gh-142518: Document thread-safety guarantees of list operations (#142519)

* Add everything to code blocks
* Improve wording around atomicity; specify exact types
* Better explain lock-free and atomicity

Author: lysnikolaou

Doc/library/stdtypes.rst

@@ -1441,6 +1441,109 @@ application).
          list appear empty for the duration, and raises :exc:`ValueError` if it can
          detect that the list has been mutated during a sort.
 
+.. admonition:: Thread safety
+
+   Reading a single element from a :class:`list` is
+   :term:`atomic <atomic operation>`:
+
+   .. code-block::
+      :class: green
+
+      lst[i]   # list.__getitem__
+
+   The following methods traverse the list and use :term:`atomic <atomic operation>`
+   reads of each item to perform their function. That means that they may
+   return results affected by concurrent modifications:
+
+   .. code-block::
+      :class: maybe
+
+      item in lst
+      lst.index(item)
+      lst.count(item)
+
+   All of the above methods/operations are also lock-free. They do not block
+   concurrent modifications. Other operations that hold a lock will not block
+   these from observing intermediate states.
+
+   All other operations from here on block using the per-object lock.
+
+   Writing a single item via ``lst[i] = x`` is safe to call from multiple
+   threads and will not corrupt the list.
+
+   The following operations return new objects and appear
+   :term:`atomic <atomic operation>` to other threads:
+
+   .. code-block::
+      :class: good
+
+      lst1 + lst2    # concatenates two lists into a new list
+      x * lst        # repeats lst x times into a new list
+      lst.copy()     # returns a shallow copy of the list
+
+   Methods that only operate on a single elements with no shifting required are
+   :term:`atomic <atomic operation>`:
+
+   .. code-block::
+      :class: good
+
+      lst.append(x)  # append to the end of the list, no shifting required
+      lst.pop()      # pop element from the end of the list, no shifting required
+
+   The :meth:`~list.clear` method is also :term:`atomic <atomic operation>`.
+   Other threads cannot observe elements being removed.
+
+   The :meth:`~list.sort` method is not :term:`atomic <atomic operation>`.
+   Other threads cannot observe intermediate states during sorting, but the
+   list appears empty for the duration of the sort.
+
+   The following operations may allow lock-free operations to observe
+   intermediate states since they modify multiple elements in place:
+
+   .. code-block::
+      :class: maybe
+
+      lst.insert(idx, item)  # shifts elements
+      lst.pop(idx)           # idx not at the end of the list, shifts elements
+      lst *= x               # copies elements in place
+
+   The :meth:`~list.remove` method may allow concurrent modifications since
+   element comparison may execute arbitrary Python code (via
+   :meth:`~object.__eq__`).
+
+   :meth:`~list.extend` is safe to call from multiple threads.  However, its
+   guarantees depend on the iterable passed to it. If it is a :class:`list`, a
+   :class:`tuple`, a :class:`set`, a :class:`frozenset`, a :class:`dict` or a
+   :ref:`dictionary view object <dict-views>` (but not their subclasses), the
+   ``extend`` operation is safe from concurrent modifications to the iterable.
+   Otherwise, an iterator is created which can be concurrently modified by
+   another thread.  The same applies to inplace concatenation of a list with
+   other iterables when using ``lst += iterable``.
+
+   Similarly, assigning to a list slice with ``lst[i:j] = iterable`` is safe
+   to call from multiple threads, but ``iterable`` is only locked when it is
+   also a :class:`list` (but not its subclasses).
+
+   Operations that involve multiple accesses, as well as iteration, are never
+   atomic. For example:
+
+   .. code-block::
+      :class: bad
+
+      # NOT atomic: read-modify-write
+      lst[i] = lst[i] + 1
+
+      # NOT atomic: check-then-act
+      if lst:
+          item = lst.pop()
+
+      # NOT thread-safe: iteration while modifying
+      for item in lst:
+          process(item)  # another thread may modify lst
+
+   Consider external synchronization when sharing :class:`list` instances
+   across threads.  See :ref:`freethreading-python-howto` for more information.
+
 
 .. _typesseq-tuple: