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gh-135171: Update documentation for the generator expression #135351

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gh-135171: Update documentation for the generator expression
serhiy-storchaka Jun 10, 2025
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Update Doc/reference/expressions.rst
serhiy-storchaka Jun 10, 2025
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5 changes: 3 additions & 2 deletions Doc/reference/expressions.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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What about async for?

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It is the same. async for is mentioned at the end of this section. What is written here applies to synchronous and asynchronous generator expressions (which are created not only by async for).

: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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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Documentation/2025-06-10-17-02-06.gh-issue-135171.quHvts.rst
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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.
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