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[3.7] bpo-33609: Document dict insertion order guarantee as of 3.7 (GH-7093) #7407

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miss-islington merged 1 commit into from
Jun 4, 2018
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[3.7] bpo-33609: Document dict insertion order guarantee as of 3.7 (GH-7093) #7407

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23 changes: 23 additions & 0 deletions Doc/library/stdtypes.rst
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Expand Up @@ -4229,6 +4229,29 @@ pairs within braces, for example: ``{'jack': 4098, 'sjoerd': 4127}`` or ``{4098:
value)`` pairs. Order comparisons ('<', '<=', '>=', '>') raise
:exc:`TypeError`.

Dict preserves insertion order. Note that updating key doesn't affects the
order. On the other hand, keys added after deletion are inserted to the
last. ::

>>> d = {"one": 1, "two": 2, "three": 3, "four": 4}
>>> d
{'one': 1, 'two': 2, 'three': 3, 'four': 4}
>>> list(d)
['one', 'two', 'three', 'four']
>>> list(d.values())
[1, 2, 3, 4]
>>> d["one"] = 42
>>> d
{'one': 42, 'two': 2, 'three': 3, 'four': 4}
>>> del d["two"]
>>> d["two"] = None
>>> d
{'one': 42, 'three': 3, 'four': 4, 'two': None}

.. versionchanged:: 3.7
Dict order is guaranteed to be insertion order. This behavior was
implementation detail of CPython from 3.6.

.. seealso::
:class:`types.MappingProxyType` can be used to create a read-only view
of a :class:`dict`.
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