Python: Dynamically adding function to a class, whose name is contained in a string

Question

What is the best way to create a new member function of a class with function name contained as string? Also, this new function is merely as pass-through for another object(helper class), which has the same function name but with variable arguments. I use lambda to achieve this, but I don't know how to handle the scenario, where my pass-through wrapper would be more than one-statement (which is my requirement)

# This is a helper class
class Compensation:
   def bonus(self):
       return 10000
   def salary(self):
       # Do something
   def stack(self):
       # Do something

# This is a employer class
class employee:
   def __init__(self):
       self.compensation = Compensation()

# This is a wrapper that creates the function
def passThru(funcName):
    fn = "employee."+funcName+"=" + "lambda self, *arg: self.compensation." + funcName +"(*arg)"
    exec(fn)

fnNames = ["bonus", "salary", "stocks"]
for items in fnNames: passThru(items)

emp = employee()
emp.bonus() # returns 1000

What are you trying to accomplish? Whatever it is there is probably a better way especially because eval (exec) is evil. — msw
– msw, Commented Oct 23, 2013 at 2:50

Tim Peters · Accepted Answer · 2013-10-23 03:59:22Z

5

All that trickery with exec gives me a headache ;-) I'm not exactly clear on what you want to do, but adding a new method with a name given by a string is really quite easy. For example,

class employee:
    pass

# Some multiline-function.
def whatever(self, a, b):
    c = a + b
    return c

e = employee()

# Make `whatever` an `employee` method with name "add".
setattr(employee, "add", whatever)
print e.add(2, 9)

Whenever you're reaching for exec, you're probably missing a straightforward way.

EDIT: an oddity here is that if someone tries to display e.add, they'll get a string claiming its name is whatever. If that bothers you, you can add, e.g.,

whatever.__name__ = "add"

Fleshing it out

Is this closer to what you want? Note that @gnibbler suggested much the same, although more telegraphically:

class Compensation:
    def bonus(self, a):
        return 10000 + a
    def salary(self):
        return 20000
    def stack(self, a=2, b=3):
        return a+b

class employee:
    def __init__(self):
        self.comp = Compensation()


e = employee()

for name in "bonus", "salary", "stack":
    def outer(name):
        def f(self, *args, **kw):
            return getattr(self.comp, name)(*args, **kw)
        f.__name__ = name
        return f
    setattr(employee, name, outer(name))

print e.bonus(9)
print e.salary()
print e.stack(b="def", a="abc")

That displays:

10009
20000
abcdef

All that said, you might want to re-think your architecture. It's strained.

edited Oct 23, 2013 at 3:59

answered Oct 23, 2013 at 2:52

Tim Peters

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2 Comments

rajachan Over a year ago

Thanks Tim. My requirement is to have whatever call another object's add function, so e.add(2,9) should call e.comp.add(2,9). I have 100s of such functions in my architecture which would merely call another object's function with a same name and I need to find better way of doing it.

rajachan Over a year ago

Thanks Tim. This really helps. I will think about reworking the architecture.

Claudiu · Accepted Answer · 2013-10-23 02:51:54Z

You want setattr. Let's say you have:

>>> inst = Foo(10)
>>> class Foo(object):
    def __init__(self, x):
        self.x = x

>>> inst = Foo(10)
>>> inst2 = Foo(50)

If you want to add a method to all instances of the class, then setattr on the class. This function will end up being an unbound method on the class, becoming bound in each instance, so it will take the self param:

>>> setattr(inst.__class__, "twice_x", lambda self: self.x * 2)
>>> inst.twice_x()
20
>>> inst2.twice_x()
100

If you want to add the function to just one instance of the class, then setattr on the instance itself. This will be a regular function which will not take the implicit self argument:

>>> setattr(inst, "thrice_x", lambda: inst.x * 3)
>>> inst.thrice_x()
30
>>> inst2.thrice_x()

Traceback (most recent call last):
  File "<pyshell#16>", line 1, in <module>
    inst2.thrice_x()
AttributeError: 'Foo' object has no attribute 'thrice_x'

John La Rooy · Accepted Answer · 2013-10-23 03:19:11Z

2

You are looking for setattr/getattr.

for func_name in fnNames:
    setattr(employee, func_name, (lambda self, *args:getattr(self.compensation, func_name)(*args)))

This still has a problem because you need the lambda function to be closed over func_name. While you could create a closure with another lambda, I'll pull it out into another function for readability

for func_name in fnNames:
    def f(func_name):  # close the lambda over "func_name"
        return lambda self, *args:getattr(self.compensation, func_name)(*args)

    setattr(employee, items, f(func_name))

edited Oct 23, 2013 at 3:19

answered Oct 23, 2013 at 2:52

John La Rooy

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