"With the `class` statement, we can create a *user-defined* data type that we also call a **class**.\n",
"\n",
"Then, to create new **instances** of the data type, we simply call the class, just as we do with the built-in constructors.\n",
"\n",
"Conceptually, a class is the **blueprint** defining the **behavior** each instance exhibits.\n",
"\n",
"In the example used throughout the chapter, the `Vector` and `Matrix` classes implement the linear algebra rules all `Vector` and `Matrix` instances follow *in general*. On the contrary, the instances **encapsulate** the **state** of *concrete* vectors and matrices.\n",
"\n",
"The `class` statement acts as a *namespace* that consists of simple *variable assignments* and *function definitions*:\n",
"1. Variables become the **class attributes** that are shared among all instances.\n",
"2. Functions may take a different role:\n",
" - By default, they become **instance methods** by going through a **binding process** where a reference to the instance on which the method is *invoked* is passed in as the first argument. By convention, the corresponding parameter is called `self`; it embodies an instance's state: That means that instance methods set and get **instance attributes** on and from `self`.\n",
" - They may be declared as **class methods**. Then, the binding process is adjusted such that the first argument passed in is a reference to the class itself and, by convention, named `cls`. A common use case is to design **alternative constructors**.\n",
" - They may also be declared as **properties**. A use case for that are *derived* attributes that follow semantically from an instance's state.\n",
" \n",
"The **Python Data Model** concerns what special methods (i.e., the ones with the dunder names) exists and how they work together.\n",
"\n",
"The instantiation process is controlled by the `.__init__()` method.\n",
"\n",
"The `__repr__()` and `__str__()` methods implement the **text representation** of an instance, which can be regarded as a Unicode encoded representation of all the state encapsulated in an instance.\n",
"\n",
"**Sequence emulation** means that a user-defined data type exhibits the same four properties as the built-in sequences, which are regarded as finite and iterable containers with a predictable order. The `.__len__()`, `.__iter__()`, `__reversed__()`, `__getitem__()`, and some others are used to implement the corresponding behaviors.\n",
"\n",
"Similarly, **number emulation** means that an instance of a user-defined data type behaves like a built-in number. For example, by implementing the `.__abs__()` method, an instance may be passed to the built-in [abs() <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/library/functions.html#abs) function.\n",
"\n",
"If different data types, built-in or user-defined, share a well-defined set of behaviors, a single function may be written to work with objects of all the data types. We describe such functions as **polymorphic**.\n",
"\n",
"Classes may specify how *operators* are **overloaded**. Examples for that are the `.__add__()`, `.__sub__()`, or `.__eq__()` methods.\n",
"\n",
"**Packages** are folders containing **modules** (i.e., \\*.py files) and a \"*\\_\\_init\\_\\_.py*\" file. We use them to design coherent libraries with reusable code."
