What am I missing about how `setattr()` works in this context? How can I use `setattr Python` properly to achieve what I’m trying to do?

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1

How do I use setattr() in Python?

I’m trying to understand the basics of how to use setattr() correctly in Python, and I encountered an issue while attempting to use a class method to return data and then pass that data into another method. My goal is to get a better understanding of how classes work, specifically in relation to setting attributes dynamically.

I thought setattr() might be the solution, but I ran into an error in my code. Here’s what I’ve tried so far:

#! /bin/python2.6

class HolyGrail(object):

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

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

        value_returned = start()  # I believe this == self.get_thing()
        use_it(value_returned)

    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()

I also tried an alternate approach:

class HolyGrail(object):

    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()

But I keep getting errors like TypeError: 'str' object is not callable and NameError: global name 'start' is not defined.

What am I missing about how setattr() works in this context? How can I use setattr Python properly to achieve what I’m trying to do?

2

You can use setattr() to dynamically assign a method name to an instance attribute. However, you need to ensure that you’re calling the method correctly.

Here’s a corrected version of your code:

class HolyGrail(object):

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

    def run_it(self):
        # Using setattr to assign the method name to an attribute
        setattr(self, 'name', self.get_thing)
        
        # Calling the method stored in 'name' attribute
        value_returned = self.name()
        self.use_it(value_returned)

    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()

Here, I have use setattr() to store a reference to the get_thing() method in the name attribute. Later, we call self.name() to execute the method.

3

You can use getattr() to retrieve a method dynamically based on its name. This is especially useful when you need to access a method whose name is stored in a variable or attribute. Here’s an example:

class HolyGrail(object):

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

    def run_it(self):
        # Using getattr to access the method dynamically
        go_do_it = getattr(self, self.start)
        first_output = go_do_it()
        self.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()

Here, we use getattr(self, self.start) to dynamically retrieve the get_thing() method based on the start attribute and then call it.

4

If you don’t need dynamic assignment of the method, you can simply call the method directly without using setattr(). This approach is more straightforward and avoids unnecessary complexity.

class HolyGrail(object):

    def __init__(self):
        pass

    def run_it(self):
        # Directly calling the method
        value_returned = self.get_thing()
        self.use_it(value_returned)

    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()

Here, we directly call self.get_thing() to get the value and pass it to self.use_it(). This method is simpler and doesn’t require setattr() or getattr().