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[3.9] bpo-39868: Add documentation for Assignment Expressions (walrus, PEP 572) (GH-18851) #21622

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Jul 25, 2020
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[3.9] bpo-39868: Add documentation for Assignment Expressions (walrus, PEP 572) (GH-18851) #21622

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21 changes: 19 additions & 2 deletions Doc/reference/expressions.rst
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Expand Up @@ -1650,9 +1650,26 @@ Assignment expressions
.. productionlist::
assignment_expression: [`identifier` ":="] `expression`

.. TODO: BPO-39868
An assignment expression (sometimes also called a "named expression" or
"walrus") assigns an :token:`expression` to an :token:`identifier`, while also
returning the value of the :token:`expression`.

See :pep:`572` for more details about assignment expressions.
One common use case is when handling matched regular expressions:

.. code-block:: python

if matching := pattern.search(data):
do_something(matching)

Or, when processing a file stream in chunks:

.. code-block:: python

while chunk := file.read(9000):
process(chunk)

.. versionadded:: 3.8
See :pep:`572` for more details about assignment expressions.


.. _if_expr:
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