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<overview>

<p> In Python 2 list comprehensions are evaluated in the enclosing scope, which means that the iteration variable of a list comprehension is visible

outside of the list comprehension. In Python 3 the iteration variable is no longer visible in the enclosing scope.

</p>

<p>

Code that uses the value of a list comprehension iteration variable after the list comprehension has finished will

behave differently under Python 2 and Python 3.

</p>

</overview>

<recommendation>

<p>Explicitly set the variable in the outer scope to the value that it would have held when run under Python 2.

Then rename the list comprehension variable for additional clarity.

</p>

</recommendation>

<example>

<p>In this example, <code>x</code> is initially assigned the value of 3.

In Python 3, <code>x</code> will be unchanged as the list comprehension is evaluated in its own scope.

In Python 2, evaluation of the list comprehension occurs in the scope of <code>two_or_three</code>, setting <code>x</code> to 2.</p>

<sample src="LeakingListComprehension.py" />

<p> The following example is the same code as above, but the list comprehension variable is renamed to ensure it does not overwrite <code>x</code>.</p>

<sample src="LeakingListComprehensionFixed.py" />

</example>

<references>

<li>Python Tutorial: <a href="https://docs.python.org/2/tutorial/datastructures.html#list-comprehensions">List Comprehensions</a>.</li>

<li>The History of Python: <a href="http://python-history.blogspot.co.uk/2010/06/from-list-comprehensions-to-generator.html">From List Comprehensions to Generator Expressions</a>.</li>

<li>Python Language Reference: <a href="https://docs.python.org/2/reference/expressions.html#list-displays">List displays</a>.</li>

</references>

</qhelp>