MetaPython: Adding Methods to a Class

Question

I would like to add some methods to a class definition at runtime. However, when running the following code, I get some surprising (to me) results.

test.py

class klass(object):
    pass

for i in [1,2]:
    def f(self):
     print(i)
    setattr(klass, 'f' + str(i), f)

I get the following when testing on the command line:

>>> import test
>>> k = test.klass()
>>> k.f1()
2
>>> k.f2()
2

Why does k.f1() return 2 instead of 1? It seems rather counter intuitive to me.

notes

This test was done using python3.0 on a kubuntu machine.

Answer 1

My guess is that it's because print (i) prints i not by value, but by reference. Thus, when leaving the for loop, i has the value 2, which will be printed both times.

Answer 2

It's the usual problem of binding -- you want early binding for the use of i inside the function and Python is doing late binding for it. You can force the earlier binding this way:

class klass(object):
    pass

for i in [1,2]:
    def f(self, i=i):
        print(i)
    setattr(klass, 'f' + str(i), f)

or by wrapping f into an outer function layer taking i as an argument:

class klass(object):
    pass

def fmaker(i):
    def f(self):
        print(i)
    return f

for i in [1,2]:
    setattr(klass, 'f' + str(i), fmaker(i))