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copied!<p>@Gabe's answer is correct, but needs to be shown clearly rather than hypothesized.</p> <p>CPython (and probably only CPython) does an in-place string append when it can. There are limitations on when it can do this.</p> <p>First, it can't do it for interned strings. That's why you'll never see this if you test with <code>a = "testing"; a = a + "testing"</code>, because assigning a string literal results in an interned string. You have to create the string dynamically, as this code does with <code>str(12345)</code>. (This isn't much of a limitation; once you do an append this way once, the <em>result</em> is an uninterned string, so if you append string literals in a loop this will only happen the first time.)</p> <p>Second, Python 2.x only does this for <code>str</code>, not <code>unicode</code>. Python 3.x does do this for Unicode strings. This is strange: it's a major performance difference--a difference in <em>complexity</em>. This discourages using Unicode strings in 2.x, when they should be encouraging it to help the transition to 3.x.</p> <p>And finally, there can be no other references to the string.</p> <pre><code>>>> a = str(12345) >>> id(a) 3082418720 >>> a += str(67890) >>> id(a) 3082418720 </code></pre> <p>This explains why the non-Unicode version is so much faster in your test than the Unicode version.</p> <p>The actual code for this is <code>string_concatenate</code> in <code>Python/ceval.c</code>, and works for both <code>s1 = s1 + s2</code> and <code>s1 += s2</code>. The function <code>_PyString_Resize</code> in <code>Objects/stringobject.c</code> also says explicitly: <em>The following function breaks the notion that strings are immutable</em>. See also <a href="http://bugs.python.org/issue980695" rel="noreferrer">http://bugs.python.org/issue980695</a>.</p>

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