How do you Dynamically Add Properties to Classes in Python?

Dynamic Property Addition in Python

In Python, dynamic property addition to classes presents a unique challenge. Initially, one might attempt to add properties directly to class instances using setattr. However, this approach assigns a property object to the attribute, defeating the desired behavior.

The Solution

To add a property dynamically, it must be added directly to the class itself. Consider the following example:

class Foo(object):
    pass

foo = Foo()
foo.a = 3

# Dynamically add property 'b' to class
Foo.b = property(lambda self: self.a + 1)

print(foo.b)  # Outputs 4

Understanding Descriptors

Properties in Python are implemented using descriptors. Descriptors are objects that handle attribute access on a specific class. They possess get__, __set__, or __delete methods that define how the attribute is accessed, set, or deleted.

Python calls Foo.b.__get__(foo, Foo) when accessing foo.b, and the return value becomes the attribute's value. In this case, the property descriptor calls its fget method, passing the instance's value.

Method Objects as Descriptors

Methods themselves are another type of descriptor. Their get method adds the calling instance as the first argument, effectively binding it to the instance.

In summary, dynamic property addition in Python requires attaching the property to the class itself, leveraging Python's descriptor system. This allows for custom handling of attributes on a per-class basis.

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