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Doc/library/stdtypes.rst

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@@ -1445,107 +1445,107 @@ application).
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.. rubric:: Thread safety for list objects
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Reading a single element from a :class:`list` is
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:term:`atomic <atomic operation>`:
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Reading a single element from a :class:`list` is
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:term:`atomic <atomic operation>`:
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.. code-block::
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:class: green
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.. code-block::
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:class: green
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lst[i] # list.__getitem__
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lst[i] # list.__getitem__
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The following methods traverse the list and use :term:`atomic <atomic operation>`
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reads of each item to perform their function. That means that they may
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return results affected by concurrent modifications:
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The following methods traverse the list and use :term:`atomic <atomic operation>`
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reads of each item to perform their function. That means that they may
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return results affected by concurrent modifications:
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.. code-block::
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:class: maybe
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.. code-block::
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:class: maybe
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item in lst
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lst.index(item)
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lst.count(item)
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item in lst
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lst.index(item)
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lst.count(item)
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All of the above operations avoid acquiring :term:`per-object locks
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<per-object lock>`. They do not block concurrent modifications. Other
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operations that hold a lock will not block these from observing intermediate
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states.
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All of the above operations avoid acquiring :term:`per-object locks
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<per-object lock>`. They do not block concurrent modifications. Other
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operations that hold a lock will not block these from observing intermediate
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states.
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All other operations from here on block using the :term:`per-object lock`.
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All other operations from here on block using the :term:`per-object lock`.
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Writing a single item via ``lst[i] = x`` is safe to call from multiple
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threads and will not corrupt the list.
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Writing a single item via ``lst[i] = x`` is safe to call from multiple
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threads and will not corrupt the list.
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The following operations return new objects and appear
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:term:`atomic <atomic operation>` to other threads:
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The following operations return new objects and appear
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:term:`atomic <atomic operation>` to other threads:
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.. code-block::
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:class: good
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.. code-block::
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:class: good
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lst1 + lst2 # concatenates two lists into a new list
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x * lst # repeats lst x times into a new list
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lst.copy() # returns a shallow copy of the list
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lst1 + lst2 # concatenates two lists into a new list
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x * lst # repeats lst x times into a new list
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lst.copy() # returns a shallow copy of the list
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Methods that only operate on a single elements with no shifting required are
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:term:`atomic <atomic operation>`:
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Methods that only operate on a single elements with no shifting required are
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:term:`atomic <atomic operation>`:
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.. code-block::
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:class: good
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.. code-block::
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:class: good
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lst.append(x) # append to the end of the list, no shifting required
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lst.pop() # pop element from the end of the list, no shifting required
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lst.append(x) # append to the end of the list, no shifting required
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lst.pop() # pop element from the end of the list, no shifting required
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The :meth:`~list.clear` method is also :term:`atomic <atomic operation>`.
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Other threads cannot observe elements being removed.
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The :meth:`~list.clear` method is also :term:`atomic <atomic operation>`.
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Other threads cannot observe elements being removed.
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The :meth:`~list.sort` method is not :term:`atomic <atomic operation>`.
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Other threads cannot observe intermediate states during sorting, but the
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list appears empty for the duration of the sort.
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The :meth:`~list.sort` method is not :term:`atomic <atomic operation>`.
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Other threads cannot observe intermediate states during sorting, but the
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list appears empty for the duration of the sort.
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The following operations may allow :term:`lock-free` operations to observe
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intermediate states since they modify multiple elements in place:
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The following operations may allow :term:`lock-free` operations to observe
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intermediate states since they modify multiple elements in place:
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.. code-block::
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:class: maybe
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.. code-block::
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:class: maybe
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lst.insert(idx, item) # shifts elements
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lst.pop(idx) # idx not at the end of the list, shifts elements
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lst *= x # copies elements in place
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lst.insert(idx, item) # shifts elements
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lst.pop(idx) # idx not at the end of the list, shifts elements
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lst *= x # copies elements in place
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The :meth:`~list.remove` method may allow concurrent modifications since
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element comparison may execute arbitrary Python code (via
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:meth:`~object.__eq__`).
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The :meth:`~list.remove` method may allow concurrent modifications since
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element comparison may execute arbitrary Python code (via
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:meth:`~object.__eq__`).
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:meth:`~list.extend` is safe to call from multiple threads. However, its
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guarantees depend on the iterable passed to it. If it is a :class:`list`, a
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:class:`tuple`, a :class:`set`, a :class:`frozenset`, a :class:`dict` or a
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:ref:`dictionary view object <dict-views>` (but not their subclasses), the
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``extend`` operation is safe from concurrent modifications to the iterable.
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Otherwise, an iterator is created which can be concurrently modified by
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another thread. The same applies to inplace concatenation of a list with
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other iterables when using ``lst += iterable``.
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:meth:`~list.extend` is safe to call from multiple threads. However, its
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guarantees depend on the iterable passed to it. If it is a :class:`list`, a
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:class:`tuple`, a :class:`set`, a :class:`frozenset`, a :class:`dict` or a
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:ref:`dictionary view object <dict-views>` (but not their subclasses), the
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``extend`` operation is safe from concurrent modifications to the iterable.
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Otherwise, an iterator is created which can be concurrently modified by
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another thread. The same applies to inplace concatenation of a list with
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other iterables when using ``lst += iterable``.
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Similarly, assigning to a list slice with ``lst[i:j] = iterable`` is safe
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to call from multiple threads, but ``iterable`` is only locked when it is
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also a :class:`list` (but not its subclasses).
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Similarly, assigning to a list slice with ``lst[i:j] = iterable`` is safe
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to call from multiple threads, but ``iterable`` is only locked when it is
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also a :class:`list` (but not its subclasses).
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Operations that involve multiple accesses, as well as iteration, are never
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atomic. For example:
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Operations that involve multiple accesses, as well as iteration, are never
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atomic. For example:
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.. code-block::
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:class: bad
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.. code-block::
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:class: bad
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# NOT atomic: read-modify-write
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lst[i] = lst[i] + 1
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# NOT atomic: read-modify-write
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lst[i] = lst[i] + 1
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# NOT atomic: check-then-act
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if lst:
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item = lst.pop()
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# NOT atomic: check-then-act
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if lst:
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item = lst.pop()
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# NOT thread-safe: iteration while modifying
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for item in lst:
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process(item) # another thread may modify lst
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# NOT thread-safe: iteration while modifying
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for item in lst:
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process(item) # another thread may modify lst
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Consider external synchronization when sharing :class:`list` instances
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across threads. See :ref:`freethreading-python-howto` for more information.
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Consider external synchronization when sharing :class:`list` instances
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across threads. See :ref:`freethreading-python-howto` for more information.
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.. _typesseq-tuple:

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