https://github.com/dimanu-py/python-custom-builtin
A custom implementation of a Python dictionary
https://github.com/dimanu-py/python-custom-builtin
learning-by-doing python python-builtin
Last synced: about 1 month ago
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A custom implementation of a Python dictionary
- Host: GitHub
- URL: https://github.com/dimanu-py/python-custom-builtin
- Owner: dimanu-py
- Created: 2024-03-18T16:57:08.000Z (about 1 year ago)
- Default Branch: main
- Last Pushed: 2024-03-18T18:47:10.000Z (about 1 year ago)
- Last Synced: 2025-02-13T21:47:27.791Z (3 months ago)
- Topics: learning-by-doing, python, python-builtin
- Language: Python
- Homepage:
- Size: 8.79 KB
- Stars: 0
- Watchers: 1
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
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README
# Creating a custom dictionary in Python
[](https://python.org)
> This exercise is extracted from the book [Python Object-Oriented Programming](https://www.amazon.es/Python-Object-Oriented-Programming-maintainable-object-oriented/dp/1801077266/ref=asc_df_1801077266/?tag=googshopes-21&linkCode=df0&hvadid=529772228567&hvpos=&hvnetw=g&hvrand=17210193121882028235&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9181140&hvtargid=pla-1286085671460&psc=1&mcid=a6e3b83f21283dadbc7c32fb85beaca3)
## Introduction
In Python, we can use different types of collections to store data. The most common ones are lists, tuples, sets, and dictionaries.
All these built-in collections implement the `Collection` protocol, which acts as an abstraction for the different types of collections.
The `Collection` at the same time depends on the `Container`, `Iterable`, and `Sized` protocols.
Container
- It defines the `__contains__` method, which is used to check if a collection contains a specific element.
- By implementing the `__contains__` method, a collection can be used in the `in` and `not` operators.
Iterable
- It defines the `__iter__` method, which is used to iterate over the elements of a collection.
- By implementing the `__iter__` method, a collection can be used in a `for` loop.
Sized
- It defines the `__len__` method, which is used to get the number of elements in a collection.
- By implementing the `__len__` method, a collection can be used in the `len` function.
## Built-in dictionary
The class diagram of the built-in dictionary is shown below.
> [!IMPORTANT]
> In the [explanation of the commits section](#explanation) we will understand why this class hierarchy is important.
> [!NOTE]
> All these class definitions can be found inside _typing.pyi_ and _builtins.pyi_ files.
In Python, we can create a dictionary in two ways using the [overload decorator](https://levelup.gitconnected.com/overload-functions-in-python-d045375cff04):
1. Passing a sequence of key-value pairs to the `dict` constructor.
```python
my_dict = dict({"a": 42, "b": 43, "c": 44})
```
2. Passing a list of tuples with key-value pairs to the `dict` constructor.
```python
my_dict = dict([("a", 42), ("b", 43), ("c", 44)])
```
## Custom dictionary
We want to create a custom immutable dictionary. The goal is to be able to load our dictionary-like mapping once with
its keys and values, and then use it to map the keys to their values.
We want to end up with a type hint like this:
```python
BaseMapping = Mapping[Comparable, Any]
```
We are going to create a dictionary-like mapping from some key to an object of any type.
We've defined the key as `Comparable` because we want to be able to compare the keys and sort them in order.
The commits of the main branch explain step by step how the process is done.
1. [Defining the `Comparable` protocol](https://github.com/dimanu-py/python-custom-builtin/commit/b628aeb0f8b05c9ec5e7106fb629d4ceeaafca48)
2. [Ho to use `@overload` decorator to allow different signatures](https://github.com/dimanu-py/python-custom-builtin/commit/3291739184e3aec22f2d416914a5c4e1f976c480)
3. [What means to inherit from `Sized`](https://github.com/dimanu-py/python-custom-builtin/commit/794d4bc9340521edc1fa5d45cf8087e85bd1ee7b)
4. [What means to inherit from `Iterable`](https://github.com/dimanu-py/python-custom-builtin/commit/aef85ece930a808fb33caf19a0eb05640b9c596c)
5. [What means to inherit from `Container`](https://github.com/dimanu-py/python-custom-builtin/commit/2a5267ac4038ff5c84e3482223d825da5253d2b2)
6. [What means to inherit from `Mapping`](https://github.com/dimanu-py/python-custom-builtin/commit/eb480e038cd3094509a603ad3e68f74c8ce3cdac)
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