Python Collections.UserList

Collections.UserList

The collections.UserList is similar in spirit to collections.UserDict, but instead of providing a base for creating custom dictionary-like objects, UserList aids in creating custom list-like objects.

Subclassing built-in types directly can sometimes be tricky because of their specific (and optimized) implementations. The UserList class is a pure Python implementation of a list that can be subclassed more easily than the built-in list.

1. Basic Usage

To create a custom list-like object, you subclass UserList:

from collections import UserList class MyList(UserList): pass lst = MyList([1, 2, 3, 4]) print(lst) # Output: [1, 2, 3, 4]

Internally, UserList does not store the items directly in the instance but in an attribute named data. So, lst.data would give you the actual list.

2. Overriding Methods

You can override methods to add custom behaviors:

class NoDuplicatesList(UserList): def append(self, item): if item not in self.data: super().append(item) lst = NoDuplicatesList([1, 2, 3]) lst.append(3) lst.append(4) print(lst) # Output: [1, 2, 3, 4]

In the above example, we've overridden the append method to ensure that no duplicate items are added to the list.

3. Additional Methods and Attributes

You can also add completely new methods or attributes:

class CountAppendList(UserList): def __init__(self, *args, **kwargs): super().__init__(*args, **kwargs) self.append_count = 0 def append(self, item): self.append_count += 1 super().append(item) lst = CountAppendList() lst.append(1) lst.append(2) print(lst.append_count) # Output: 2

In this example, we added an append_count attribute to track how many times an item has been appended to the list.

4. Benefits Over Subclassing list

  • The UserList class does not have some of the pitfalls associated with subclassing built-in types directly.
  • It ensures that any custom behaviors added through overridden methods are respected and executed as expected.

Summary:

  • collections.UserList is a list-like class designed for easy subclassing.
  • The actual items are stored in the data attribute of UserList instances.
  • It provides a more predictable and Pythonic way to create custom list-like behaviors compared to subclassing the built-in list.
  • It's ideal when you need list-like behavior but with specific modifications or additional methods and attributes.

Remember, while UserList is helpful for specific use cases, in many scenarios, the built-in list (or simple subclassing of list) might be sufficient. Use UserList when you anticipate the complexities of custom behaviors to be more easily handled with it than with the built-in list.

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