gh-142518: Document thread-safety guarantees of dict operations

Doc/library/stdtypes.rst

@@ -5543,6 +5543,144 @@ can be used interchangeably to index the same dictionary entry.
    of a :class:`dict`.
 
 
+.. admonition:: Thread safety
+
+   Creating a dictionary with the :class:`dict` constructor is atomic when the
+   argument to it is a :class:`dict` or a :class:`tuple`. When using the
+   :meth:`dict.fromkeys` method, dictionary creation is atomic when the
+   argument is a :class:`dict`, :class:`tuple`, :class:`set` or
+   :class:`frozenset`.
+
+   The following operations and functions are lock-free and
+   :term:`atomic <atomic operation>`.
+
+   .. code-block::
+      :class: good
+
+      d[key]       # dict.__getitem__
+      d.get(key)   # dict.get
+      key in d     # dict.__contains__
+      len(d)       # dict.__len__
+
+   All other operations from here on hold the per-object lock.
+
+   Writing or removing a single item is safe to call from multiple threads
+   and will not corrupt the dictionary:
+
+   .. code-block::
+      :class: good
+
+      d[key] = value        # write
+      del d[key]            # delete
+      d.pop(key)            # remove and return
+      d.popitem()           # remove and return last item
+      d.setdefault(key, v)  # insert if missing
+
+   These operations may compare keys using :meth:`~object.__eq__`, which can
+   execute arbitrary Python code. During such comparisons, the dictionary may
+   be modified by another thread. For built-in types like :class:`str`,
+   :class:`int`, and :class:`float`, that implement :meth:`~object.__eq__` in C,
+   the underlying lock is not released during comparisons and this is not a
+   concern.
+
+   The following operations return new objects and hold the per-object lock
+   for the duration of the operation:
+
+   .. code-block::
+      :class: good
+
+      d.copy()      # returns a shallow copy of the dictionary
+      d | other     # merges two dicts into a new dict
+      d.keys()      # returns a new dict_keys view object
+      d.values()    # returns a new dict_values view object
+      d.items()     # returns a new dict_items view object
+
+   The :meth:`~dict.clear` method holds the lock for its duration. Other
+   threads cannot observe elements being removed.
+
+   The following operations lock both dictionaries. For :meth:`~dict.update`
+   and ``|=``, this applies only when the other operand is a :class:`dict`
+   that uses the standard dict iterator (but not subclasses that override
+   iteration). For equality comparison, this applies to :class:`dict` and
+   its subclasses:
+
+   .. code-block::
+      :class: good
+
+      d.update(other_dict)  # both locked when other_dict is a dict
+      d |= other_dict       # both locked when other_dict is a dict
+      d == other_dict       # both locked for dict and subclasses
+
+   All comparison operations also compare values using :meth:`~object.__eq__`,
+   so for non-built-in types the lock may be released during comparison.
+
+   :meth:`~dict.fromkeys` locks both the new dictionary and the iterable
+   when the iterable is exactly a :class:`dict`, :class:`set`, or
+   :class:`frozenset` (not subclasses):
+
+   .. code-block::
+      :class: good
+
+      dict.fromkeys(a_dict)      # locks both
+      dict.fromkeys(a_set)       # locks both
+      dict.fromkeys(a_frozenset) # locks both
+
+   When updating from a non-dict iterable, only the target dictionary is
+   locked. The iterable may be concurrently modified by another thread:
+
+   .. code-block::
+      :class: maybe
+
+      d.update(iterable)        # iterable is not a dict: only d locked
+      d |= iterable             # iterable is not a dict: only d locked
+      dict.fromkeys(iterable)   # iterable is not a dict/set/frozenset: only result locked
+
+   Operations that involve multiple accesses, as well as iteration, are never
+   atomic:
+
+   .. code-block::
+      :class: bad
+
+      # NOT atomic: read-modify-write
+      d[key] = d[key] + 1
+
+      # NOT atomic: check-then-act (TOCTOU)
+      if key in d:
+          del d[key]
+
+      # NOT thread-safe: iteration while modifying
+      for key, value in d.items():
+          process(key)  # another thread may modify d
+
+   To avoid time-of-check to time-of-use (TOCTOU) issues, use atomic
+   operations or handle exceptions:
+
+   .. code-block::
+      :class: good
+
+      # Use pop() with default instead of check-then-delete
+      d.pop(key, None)
+
+      # Or handle the exception
+      try:
+          del d[key]
+      except KeyError:
+          pass
+
+   To safely iterate over a dictionary that may be modified by another
+   thread, iterate over a copy:
+
+   .. code-block::
+      :class: good
+
+      # Make a copy to iterate safely
+      for key, value in d.copy().items():
+          process(key)
+
+   Consider external synchronization when sharing :class:`dict` instances
+   across threads. See :ref:`freethreading-python-howto` for more information.
+
+
 .. _dict-views:
 
 Dictionary view objects