Python extension methods

Question

OK, in C# we have something like:

public static string Destroy(this string s) { 
    return "";
}

So basically, when you have a string you can do:

str = "This is my string to be destroyed";
newstr = str.Destroy()
# instead of 
newstr = Destroy(str)

Now this is cool because in my opinion it's more readable. Does Python have something similar? I mean instead of writing like this:

x = SomeClass()
div = x.getMyDiv()
span = x.FirstChild(x.FirstChild(div)) # so instead of this

I'd like to write:

span = div.FirstChild().FirstChild() # which is more readable to me

Any suggestion?

Answer 1

You can just modify the class directly, sometimes known as monkey patching.

def MyMethod(self):
      return self + self

MyClass.MyMethod = MyMethod
del(MyMethod)#clean up namespace

I'm not 100% sure you can do this on a special class like str, but it's fine for your user-defined classes.

Update

You confirm in a comment my suspicion that this is not possible for a builtin like str. In which case I believe there is no analogue to C# extension methods for such classes.

Finally, the convenience of these methods, in both C# and Python, comes with an associated risk. Using these techniques can make code more complex to understand and maintain.

Answer 2

I would use the Adapter pattern here. So, let's say we have a Person class and in one specific place we would like to add some health-related methods.

from dataclasses import dataclass


@dataclass
class Person:
    name: str
    height: float  # in meters
    mass: float  # in kg


class PersonMedicalAdapter:
    person: Person

    def __init__(self, person: Person):
        self.person = person

    def __getattr__(self, item):
        return getattr(self.person, item)

    def get_body_mass_index(self) -> float:
        return self.person.mass / self.person.height ** 2


if __name__ == '__main__':
    person = Person('John', height=1.7, mass=76)
    person_adapter = PersonMedicalAdapter(person)

    print(person_adapter.name)  # Call to Person object field
    print(person_adapter.get_body_mass_index())  # Call to wrapper object method

I consider it to be an easy-to-read, yet flexible and pythonic solution.

Answer 3

You can do what you have asked like the following:

def extension_method(self):
    #do stuff
class.extension_method = extension_method

Answer 4

You can achieve this nicely with the following context manager that adds the method to the class or object inside the context block and removes it afterwards:

class extension_method:

    def __init__(self, obj, method):
        method_name = method.__name__
        setattr(obj, method_name, method)
        self.obj = obj
        self.method_name = method_name

    def __enter__(self):
        return self.obj

    def __exit__(self, type, value, traceback):
        # remove this if you want to keep the extension method after context exit
        delattr(self.obj, self.method_name)

Usage is as follows:

class C:
    pass

def get_class_name(self):
    return self.__class__.__name__

with extension_method(C, get_class_name):
    assert hasattr(C, 'get_class_name') # the method is added to C
    c = C()
    print(c.get_class_name()) # prints 'C'

assert not hasattr(C, 'get_class_name') # the method is gone from C

Answer 5

You can change the built-in classes by monkey-patching with the help of forbidden fruit

But installing forbidden fruit requires a C compiler and unrestricted environment so it probably will not work or needs hard effort to run on Google App Engine, Heroku, etc.

I changed the behaviour of unicode class in Python 2.7 for a Turkish i,I uppercase/lowercase problem by this library.

# -*- coding: utf8 -*-
# Redesigned by @guneysus

import __builtin__
from forbiddenfruit import curse

lcase_table = tuple(u'abcçdefgğhıijklmnoöprsştuüvyz')
ucase_table = tuple(u'ABCÇDEFGĞHIİJKLMNOÖPRSŞTUÜVYZ')


def upper(data):
    data = data.replace('i',u'İ')
    data = data.replace(u'ı',u'I')
    result = ''
    for char in data:
        try:
            char_index = lcase_table.index(char)
            ucase_char = ucase_table[char_index]
        except:
            ucase_char = char
        result += ucase_char
    return result

curse(__builtin__.unicode, 'upper', upper)
class unicode_tr(unicode):
    """For Backward compatibility"""
    def __init__(self, arg):
        super(unicode_tr, self).__init__(*args, **kwargs)

if __name__ == '__main__':
    print u'istanbul'.upper()

Answer 6

I'd like to think that extension methods in C# are pretty much the same as normal method call where you pass the instance then arguments and stuff.

instance.method(*args, **kwargs)
method(instance, *args, **kwargs) # pretty much the same as above, I don't see much benefit of it getting implemented in python.

Answer 7

C# implemented extension methods because it lacks first class functions, Python has them and it is the preferred method for "wrapping" common functionality across disparate classes in Python.

There are good reasons to believe Python will never have extension methods, simply look at the available built-ins:

len(o) calls o.__len__
iter(o) calls o.__iter__
next(o) calls o.next
format(o, s) calls o.__format__(s)

Basically, Python likes functions.

Answer 8

After a week, I have a solution that is closest to what I was seeking for. The solution consists of using getattr and __getattr__. Here is an example for those who are interested.

class myClass:

    def __init__(self): pass

    def __getattr__(self, attr): 
        try:
            methodToCall = getattr(myClass, attr)
            return methodToCall(myClass(), self)
        except:
            pass

    def firstChild(self, node):
        # bla bla bla
    def lastChild(self, node):
        # bla bla bla 

x = myClass()
div = x.getMYDiv()
y = div.firstChild.lastChild 

I haven't test this example, I just gave it to give an idea for who might be interested. Hope that helps.