add: TIL post - Typing.List

_posts/2023-12-20-til-list-from-typing-module-is-inheritable.md

@@ -0,0 +1,102 @@
+---
+title: List from Typing module is inheritable
+date: 2023-12-20 11:55:00 +05:30
+categories: [Python, TIL, List, Typing]
+tags: [python, til, list, typing]     # TAG names should always be lowercase
+---
+
+> TIL - "Today I learned" series of Posts are short notes of new things I encounter with Python.
+
+While browsing the source of SQLAlchemy, I came across a [custom list class](https://github.com/sqlalchemy/sqlalchemy/blob/b12b7b120559d07a6f24fb2d6d29c7049084b4a5/lib/sqlalchemy/ext/orderinglist.py#L231)
+
+The custom class is created by inheriting from `List` from `typing`, rather than the builtin `list` or the `UserList` class provided by [collections](https://docs.python.org/3/library/collections.html#userlist-objects)
+
+It works.
+
+```python
+from typing import List
+
+
+class CustomList(List):
+    pass
+
+
+print(CustomList.__mro__)
+
+# (<class '__main__.CustomList'>, <class 'list'>, <class 'typing.Generic'>, <class 'object'>)
+```
+
+The builtin `list` can be seen as a parent class of CustomList.
+
+Normal list operations also work as expected.
+
+```python
+from typing import List
+
+
+class CustomList(List):
+    pass
+
+
+c = CustomList()
+c.append("foo")
+print(c)
+
+# ['foo']
+```
+
+Now, what if we try to instantiate `List` directly?
+
+```python
+list_obj1 = list()  # Works
+list_obj2 = List()  # TypeError: Type List cannot be instantiated; use list() instead
+```
+
+Let's see why this happens.
+
+The `List` class is defined in typing as an alias of the `list` class.
+
+```python
+# CPython - typing.py (https://github.com/python/cpython/blob/4afa7be32da32fac2a2bcde4b881db174e81240c/Lib/typing.py#L2609)
+
+List = _alias(list, 1, inst=False, name='List')
+```
+
+This `_alias` creates `List` as a child instance of `_BaseGenericAlias`. See that the `inst` attribute is set to False above.
+
+
+Now, let's take a look at the `_BaseGenericAlias` class:
+```python
+# CPython - typing.py (https://github.com/python/cpython/blob/4afa7be32da32fac2a2bcde4b881db174e81240c/Lib/typing.py#L1107)
+
+class _BaseGenericAlias(_Final, _root=True):
+    """The central part of internal API.
+
+    This represents a generic version of type 'origin' with type arguments 'params'.
+    There are two kind of these aliases: user defined and special. The special ones
+    are wrappers around builtin collections and ABCs in collections.abc. These must
+    have 'name' always set. If 'inst' is False, then the alias can't be instantiated,
+    this is used by e.g. typing.List and typing.Dict.
+    """
+    def __init__(self, origin, *, inst=True, name=None):
+        self._inst = inst
+        self._name = name
+        self.__origin__ = origin
+        self.__slots__ = None  # This is not documented.
+
+    def __call__(self, *args, **kwargs):
+        if not self._inst:
+            raise TypeError(f"Type {self._name} cannot be instantiated; "
+                            f"use {self.__origin__.__name__}() instead")
+        ...
+```
+
+The call dunder is overridden to raise a type error if the `_inst` attribute of the alias is False. The `__call__` function gets invoked when we call the children of the `_BaseGenericAlias` class, in our case, the `List`.
+
+As you can see in the docstring of `_BaseGenericAlias`:
+
+> If 'inst' is False, then the alias can't be instantiated, this is used by e.g. typing.List and typing.Dict.
+
+Summarizing, 
+- `typing.List` behaves like a `list` when used for inheritance 
+- But can't be used directly, just like the `list` for instantiation.