Python 3.13.3 runs __iter__ twice in a list comprehension

[fb2078]

Bug report

Bug description:

I can't upload comprehensionTestbed2.py because I am told the .py type is not allowed so I here is the script I used to reproduce this issue:

===========================================

# TestClass: Class that interacts with the DVD data file.

class TestClass:
    # Initialize the file timestamp.
    def __init__(self):
        pass
    # The iterator method.
    def __iter__(self):
        print('S1')
        self._index = 0
        return self
    # The next method.
    def __next__(self):
        print('S5')
        if self._index == 5:
            raise StopIteration
        value = self._index
        self._index += 1
        return value


# Test code.

print('#1')
for n in TestClass():
    print(n)

print('#2')
numbers = [n for n in TestClass()]
print(numbers)

==============================================

Here is the output of the Python script above:

D:\Files\Testing> comprehensionTestbed2.py                
#1                                                        
S1                                                        
S5                                                        
0                                                        
S5                                                        
1                                                        
S5                                                        
2                                                        
S5                                                        
3                                                        
S5                                                        
4                                                        
S5                                                        
#2                                                        
S1                                                        
S1  <-----  The __iter__ method is invoked twice in the list comprehension when using 3.13.3 (and 3.13.2).                                            
S5                                                        
S5                                                        
S5                                                        
S5                                                        
S5                                                        
S5                                                        
[0, 1, 2, 3, 4]                                           

This does not happen with python 3.12.9 and is a real problem when the iter method creates a mutex.

Edward

CPython versions tested on:

3.13

Operating systems tested on:

Windows

[picnixz]

cc @carljm (I don't know if you're the one to ping but IIRC, you worked on list comprehensions recently)

[picnixz]

Ah, could it be a duplicate of #127682?

[efimov-mikhail]

Ah, could it be a duplicate of #127682?

I think so, this is a duplicate.