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copied!<p>Python distinguishes between the <code>+</code> and <code>+=</code> operators and provides separate hooks for these; <a href="http://docs.python.org/2/reference/datamodel.html#object.__add__" rel="noreferrer"><code>__add__</code></a> and <a href="http://docs.python.org/2/reference/datamodel.html#object.__iadd__" rel="noreferrer"><code>__iadd__</code></a>. The <code>list()</code> type simply provides a different implementation for the latter.</p> <p>It is more efficient for lists to implement these separately; <code>__add__</code> has to return a completely new list, while <code>__iadd__</code> can just extend <code>self</code> then return <code>self</code>.</p> <p>In the C code, <code>__iadd__</code> is implemented by <a href="http://hg.python.org/cpython/file/b0935ef48186/Objects/listobject.c#l914" rel="noreferrer"><code>list_inplace_concat()</code></a>, which simply calls <code>listextend()</code>, or, in python code, <code>[].extend()</code>. The latter takes <em>any</em> sequence, by design.</p> <p>The <code>__add__</code> method on the other hand, represented in C by <a href="http://hg.python.org/cpython/file/b0935ef48186/Objects/listobject.c#l506" rel="noreferrer"><code>list_concat</code></a>, only takes a <code>list</code> as input, probably for efficiency's sake; it can loop directly over the internal C array and copy items over to the new list.</p> <p>In conclusion, the reason <code>__iadd__</code> accepts any sequence is because when <a href="http://www.python.org/dev/peps/pep-0203/" rel="noreferrer">PEP 203</a> (the Augmented Add proposal) was implemented, for lists it was simplest just to reuse the <code>.extend()</code> method.</p>

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