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gh-157747: Document that ** unpacking accepts any object with keys() #157771

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19 changes: 11 additions & 8 deletions Doc/reference/expressions.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -629,8 +629,10 @@ For example::

Instead of a key-value pair, a dict item may be an expression prefixed by
a double asterisk ``**``. This denotes :dfn:`dictionary unpacking`.
At runtime, the expression must evaluate to a :term:`mapping`;
each item of the mapping is added to the new dictionary.
At runtime, the expression must evaluate to an object with a ``keys()`` method,
such as a :term:`mapping`; :meth:`~object.__getitem__` is called with
every key returned from the method, and each resulting
key-value pair is added to the new dictionary.
As with key-value pairs, later values replace values already set by
earlier items and unpackings.
This may be used to override a set of defaults::
Expand Down Expand Up @@ -1648,14 +1650,15 @@ used in the same call, so in practice this confusion does not often arise.
single: **; in function calls

If the syntax ``**expression`` appears in the function call, ``expression`` must
evaluate to a :term:`mapping`, the contents of which are treated as
additional keyword arguments. If a parameter matching a key has already been
given a value (by an explicit keyword argument, or from another unpacking),
evaluate to an object with a ``keys()`` method, such as a :term:`mapping`;
:meth:`~object.__getitem__` is called with every key returned from the method,
and the resulting key-value pairs are treated as additional keyword arguments.
If a parameter matching a key has already been given a value
(by an explicit keyword argument, or from another unpacking),
a :exc:`TypeError` exception is raised.

When ``**expression`` is used, each key in this mapping must be
a string.
Each value from the mapping is assigned to the first formal parameter
When ``**expression`` is used, each key must be a string.
Each value is assigned to the first formal parameter
eligible for keyword assignment whose name is equal to the key.
A key need not be a Python identifier (e.g. ``"max-temp °F"`` is acceptable,
although it will not match any formal parameter that could be declared).
Expand Down
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