How to dynamically define class variables?

July 9, 2024

Let’s say I want to create a class:

class Foo:
    hello = "world"
    goodbye = "moon"

But both of those class variables are dynamically provided.

attrs = [("hello", "world"), ("goodbye", "moon")]


def create_class(attrs):

    class Foo:
        # INSERT DYNAMIC ATTRS HERE
        pass


create_class(attrs)

How can I dynamically set those attributes as class variables at the time the class is defined? I know I can use setattr to set them after the class is created, but this is not what I’m looking for.

I’m thinking this is possible with __build_class__ but I’m struggling to implement it myself.

>Solution :

You can do this by using the type function. It looks like this….

attrs = [("hello", "world"), ("goodbye", "moon")]

def create_class(attrs):
    # Create a dictionary of the attributes
    attr_dict = dict(attrs)
    
    # Use the type function to create the class dynamically
    Foo = type('Foo', (object,), attr_dict)
    
    return Foo

# Create the class
my_class = create_class(attrs)

# Check the attributes
print(my_class.hello)    # Output: world
print(my_class.goodbye)  # Output: moon

If you want to inherit then you just pass that to the types arg in the second tuple.

# Define a base class
class BaseClass:
    base_attr = "base"

    def base_method(self):
        return "This is a method from BaseClass"

# Attributes to be added to the new class
attrs = [("hello", "world"), ("goodbye", "moon")]

def create_class(attrs, base_classes):
    # Create a dictionary of the attributes
    attr_dict = dict(attrs)
    
    # Use the type function to create the class dynamically with inheritance
    Foo = type('Foo', base_classes, attr_dict)
    
    return Foo

# Create the class, inheriting from BaseClass
my_class = create_class(attrs, (BaseClass,))

# Check the attributes
print(my_class.hello)    # Output: world
print(my_class.goodbye)  # Output: moon

# Check inherited attributes and methods
print(my_class.base_attr)               # Output: base
print(my_class().base_method())         # Output: This is a method