Python Cookbook

Adding a Method to a Class Instance at Runtime

Credit: an anonymous contributor, Moshe Zadka

Problem

During debugging, you want to identify certain specific instance objects so that print statements display more information when applied to those objects.

Solution

The print statement implicitly calls an object’s _ _str_ _ special method, so we can rebind the _ _str_ _ attribute of the object to a suitable new bound method, which the new module lets us build:

import string
import new

def rich_str(self):
    classStr = ''
    for name, value in self._ _class_ _._ _dict_ _.items(
            ) + self._ _dict_ _.items( ):
        classStr += string.ljust(name, 15) + '\t' + str(value) + '\n'
    return classStr

def addStr(anInstance):
    anInstance._ _str_ _ = new.instancemethod(rich_str,
        anInstance, anInstance._ _class_ _)

# Test it
class TestClass:
    classSig = 'My Sig'
    def _ _init_ _(self, a = 1, b = 2, c = 3):
        self.a = a
        self.b = b
        self.c = c

test = TestClass(  )
addStr(test)
print test

Discussion

This recipe demonstrates the runtime addition of a _ _str_ _ special method to a class instance. Python calls obj._ _str_ _ when you ask for str(obj) or when you print obj. Changing the _ _str_ _ special method of obj lets you display more information for the specific instance object in question when the instance is printed during debugging.

The recipe as shown is very simple and demonstrates the use of the special attributes _ _dict_ _ and _ _class_ _. A serious defect of this approach is that it creates a reference cycle in the ...