Adding Dynamic Properties to Classes

In the quest to simulate a database result set using mock classes, a challenge arises: how to assign dynamic properties to an instance that resemble those of a dictionary. This involves creating properties that behave like attributes with specific values.

Initially, a promising approach involved assigning properties using:

setattr(self, k, property(lambda x: vs[i], self.fn_readyonly))

However, this yielded property objects instead of the desired behavior.

The solution lies in adding properties to the class itself, rather than individual instances. Here's how it works:

class Foo(object):
    pass

foo = Foo()
foo.a = 3
Foo.b = property(lambda self: self.a   1)
print(foo.b)  # Prints 4

In this example, we add a property b to the class Foo using the property descriptor. When accessing foo.b, Python calls the __get__ method of the descriptor and passes the class instance as an argument. The __get__ method then returns the value of the property, which is calculated as self.a 1.

Properties provide a convenient way to define custom behavior for attributes, effectively exposing the plumbing of Python's OO system.