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gh-135171: Update documentation for the generator expression (GH-135351)

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* gh-135171: Update documentation for the generator expression

Document that the iterator for the leftmost "for" clause is created
immediately.

* Update Doc/reference/expressions.rst

Co-authored-by: Brian Skinn <brian.skinn@gmail.com>

---------

Co-authored-by: Brian Skinn <brian.skinn@gmail.com>
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Serhiy Storchaka 2025-06-15 03:32:44 +03:00 • committed by GitHub
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Doc/reference/expressions.rst
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@ -406,8 +406,9 @@ brackets or curly braces.
Variables used in the generator expression are evaluated lazily when the
:meth:`~generator.__next__` method is called for the generator object (in the same
fashion as normal generators). However, the iterable expression in the
leftmost :keyword:`!for` clause is immediately evaluated, so that an error
produced by it will be emitted at the point where the generator expression
leftmost :keyword:`!for` clause is immediately evaluated, and the
:term:`iterator` is immediately created for that iterable, so that an error
produced while creating the iterator will be emitted at the point where the generator expression
is defined, rather than at the point where the first value is retrieved.
Subsequent :keyword:`!for` clauses and any filter condition in the leftmost
:keyword:`!for` clause cannot be evaluated in the enclosing scope as they may

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Misc/NEWS.d/next/Documentation/2025-06-10-17-02-06.gh-issue-135171.quHvts.rst Normal file
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@ -0,0 +1,2 @@
Document that the :term:`iterator` for the leftmost :keyword:`!for` clause
in the generator expression is created immediately.