a.extend(b) better than a+=b ?

Raymond Hettinger python at rcn.com
Thu Apr 22 12:20:56 CEST 2010

On Apr 22, 12:10 am, candide <cand... at free.invalid> wrote:
> Suppose a and b are lists.
> What is more efficient in order to extend the list a by appending all
> the items in the list b ?
> I imagine a.extend(b)to be more efficient for only appendinding the
> items from b while a+=b creates a copy of a before appending, right ?

The a+=b form invokes list.__iadd__() which is implemented using
list.extend(), so the two are basically the same.  Looking at the
source in http://svn.python.org/view/python/trunk/Objects/listobject.c?revision=78522&view=markup
we see:

static PyObject *
list_inplace_concat(PyListObject *self, PyObject *other)
	PyObject *result;

	result = listextend(self, other);
	if (result == NULL)
		return result;
	return (PyObject *)self;

There is a slight and constant difference is the overhead for making
the call.  The a.extend(b) does a dictionary lookup for the "extend"
method and creates a bound method.  Using a+=b is a little more


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