Does Python have something like anonymous inner classes of Java?

Java uses anonymous classes mostly to imitate closures or simply code blocks. Since in Python you can easily pass around methods there’s no need for a construct as clunky as anonymous inner classes:

def printStuff():
   print "hello"

def doit(what):
   what()

doit(printStuff) 

Edit: I’m aware that this is not what is needed in this special case. I just described the most common python solution to the problem most commonly by anonymous inner classes in Java.

In python you have anonymous functions, declared using lambda statement. I do not like them very much – they are not so readable, and have limited functionality.

However, what you are talking about may be implemented in python with a completely different approach:

class a(object):
  def meth_a(self):
    print "a"

def meth_b(obj):
  print "b"

b = a()
b.__class__.meth_a = meth_b

Python probably has better ways to solve your problem. If you could provide more specific details of what you want to do it would help.

For example, if you need to change the method being called in a specific point in code, you can do this by passing the function as a parameter (functions are first class objects in python, you can pass them to functions, etc). You can also create anonymous lambda functions (but they’re restricted to a single expression).

Also, since python is very dynamic, you can change methods of an object after it’s been created object.method1 = alternative_impl1, although it’s actually a bit more complicated, see gnud’s answer

You can accomplish this in three ways:

1. Proper subclass (of course)
2. a custom method that you invoke with the object as an argument
3. (what you probably want) — adding a new method to an object (or replacing an existing one).

Example of option 3 (edited to remove use of “new” module — It’s deprecated, I did not know ):

import types
class someclass(object):
    val = "Value"
    def some_method(self):
        print self.val

def some_method_upper(self):
    print self.val.upper()

obj = someclass()
obj.some_method()

obj.some_method = types.MethodType(some_method_upper, obj)
obj.some_method()

Well, classes are first class objects, so you can create them in methods if you want. e.g.

from optparse import OptionParser
def make_custom_op(i):
  class MyOP(OptionParser):
    def exit(self):
      print 'custom exit called', i
  return MyOP

custom_op_class = make_custom_op(3)
custom_op = custom_op_class()

custom_op.exit()          # prints 'custom exit called 3'
dir(custom_op)            # shows all the regular attributes of an OptionParser

But, really, why not just define the class at the normal level? If you need to customise it, put the customisation in as arguments to __init__.

(edit: fixed typing errors in code)

Python doesn’t support this directly (anonymous classes) but because of its terse syntax it isn’t really necessary:

class MyOptionParser(OptionParser):
    def exit(self, status=0, msg=None):
        # body of method

p = MyOptionParser()

The only downside is you add MyOptionParser to your namespace, but as John Fouhy pointed out, you can hide that inside a function if you are going to do it multiple times.

You can always hide class by variables:

 class var(...):
     pass
 var = var()

instead of

 var = new ...() {};

This is what you would do in Python 3.7

#!/usr/bin/env python3
class ExmapleClass:
    def exit(self):
        print('this should NOT print since we are going to override')

ExmapleClass= type('', (ExmapleClass,), {'exit': lambda self: print('you should see this printed only')})()
ExmapleClass.exit()

I do this in python3 usually with inner classes

class SomeSerializer():
    class __Paginator(Paginator):
        page_size = 10

    # defining it for e.g. Rest:
    pagination_class = __Paginator

    # you could also be accessing it to e.g. create an instance via method:
    def get_paginator(self):
        return self.__Paginator()

as i used double underscore, this mixes the idea of “mangling” with inner classes, from outside you can still access the inner class with SomeSerializer._SomeSerializer__Paginator, and also subclasses, but SomeSerializer.__Paginator will not work, which might or might not be your whish if you want it a bit more “anonymous”.

However I suggest to use “private” notation with a single underscore, if you do not need the mangling.

In my case, all I need is a fast subclass to set some class attributes, followed up by assigning it to the class attribute of my RestSerializer class, so the double underscore would denote to “not use it at all further” and might change to no underscores, if I start reusing it elsewhere.

Being perverse, you could use the throwaway name _ for the derived class name:

class _(OptionParser):

    def exit(self):
        pass  # your override impl

Here is a more fancy way of doing Maciej’s method.
I defined the following decorator:

def newinstance(*args, **kwargs):
    def decorator(cls):
        return cls(*args, **kwargs)
    return decorator

The following codes are roughly equivalent (also works with args!)

// java

MyClass obj = new MyClass(arg) {
    public void method() {
        // body of the method
    }
};

# python

@newinstance(arg)
class obj(MyClass):
    def method(self):
        pass # body of the method

You can use this code from within a class/method/function if you want to define an "inner" class instance.