lue">
&gt;&gt;&gt; t1 = [1, 2]
&gt;&gt;&gt; t2 = t1.append(3)
&gt;&gt;&gt; print t1
[1, 2, 3]
&gt;&gt;&gt; print t2
None

&gt;&gt;&gt; t3 = t1 + [3]
&gt;&gt;&gt; print t3
[1, 2, 3]
&gt;&gt;&gt; t2 is t3
False
</font></pre>
<font color="black">This difference is important when you write functions that
are supposed to modify lists. For example, this function
<em>does not</em> delete the head of a list:
</font><pre><font size="4" color="blue">
def bad_delete_head(t):
    t = t[1:]              # WRONG!
</font>