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> Pretty sure it was a bug that they kept for backwards compat, not intentional

Yep, as demonstrated by only list comprehensions exposing this behaviour (on python 2) whereas none of generator comprehensions, dictionary comprehensions or set comprehensions do:

    >>> [x for x  in range(5)]
    [0, 1, 2, 3, 4]
    >>> x
    4
    >>> {y for y  in range(5)}
    set([0, 1, 2, 3, 4])
    >>> y
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'y' is not defined
    >>> {y:y for y  in range(5)}
    {0: 0, 1: 1, 2: 2, 3: 3, 4: 4}
    >>> y
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'y' is not defined
    >>> list(y for y  in range(5))
    [0, 1, 2, 3, 4]
    >>> y
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'y' is not defined
    >>>