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    <p>Yes, list comprehensions "leak" their variable in Python 2.x, just like for loops.</p> <p>In retrospect, this was recognized to be a mistake, and it was avoided with generator expressions. EDIT: As <a href="https://stackoverflow.com/questions/4198906/python-list-comprehension-rebind-names-even-after-scope-of-comprehension-is-thi/4199344#comment10208805_4199355">Matt B. notes</a> it was also avoided when set and dictionary comprehension syntaxes were backported from Python 3.</p> <p>List comprehensions' behavior had to be left as it is in Python 2, but it's fully fixed in Python 3.</p> <p>This means that in all of:</p> <pre><code>list(x for x in a if x&gt;32) set(x//4 for x in a if x&gt;32) # just another generator exp. dict((x, x//16) for x in a if x&gt;32) # yet another generator exp. {x//4 for x in a if x&gt;32} # 2.7+ syntax {x: x//16 for x in a if x&gt;32} # 2.7+ syntax </code></pre> <p>the <code>x</code> is always local to the expression while these:</p> <pre><code>[x for x in a if x&gt;32] set([x//4 for x in a if x&gt;32]) # just another list comp. dict([(x, x//16) for x in a if x&gt;32]) # yet another list comp. </code></pre> <p>in Python 2.x all leak the <code>x</code> variable to the surrounding scope.</p> <hr> <p><strong>UPDATE for Python 3.8(?)</strong>: <a href="https://www.python.org/dev/peps/pep-0572/" rel="nofollow noreferrer">PEP 572</a> will introduce <code>:=</code> assignment operator that <strong>deliberately leaks</strong> out of comprehensions and generator expressions! It's motivated by essentially 2 use cases: capturing a "witness" from early-terminating functions like <code>any()</code> and <code>all()</code>:</p> <pre><code>if any((comment := line).startswith('#') for line in lines): print("First comment:", comment) else: print("There are no comments") </code></pre> <p>and updating mutable state:</p> <pre><code>total = 0 partial_sums = [total := total + v for v in values] </code></pre> <p>See <a href="https://www.python.org/dev/peps/pep-0572/#appendix-b-rough-code-translations-for-comprehensions" rel="nofollow noreferrer">Appendix B</a> for exact scoping. The variable is assigned in closest surrounding <code>def</code> or <code>lambda</code>, unless that function declares it <code>nonlocal</code> or <code>global</code>.</p>
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