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    <p>List comprehensions leak the loop control variable in Python 2 but not in Python 3. Here's Guido van Rossum (creator of Python) <a href="http://python-history.blogspot.com/2010/06/from-list-comprehensions-to-generator.html" rel="noreferrer">explaining</a> the history behind this:</p> <blockquote> <p>We also made another change in Python 3, to improve equivalence between list comprehensions and generator expressions. In Python 2, the list comprehension "leaks" the loop control variable into the surrounding scope:</p> <pre><code>x = 'before' a = [x for x in 1, 2, 3] print x # this prints '3', not 'before' </code></pre> <p>This was an artifact of the original implementation of list comprehensions; it was one of Python's "dirty little secrets" for years. It started out as an intentional compromise to make list comprehensions blindingly fast, and while it was not a common pitfall for beginners, it definitely stung people occasionally. For generator expressions we could not do this. Generator expressions are implemented using generators, whose execution requires a separate execution frame. Thus, generator expressions (especially if they iterate over a short sequence) were less efficient than list comprehensions.</p> <p>However, in Python 3, we decided to fix the "dirty little secret" of list comprehensions by using the same implementation strategy as for generator expressions. Thus, in Python 3, the above example (after modification to use print(x) :-) will print 'before', proving that the 'x' in the list comprehension temporarily shadows but does not override the 'x' in the surrounding scope.</p> </blockquote>
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    1. POPython list comprehension rebind names even after scope of comprehension. Is this right?
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    1. USSteven Rumbalski
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    1. COI'll add that although Guido calls it a "dirty little secret", many considered it a feature, not a bug.
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      1. USSteven Rumbalski
    2. COAlso note that now in 2.7, set and dictionary comprehensions (and generators) have private scopes, but list comprehensions still don't. While this makes some sense in that the former were all back-ported from Python 3, it really makes the contrast with list comprehensions jarring.
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      1. USMatt B.
 

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