Using setattr() in python

Question

I am looking for someone to explain the basics of how to use, and not use setattr().

My problem arose trying to use one class method/function to return data that is then put in another method/function. Perhaps a simpler approach would be much better in this case, but I'm trying to understand how classes work/are used. This problem seems to hinge on setattr(), and this is my attempt to make a fairly simple use of this.

Though it's not quite the same problem, I was following Python The Hard Way, ex42—the while loop @ lines 18-41.

I tried writing an \__init__(), and using getattr() instead, thinking perhaps something needed to be in the class' namespace, but this doesn't seem to help.

#! /bin/python2.6

class HolyGrail(object):

    def __init__(self):
        self.start = 'start_at_init'

    # function definition in question:
    # TypeError: 'str' object is not callable

    def run_it(self):
        start = setattr(self, 'name', 'get_thing')
        start = self.name

        # Something wrong here?
        value_returned = start() #I believe this == self.get_thing()
        use_it(value_returned)

    """
    # alternate function definitions
    # NameError: global name 'start' is not defined

    def __init__(self):
        self.start = 'get_thing'

    def run_it(self):
        go_do_it = getattr(self, start)
        first_output = go_do_it()
        use_it(first_output)
    """

    def get_thing(self):
        return "The Knights Who Say ... Ni!"

    def use_it(self, x):
        print x
        print "We want a shrubbery!"

my_instance = HolyGrail()
my_instance.run_it()

---

@Karl Knechtel, @Amber , @Chris Morgan thanks for your help.

I think I can now explain my own answer! This required a better grasp of self as an object for me. It's an instance name that gets tagged up with stuff like attributes.

The class could be a Town, and then. getattr looks for a house using it's name so you are ready to call on it soon, and comes up with a different place if you don't find the house --With getattr a 'name' exists, and you go find it. Makes the step from one function to another dynamic As a bonus you may have a default value, useful to get a fallback default method--connection failed or something?

setattr builds a house and gives it a name so you can call in on it later. You could potentially rebuild this house, or go to a particular place if you are unable to find it. --setattr makes an attribute name and gives, or changes it's value, to be called on later Perhaps a user turns sound off, then future methods don't output any audio.

I could have written my function a number of ways, but there's no need to change any attributes:

def run_it(self):
    yo = getattr(self, 'get_thing')
    answer = yo()
    setattr(self, 'deal_accepted', self.use_it) #really ott
    no = getattr(self, 'deal_accepted')
    no(answer)

Properly corrected code:

def run_it(self):
    value_returned = self.get_thing()
    self.use_it(value_returned)

Answer 1

The Python docs say all that needs to be said, as far as I can see.

setattr(object, name, value)

This is the counterpart of getattr(). The arguments are an object, a string and an arbitrary value. The string may name an existing attribute or a new attribute. The function assigns the value to the attribute, provided the object allows it. For example, setattr(x, 'foobar', 123) is equivalent to x.foobar = 123.

Answer 2

You are setting self.name to the string "get_thing", not the function get_thing.

If you want self.name to be a function, then you should set it to one:

setattr(self, 'name', self.get_thing)

However, that's completely unnecessary for your other code, because you could just call it directly:

value_returned = self.get_thing()

Answer 3

Setattr: We use setattr to add an attribute to our class instance. We pass the class instance, the attribute name, and the value. and with getattr we retrive these values

For example

Employee = type("Employee", (object,), dict())

employee = Employee()

# Set salary to 1000
setattr(employee,"salary", 1000 )

# Get the Salary
value = getattr(employee, "salary")

print(value)