Writing a Python class that provides default instance succinctly
I'd say a much easier way to accomplish this is to use a module instead of a class.  In a language like Java the primary form of encapsulation is a class, while in Python, the primary form of encapsulation is a module.  For example,

foo.py

class Example:

    def __init__(self, customizable):
        self.customizable = customizable

    def do_something(self):
        print("*** {} ***".format(self.customizable*2))

default_instance = Example(0)


main.py

import foo

my_foo = foo.Example(20)
my_foo.do_something()

foo.default_instance.do_something()


Output:

*** 20 ***
*** 0 ***


If you really wanted the default instance to be a class level attribute of Example, then you could add this line to foo.py

Example.default_instance = Example(0)


Personally, I don't think making this read only adds that much value; in general we let people do what they want in Python (that's why we don't have language defined private/protected/internal/public/etc features).  If the user wants to mess with something they shouldn't, that's on them.