Creating Custom List-Like Classes in Python
Python provides developers with a wide array of built-in data structures, methods, and classes. One of the popular built-in structures is the list.
Lists are widely used in Python programming because they allow you to store a collection of items of different data types in a single variable. But sometimes, the list built-in class in Python may not offer all the functionality you desire.
In this case, it’s possible to create a custom list-like class that suits your specific needs. This article will explore how to create custom list-like classes in Python by inheriting from an abstract base class, Python’s built-in list class, and UserList.
Inheriting from an Abstract Base Class
One way to create a custom list-like class is to inherit from an abstract base class in Python. In this case, the custom list-like classes inherit a specific set of methods that provide a level of consistency and standardization across the classes.
One such abstract base class is MutableSequence. MutableSequence is a built-in abstract base class that provides methods for classes that are sequences whose elements can be modified.
When you inherit from MutableSequence, you get the required methods that enable the implementation of a list-like class.
Inheriting from Python’s Built-in list Class
Another way to create a custom list-like class is by subclassing Python’s built-in list class.
In this approach, the custom list-like class inherits all the built-in methods of the list class, plus any custom methods you define. This method is particularly useful when you need to create a list-like class that has the standard functionality of a Python list but with additional functionality.
For example, you may need a list-like class that performs operations such as concatenating two lists or filtering items in a list.
Subclassing UserList from Collections
The UserList class is another option for creating a custom list-like class in Python. It is a subclass of the collections module.
The UserList class provides a handy way to create list-like classes that don’t require you to inherit from MutableSequence explicitly. In fact, the UserList class is itself a subclass of MutableSequence.
Building a List-Like Class From an Abstract Base Class
When building a custom list-like class from an abstract base class such as MutableSequence, it’s essential to implement the required methods for the sequence to function correctly. The required methods include __getitem__(), __setitem__(), __delitem__, __len__(), and .insert().
__getitem__() is responsible for getting an item from the sequence. The __setitem__() method allows you to set or change an item.
__delitem__() is used to delete an item from the sequence. The __len__() method returns the length of the sequence, and the .insert() method inserts an item into the sequence at the desired index.
Advantages and Disadvantages of This Method
Creating custom list-like classes by inheriting from an abstract base class has several benefits. One of the benefits is improved performance.
Inheriting from an abstract base class allows you to reuse existing code, which can improve the performance of your application. Inheriting from an abstract base class also ensures you build code that is consistent with other Python classes, which also saves time and improves maintainability.
However, there are also some drawbacks to this approach. The first is that it requires advanced knowledge of Python programming.
If you’re new to Python programming, this approach may be challenging to implement, and you may run into errors. Secondly, since you have to implement all required methods for the sequence to work, it is error-prone.
Conclusion
In conclusion, creating custom list-like classes in Python can be done by inheriting from an abstract base class, Python’s built-in list class, or the UserList class. When building a list-like class from an abstract base class, it is crucial to implement the required methods to ensure that the sequence works as intended.
Nevertheless, this approach requires advanced knowledge of Python programming. While there are multiple approaches to creating a custom list-like class, developers must carefully consider their use case before choosing which approach to use.
By embracing the flexibility of Python, developers can tailor their code to meet specific requirements, creating more efficient and effective solutions.
3) Inheriting From Python’s Built-in List Class
Inheriting from Python’s built-in list class is a popular approach used to create custom list-like classes.
Directly subclassing built-in types has many technical advantages over other approaches. One of the primary advantages is that you can define the methods of the subclass with method definitions, which are written in an efficient C code.
This leads to a performance boost when working with large datasets and computation-heavy workloads. Another advantage of directly subclassing built-in types is the use of __slots__.
__slots__ is a special attribute that can be used to minimize the memory footprint of the class instance. This is achieved by defining a fixed set of attributes at the class level, and therefore the size of the class instances is reduced.
Classes that use __slots__ can be faster in both memory usage and computation speed.
Creating a StringList Class
One of the most common applications of directly subclassing Python’s built-in list class is creating a StringList class. The StringList class is essentially a list-like class that automatically converts all items added to it into strings.
This is beneficial because it saves time in cases where you need to ensure that all items in the list are strings.
To create a StringList class, you would start by defining a new class that inherits from Python’s built-in list class.
You then define a __init__() method for the StringList class. In the __init__() method, you call the __init__() method of the list class and set any other required attributes specific to the StringList class.
Finally, you override the append() method to automatically convert any appended items to a string.
class StringList(list):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
def append(self, item):
super().append(str(item))
With this implementation, calling the append() method with any other data type will automatically convert the data type to a string.
4) Subclassing UserList from Collections
Unlike directly subclassing built-in types, using UserList to create custom list-like classes requires you to subclass the UserList class from the collections module. UserList is a variant of the built-in list class and provides a more practical and readable approach to creating a list-like class.
One advantage of using UserList is that it has a .data attribute that makes it simpler to create a list-like class. The .data attribute preserves a list of elements to which the new subclass can delegate most of its work.
By taking advantage of this attribute, you can simplify coding while still having access to the Python list methods you may need. Another advantage is the use of delegation.
With UserList, you don’t have to create methods that mimic the existing Python list methods. Instead, you delegate the work to the internal .data attribute.
This can make designing and implementing custom list classes more straightforward.
Creating a StringList Class with UserList
To create a StringList class with UserList, you start by subclassing UserList. You then define a __init__() method that calls the __init__() method of the UserList class.
In the __init__() method, you can set any additional attributes specific to the StringList class.
Next, you need to override the __setitem__() method to ensure that any new items added to the list are automatically converted to strings.
class StringList(UserList):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
def __setitem__(self, index, value):
super().__setitem__(index, str(value))
With this implementation, every time a new item is added to the StringList, it is automatically converted to a string.
Conclusion
Creating custom list-like classes in Python can be achieved by inheriting from Python’s built-in list class or by using UserList from the collections module. Although both approaches have their advantages and disadvantages, understanding when to use each approach can lead to more efficient and streamlined design.
Creating a custom list-like class allows you to create more focused and specific data structures that can improve the overall performance of your code. As with every Python development project, be sure to weigh the trade-offs to determine the optimal approach for your use case.
5) Coding List-Like Classes: Practical Examples
In the previous sections, we explored how to create custom list-like classes in Python by inheriting from an abstract base class, Python’s built-in list class, or UserList from the collections module. In this section, we’ll explore some practical examples of creating custom list-like classes and how they can be used to create more specific and efficient data structures.
A List That Accepts Numeric Data Only
Often, data validation is crucial when working with custom list-like classes. One practical example is creating a List that accepts only numeric data.
In this case, if the list is intended for purely numerical data, any attempt to add non-numeric data to the list will create an error.
To achieve this, we can create a custom list-like class called NumberList that only accepts numeric values.
class NumberList(list):
def __init__(self, *args):
for arg in args:
if not isinstance(arg, (int, float, complex)):
raise TypeError('NumberList accepts numeric values only')
super().__init__(args)
def append(self, item):
if not isinstance(item, (int, float, complex)):
raise TypeError('NumberList accepts numeric values only')
super().append(item)
In the code above, we defined the NumberList class with the built-in list class as a parent class. We then define the __init__() method, which ensures that any data passed to the list is numeric only.
If the data passed is not numeric, a TypeError is raised. The overridden append() method also validates the item added to the NumberList instance before appending it.
This implementation ensures that every NumberList instance is consistent and can only accept numeric values. In cases where the list is intended for numerical data, this guarantees consistency and prevents possible errors or bugs.
Implementing NumberList using UserList
Another approach to implementing a NumberList is by using the UserList from the collections module. Unlike direct subclassing from list, using UserList comes with several advantages such as automated validation of inputs.
class NumberList(UserList):
def __init__(self, *args):
for arg in args:
if not isinstance(arg, (int, float, complex)):
raise TypeError('NumberList accepts numeric values only')
super().__init__(args)
def __setitem__(self, index, item):
if not isinstance(item, (int, float, complex)):
raise TypeError('NumberList accepts numeric values only')
super().__setitem__(index, item)
def append(self, item):
if not isinstance(item, (int, float, complex)):
raise TypeError('NumberList accepts numeric values only')
super().append(item)
In this implementation, we created the NumberList class that inherits from UserList. We then defined the __init__() method, which ensures that any data passed to the list is numeric only.
The overridden __setitem__() method and append() method check that the input data is numeric before setting or appending the item to the list. This approach ensures that any instance of NumberList is always consistent and can only accept numeric values.
Conclusion
Custom list-like classes are a powerful tool that allows developers to create specific and efficient data structures. In this section, we explored the practical example of creating a custom list-like class that accepts only numeric values.
We saw how we could implement this custom list-like class by subclassing from Python’s built-in list class or by using UserList from the collections module. Regardless of the approach taken, we emphasized the importance of data validation when defining custom list-like classes.
In this article, we explored how to create custom list-like classes in Python by inheriting from an abstract base class, Python’s built-in list class, or UserList from the collections module. We discussed the technical advantages of directly subclassing built-in types, such as defining the methods of the subclass with method definitions, using __slots__ to minimize memory footprint, and C code for performance.
We also provided practical examples of custom list-like classes, including a NumberList that only accepts numeric data and demonstrated how to implement NumberList using both direct inheritance from list and UserList. Overall, creating custom list-like classes can improve the efficiency and effectiveness of Python code, and it is important to be aware of the various approaches available.