Python Property Access using PyGetSetDef

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Now, before we dive into the details of this magical technique, let’s first take a moment to appreciate how much easier our lives would be if we could just magically make properties appear out of thin air. Imagine being able to write code like this:

# Define a class called MyClass, inheriting from the object class
class MyClass(object):
    # Define a constructor method that takes in the self parameter
    def __init__(self):
        # Set the attribute _x to 0
        self._x = 0
        
    # Define a property called x
    @property
    # Define a method called x that takes in the self parameter
    def x(self):
        # Return the value of the attribute _x
        return self._x
    
    # Define a setter method for the property x
    @x.setter
    # Define a method called x that takes in the self parameter and a value parameter
    def x(self, value):
        # Check if the value is an integer and greater than or equal to 0
        if isinstance(value, int) and value >= 0:
            # Set the attribute _x to the value
            self._x = value

But alas, we can’t have everything in life. Instead, we need to use PyGetSetDef to create our properties. And let me tell you, its not as bad as it sounds! In fact, it’s pretty ***** easy once you get the hang of it.

Before anything else what is a property? A property is essentially an object attribute that can be accessed like any other attribute, but with some added functionality (like validation or caching). To create a property using PyGetSetDef, we need to define three functions: `getter`, `setter`, and `deleter`.

Here’s what our example from earlier would look like if we were using PyGetSetDef instead of the @property decorator:

# The following script creates a class called MyClass with a property called x that can be accessed, set, and deleted.

class MyClass(object):
    def __init__(self):
        self._x = 0 # Initialize the private variable _x to 0
        
    # Define a getter function for the property x using PyGetSetDef
    # The getter function uses the lambda expression to return the value of the private variable _x
    # The docstring provides a brief description of the function's purpose
    x_getter = property(fget=lambda obj: getattr(obj, '_x'), doc="Get the value of x")
    
    # Define a setter function for the property x using PyGetSetDef
    # The setter function uses the lambda expression to set the value of the private variable _x to an integer
    # The docstring provides a brief description of the function's purpose
    x_setter = property(fset=lambda obj, value: setattr(obj, '_x', int(value)), doc="Set the value of x to an integer greater than or equal to zero")
    
    def __del__(self):
        del self._x # Delete the private variable _x when the object is deleted

Now, let’s break down what we did here. First, we defined two new functions: `x_getter` and `x_setter`. These are our getter and setter functions respectively.

The `fget` parameter of the property constructor is used to specify the function that should be called when accessing the property (i.e., getting its value). In this case, we’re using a lambda function to call the `_x` attribute directly on our object and return it. This allows us to avoid having to write out the full `getter` function every time we want to create a new property.

The `fset` parameter of the property constructor is used to specify the function that should be called when setting the value of the property (i.e., assigning it). In this case, we’re using another lambda function to call the `_x` attribute directly on our object and set its value to an integer greater than or equal to zero. Again, this allows us to avoid having to write out the full `setter` function every time we want to create a new property.

Finally, we defined a `__del__` method for our class that simply deletes the `_x` attribute when it’s no longer needed (i.e., when the object is being destroyed). This allows us to avoid having to write out this code every time we want to create a new property as well.

It may not be as elegant or concise as using the @property decorator, but it’s definitely more flexible and customizable (not to mention, it works on older versions of Python that don’t support property access).

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