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Document ReadOnly (PEP 705) (#17905)
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Add basic documentation for mypy 1.12 release.

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Co-authored-by: sobolevn <[email protected]>
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JukkaL and sobolevn committed Oct 9, 2024
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Expand Up @@ -236,6 +236,46 @@ another ``TypedDict`` if all required keys in the other ``TypedDict`` are requir
first ``TypedDict``, and all non-required keys of the other ``TypedDict`` are also non-required keys
in the first ``TypedDict``.

Read-only items
---------------

You can use ``typing.ReadOnly``, introduced in Python 3.13, or
``typing_extensions.ReadOnly`` to mark TypedDict items as read-only (:pep:`705`):

.. code-block:: python
from typing import TypedDict
# Or "from typing ..." on Python 3.13+
from typing_extensions import ReadOnly
class Movie(TypedDict):
name: ReadOnly[str]
num_watched: int
m: Movie = {"name": "Jaws", "num_watched": 1}
m["name"] = "The Godfather" # Error: "name" is read-only
m["num_watched"] += 1 # OK
A TypedDict with a mutable item can be assigned to a TypedDict
with a corresponding read-only item, and the type of the item can
vary :ref:`covariantly <variance-of-generics>`:

.. code-block:: python
class Entry(TypedDict):
name: ReadOnly[str | None]
year: ReadOnly[int]
class Movie(TypedDict):
name: str
year: int
def process_entry(i: Entry) -> None: ...
m: Movie = {"name": "Jaws", "year": 1975}
process_entry(m) # OK
Unions of TypedDicts
--------------------

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