Apr 29, 2025 · Apr 11, 2025
diff --git a/Lib/test/test_dis.py b/Lib/test/test_dis.py
              LOAD_CONST               1 (<code object <genexpr> at 0x..., file "%s", line %d>)
              MAKE_FUNCTION
              LOAD_FAST                0 (x)
              GET_ITER
              CALL                     0

 %3d           LOAD_CONST               2 (1)
               MAKE_FUNCTION
               SET_FUNCTION_ATTRIBUTE   8 (closure)
               LOAD_DEREF               1 (y)
               GET_ITER
               CALL                     0
               CALL                     1
               RETURN_VALUE
diff --git a/Lib/test/test_generators.py b/Lib/test/test_generators.py
        #This should not raise
        loop()

    def test_genexpr_only_calls_dunder_iter_once(self):

        class Iterator:

            def __init__(self):
                self.val = 0

            def __next__(self):
                if self.val == 2:
                    raise StopIteration
                self.val += 1
                return self.val

            # No __iter__ method

        class C:

            def __iter__(self):
                return Iterator()

        self.assertEqual([1,2], list(i for i in C()))


 class ModifyUnderlyingIterableTest(unittest.TestCase):
    iterables = [
diff --git a/Lib/test/test_genexps.py b/Lib/test/test_genexps.py
    >>> list(g)
    [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

 Verify that the outermost for-expression makes an immediate check
 for iterability

    >>> (i for i in 6)
    Traceback (most recent call last):
      File "<pyshell#4>", line 1, in -toplevel-
        (i for i in 6)
    TypeError: 'int' object is not iterable

 Verify late binding for the outermost if-expression

    >>> include = (2,4,6,8)
diff --git a/Misc/NEWS.d/next/Core and Builtins/2025-04-10-10-29-45.gh-issue-127682.X0HoGz.rst b/Misc/NEWS.d/next/Core and Builtins/2025-04-10-10-29-45.gh-issue-127682.X0HoGz.rst
 No longer call ``__iter__`` twice when creating and executing a generator expression.
 Creating a generator expression from a non-interable will raise only when the
 generator expression is executed.
 This brings the behavior of generator expressions in line with other generators.
diff --git a/Python/compile.c b/Python/compile.c
    }
    Py_CLEAR(co);

    if (compiler_comprehension_iter(c, outermost)) {
        goto error;
    }
    VISIT(c, expr, outermost->iter);

    ADDOP_I(c, loc, CALL, 0);
Original file line number	Diff line number	Diff line change
Expand Up		@@ -184,7 +184,6 @@ def bug1333982(x=[]):
		LOAD_CONST 1 (<code object <genexpr> at 0x..., file "%s", line %d>)
		MAKE_FUNCTION
		LOAD_FAST 0 (x)
		GET_ITER
		CALL 0

		%3d LOAD_CONST 2 (1)
Expand DownExpand Up		@@ -765,7 +764,6 @@ def foo(x):
		MAKE_FUNCTION
		SET_FUNCTION_ATTRIBUTE 8 (closure)
		LOAD_DEREF 1 (y)
		GET_ITER
		CALL 0
		CALL 1
		RETURN_VALUE
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -246,6 +246,28 @@ def loop():
		#This should not raise
		loop()

		def test_genexpr_only_calls_dunder_iter_once(self):

		class Iterator:

		def __init__(self):
		self.val = 0

		def __next__(self):
		if self.val == 2:
		raise StopIteration
		self.val += 1
		return self.val

		# No __iter__ method

		class C:

		def __iter__(self):
		return Iterator()

		self.assertEqual([1,2], list(i for i in C()))


		class ModifyUnderlyingIterableTest(unittest.TestCase):
		iterables = [
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -123,15 +123,6 @@
		>>> list(g)
		[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

		Verify that the outermost for-expression makes an immediate check
		for iterability

		>>> (i for i in 6)
		Traceback (most recent call last):
		File "<pyshell#4>", line 1, in -toplevel-
		(i for i in 6)
		TypeError: 'int' object is not iterable

		Verify late binding for the outermost if-expression

		>>> include = (2,4,6,8)
Expand Down
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,4 @@
		No longer call ``__iter__`` twice when creating and executing a generator expression.
		Creating a generator expression from a non-interable will raise only when the
		generator expression is executed.
		This brings the behavior of generator expressions in line with other generators.
Original file line number	Diff line number	Diff line change
Expand Up		@@ -5910,9 +5910,7 @@ compiler_comprehension(struct compiler *c, expr_ty e, int type,
		}
		Py_CLEAR(co);

		if (compiler_comprehension_iter(c, outermost)) {
		goto error;
		}
		VISIT(c, expr, outermost->iter);

		ADDOP_I(c, loc, CALL, 0);

Expand Down