type 'list'

Answered By: fletom

Answer 2

No. Types defined in C cannot be monkeypatched.

Answered By: Ignacio Vazquez-Abrams

Answer 3

The best you can do appears to be deriving a class from the built-in type. For example:

class mylist(list):
    def myfunc(self, x):
        self.append(x)

test = mylist([1,2,3,4])
test.myfunc(99)

(You could even name it “list” so as to get the same constructor, if you wanted.) However, you cannot directly modify a built-in type like the example in your question.

Answered By: user812786

Answer 4

Nope, you gotta subclass!

>>> import string
>>> class MyString(str):
...     def disemvowel(self):
...         return MyString(string.translate(self, None, "aeiou"))
... 
>>> s = MyString("this is only a test")
>>> s.disemvowel()
'ths s nly  tst'

Or more specific to your example

>>> class MyList(list):
...     pass
... 
>>> MyList.myExtension = lambda self,x:x * 2
>>> l = MyList()
>>> l.myExtension(10)
20

Answered By: ironchefpython

Answer 5

It can be done in pure Python with this incredibly clever module:

https://pypi.python.org/pypi/forbiddenfruit

For example:

import functools
import ctypes
import __builtin__
import operator

class PyObject(ctypes.Structure):
    pass

Py_ssize_t = hasattr(ctypes.pythonapi, 'Py_InitModule4_64') and ctypes.c_int64 or ctypes.c_int

PyObject._fields_ = [
    ('ob_refcnt', Py_ssize_t),
    ('ob_type', ctypes.POINTER(PyObject)),
]

class SlotsPointer(PyObject):
    _fields_ = [('dict', ctypes.POINTER(PyObject))]

def proxy_builtin(klass):
    name = klass.__name__
    slots = getattr(klass, '__dict__', name)

    pointer = SlotsPointer.from_address(id(slots))
    namespace = {}

    ctypes.pythonapi.PyDict_SetItem(
        ctypes.py_object(namespace),
        ctypes.py_object(name),
        pointer.dict,
    )

    return namespace[name]

def die(message, cls=Exception):
    """
        Raise an exception, allows you to use logical shortcut operators to test for object existence succinctly.

        User.by_name('username') or die('Failed to find user')
    """
    raise cls(message)

def unguido(self, key):
    """
        Attempt to find methods which should really exist on the object instance.
    """
    return functools.partial((getattr(__builtin__, key, None) if hasattr(__builtin__, key) else getattr(operator, key, None)) or die(key, KeyError), self)

class mapper(object):
    def __init__(self, iterator, key):
        self.iterator = iterator
        self.key = key
        self.fn = lambda o: getattr(o, key)

    def __getattribute__(self, key):
        if key in ('iterator', 'fn', 'key'): return object.__getattribute__(self, key)
        return mapper(self, key)

    def __call__(self, *args, **kwargs):
        self.fn = lambda o: (getattr(o, self.key, None) or unguido(o, self.key))(*args, **kwargs)
        return self

    def __iter__(self):
        for value in self.iterator:
            yield self.fn(value)

class foreach(object):
    """
        Creates an output iterator which will apply any functions called on it to every element
        in the input iterator. A kind of chainable version of filter().

        E.g:

        foreach([1, 2, 3]).__add__(2).__str__().replace('3', 'a').upper()

        is equivalent to:

        (str(o + 2).replace('3', 'a').upper() for o in iterator)

        Obviously this is not 'Pythonic'.
    """
    def __init__(self, iterator):
        self.iterator = iterator

    def __getattribute__(self, key):
        if key in ('iterator',): return object.__getattribute__(self, key)
        return mapper(self.iterator, key)

    def __iter__(self):
        for value in self.iterator:
            yield value

proxy_builtin(list)['foreach'] = property(foreach)

import string

print string.join([1, 2, 3].foreach.add(2).str().add(' cookies').upper(), ', ')

>>> 3 COOKIES, 4 COOKIES, 5 COOKIES

There, doesn’t that feel good?

Answered By: toothygoose

Extension method for python built-in types

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