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bpo-35224: Add PEP 572 (assignment expressions) to What's New #12941

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gvanrossum merged 1 commit into from
Apr 24, 2019
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bpo-35224: Add PEP 572 (assignment expressions) to What's New #12941

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16 changes: 16 additions & 0 deletions Doc/whatsnew/3.8.rst
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Expand Up @@ -67,6 +67,22 @@ Summary -- Release highlights
New Features
============

Assignment expressions
----------------------

There is new syntax (the "walrus operator", ``:=``) to assign values
to variables as part of an expression. Example::

if (n := len(a)) > 10:
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@serhiy-storchaka serhiy-storchaka Apr 24, 2019

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Is this the recommended way of writing such code? Is it more preferable than a separate statement n = len(a)?

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@gvanrossum gvanrossum Apr 24, 2019

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Writing it this way indicates to the human reader (though not to the interpreter -- see the PEP's fine print about scopes :-) that n is only relevant inside the if block.

You are really free to write it either way, there is no recommendation intended -- this is just a quick example. (I strongly advise everyone to try and come up with examples that are meaningful, rather than using foo, bar, spam and ham. This one was a compromise. Perhaps someone can come up with a more realistic example where data over (or under!) a certain length must be rejected.

print(f"List is too long ({n} elements, expected <= 10)")

See :pep:`572` for a full description.

(Contributed by Emily Morehouse in :issue:`35224`.)

.. TODO: Emily will sprint on docs at PyCon US 2019.


Parallel filesystem cache for compiled bytecode files
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