Add a method to a Built-In Function

ghz 13hours ago ⋅ 1 views

How can I add a method to a built-in function?

For example:

Adding push function (Inspired from deque module) to a list

def push(self, element):
    self.insert(0, self)  # First way I could think of

# And adding push to built-in `list`
# Example:

class list:
    def __init__(self, iterable):
        # Do something with iterable
        self.push = push

Not necessarily push method but add any method to any built-in function. I'm just giving an example.

Answer

In Python, modifying built-in types directly (such as adding methods to list, int, or other built-in types) is not straightforward due to how Python's internal objects are managed. Built-in types are implemented in C and are immutable in terms of adding new attributes or methods at runtime. However, there are a few strategies you can use to achieve the effect you're looking for, depending on your needs.

Here’s an overview of your options:

1. Subclassing Built-in Types

You can subclass the built-in list type and add methods to the subclass. This is the most straightforward and Pythonic way to extend the behavior of built-in types.

Example: Subclassing list to add a push method

class MyList(list):
    def push(self, element):
        self.insert(0, element)  # Insert at the beginning (like `deque`)

# Usage
my_list = MyList([1, 2, 3])
print(my_list)  # [1, 2, 3]
my_list.push(0)
print(my_list)  # [0, 1, 2, 3]

In this example, MyList is a subclass of list, and we've added a push method that inserts an element at the beginning of the list (similar to a deque).

2. Monkey Patching

Monkey patching allows you to modify or extend existing classes at runtime. You can add new methods or attributes to existing classes like list by directly modifying their definition. While this approach is less safe and less recommended for production code (since it can have unintended side effects), it's possible.

Example: Monkey patching the built-in list type

def push(self, element):
    self.insert(0, element)  # Insert at the beginning

# Monkey patch the `list` class
list.push = push

# Usage
my_list = [1, 2, 3]
print(my_list)  # [1, 2, 3]
my_list.push(0)
print(my_list)  # [0, 1, 2, 3]

In this example, we add the push method to the built-in list class by directly assigning it to list.push. Now, all list instances will have the push method.

Note: This will work for list objects globally, but be careful, as modifying built-in classes can lead to subtle bugs, especially if you are working in larger codebases or libraries that expect standard behavior.

3. Using types.MethodType to Add Methods

You can use types.MethodType to dynamically add methods to instances of a class. This allows you to add instance methods to an object rather than the entire class.

Example: Adding a method to an instance of list

import types

def push(self, element):
    self.insert(0, element)

# Create a normal list
my_list = [1, 2, 3]

# Add `push` method to this instance only
my_list.push = types.MethodType(push, my_list)

# Usage
print(my_list)  # [1, 2, 3]
my_list.push(0)
print(my_list)  # [0, 1, 2, 3]

In this example, the push method is only added to the my_list instance, not to all list objects. This gives you a more controlled way to add methods without affecting the global list class.

4. Using a Wrapper Class

You can also create a wrapper class around a built-in object, where you define methods that delegate to the original object. This approach gives you full control over what methods are available and allows you to "add" methods without modifying the original class.

Example: Wrapper Class

class ListWrapper:
    def __init__(self, iterable):
        self._list = list(iterable)
    
    def push(self, element):
        self._list.insert(0, element)
    
    def __getitem__(self, index):
        return self._list[index]
    
    def __setitem__(self, index, value):
        self._list[index] = value
    
    def __repr__(self):
        return repr(self._list)

# Usage
wrapped_list = ListWrapper([1, 2, 3])
print(wrapped_list)  # [1, 2, 3]
wrapped_list.push(0)
print(wrapped_list)  # [0, 1, 2, 3]

Here, ListWrapper is a class that wraps around the built-in list object and provides a push method. You can delegate other list operations like __getitem__, __setitem__, etc., to the internal list object.

Summary of Approaches

  • Subclassing: Best for creating custom list-like objects with additional methods.
  • Monkey Patching: Allows modification of the built-in list class, but should be used cautiously.
  • MethodType: Adds methods to individual instances of a class without modifying the class itself.
  • Wrapper Class: Encapsulates a built-in object and extends its functionality in a controlled way.

Conclusion

If you simply want to add a method like push to list, subclassing or monkey patching are the most common and easiest ways. If you need to limit the method to specific instances, MethodType is a good choice. For more control and flexibility, wrapping the list in another class (a wrapper class) is an effective approach.

#py